Ignimbrites and Block-And-Ash

Ignimbrites and
Block-And-Ash
Flow Deposits
Welded-ignimbrite vent
Outflow
Vent fill
Shattered
country rock
Marginal vitrophyre
Country rock
A. FREUNDT
Conduit
GEOMAR Research Center for Marine Geosciences
C. J. N. WILSON
Institute of Geological & Nuclear Sciences
S. N. CAREY
University of Rhode Island
I.
II.
III.
IV.
V.
VI.
VII.
VIII.
flow moving along the ground. This term includes both
pyroclastic flows and pyroclastic surges but has no connotation of particle concentration or flow steadiness.
pyroclastic flow deposit Predominantly massive, poorly
sorted, ash-rich deposit laid down by a particulate gaseous flow. This term is mostly used to imply a depositing
flow of high particle concentration (tens of volume percent) in contrast to a low particle concentration pyroclastic surge.
pyroclastic surge deposit Strongly bedded (laminated, lowangle cross-bedded) deposits laid down by fast-moving
gaseous flows of low particle concentration.
General Features of Ignimbrites
Welded Ignimbrites
Block-and-Ash Flow Deposits
Pyroclastic Flows That Entered the Sea
Ignimbrite Source Vents
Products of Secondary Processes
Generation and Transport Processes
Future Research
Glossary
ash flow/ash-flow deposit or ash-flow tuff Deposit of pyroclastic density current(s) consisting predominantly (⬎50
wt%) of ash-size material. In practice, ash-flow tuff is
used for moderate- to large-volume ignimbrites in the
U.S. literature.
block-and-ash flow deposit Small-volume pyroclastic flow
deposit characterized by a large fraction of dense to
moderately vesicular juvenile blocks in a medium to
coarse ash matrix of the same composition.
ignimbrite Welded or unwelded, pumiceous, ash-rich deposit of pyroclastic density current(s). This term was
formerly used for strongly welded deposits only.
pyroclastic density current A particulate gaseous volcanic
Encyclopedia of Volcanoes
T
HE PERCEPTION OF pyroclastic flows as a
highly dangerous and devastating type of explosive volcanic activity was brought about dramatically by
the 1902 nuee ardente (glowing cloud) of Mt. Pelée,
Martinique, which killed 30,000 people. Studies of recent historic eruptions facilitated the recognition of
abundant ignimbrites in the geological record, many of
them orders of magnitude larger in volume. It took until
the 1960s, however, before large welded ignimbrites
with lavalike field characteristics but a clearly fragmental
texture became widely accepted as deposits of pyroclastic
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Copyright  2000 by Academic Press
All rights of reproduction in any form reserved.
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flows. The deposits of pyroclastic flows are highly variable in bulk volume (⬍0.1 to ⬎1000 km3), runout distance (⬍1 to ⬎100 km), deposit geometry and response
to topography (channel confined to radially symmetrical), internal structure (massive through layered; Figs.
1a–c), degree of welding (unwelded to lavalike; Fig.
1d), clast type and size (wall rock lithic- to pumicedominated, ash- to block-rich; Figs. 1a and 1c; see also
color inserts), and chemical composition (mafic through
felsic, often compositionally zoned; Fig. 1b). Gradations, rather than sharp boundaries, for all these characteristics occur between different ignimbrites and
FIGURE 1 (a) Flow units near canyon wall in Laacher See ignimbrite. Sharp boundaries between lower lithic-rich and upper pumicerich, reversely graded zones reflect local detachment of respective zones in the flows. (See also color insert.) (b) C. 60-m-thick section
of ignimbrite at Crater Lake, Oregon, compositionally zoned from lower rhyodacite (white) through upper mafic andesite (gray) with
pink oxidation zone near top. Pinnacles are cemented fumaroles that resisted erosion. (c) Two depositional units of block-and-ash
flows at Mt. Merapi, Indonesia. Note meter-sized blocks. Person for scale. (See also color insert.) (d) Densely welded ignimbrite from
Gran Canaria with highly flattened and sheared fiamme, some pulled apart and rotated.
I GNIMBRITES
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between different facies within one ignimbrite. Furthermore, there are transitions between pyroclastic flow and
pyroclastic surge deposits. Such diversity in the deposits
has caused a similar diversity in the inferred transport
and emplacement processes. Most authors use the term
pyroclastic flow to imply a current of high particle concentration as opposed to pyroclastic surge of low particle
concentration. We use the term pyroclastic density current
here to avoid specific implications on the nature of the
flow other than that it was a gas–pyroclast mixture moving along the ground driven mainly by gravity.
(a)
Valley-fill
VTTS Ignimbrite
500
500
MSH blast
Fisher, Alaska
700
(c)
Aniakchak, Alaska
800
Ata, Japan
1500
Taupo, NZ
(b)
0
20
40
60
80
Runout distance (km)
5 km
103
Taupo Ignimbrite
A. Areal Distribution
50 km
Ito
100
Campanian
(d)
Bishop
Volume (km3)
I. General Features
of Ignimbrites
Moving pyroclastic density currents are gravity-controlled and preferentially follow basins and channels.
This topographic control shows in the areal distribution
of ignimbrites (Fig. 2a), which typically pond to great
thickness in valleys and grade into thin deposits or disappear on topographic highs. Large ignimbrites can, however, form extensive radial sheets that completely bury
the pre-eruptive topography (Fig. 2b). Many ignimbrites
were emplaced by pyroclastic currents that were able to
surmount high topographic barriers at tens of kilometers
distance from the vent (Fig. 2c). On the other hand,
small pyroclastic flows moving across a plain can confine
themselves by forming lateral block levees, similar to
lava or debris flows, and terminate in lobes with steep
levee fronts (e.g., Mount St. Helens post-May 18 flows;
also typical of many block-and-ash flows).
The dimensions of pyroclastic flow deposits vary
greatly, and the maxima for any parameter are still being
extended. Distances traveled range from a few hundred
meters for small block-and-ash flow deposits (Section
III) to ca. 200 km for the most widespread ignimbrites.
Areas covered are from a few hundred square meters to
45,000 km2, and volumes from a few thousand cubic
meters to about 5000 km3. The three-dimensional geometry of ignimbrites has been little documented, in
part because erosion since deposition has often removed
large amounts of material, especially in thinner deposits.
A simple quantitative descriptor of the overall shape of
a deposit is the aspect ratio, defined as the ratio of the
average thickness to the diameter of a circle equal in
area to the deposit. Aspect ratios range from ⬍10⫺5 (lowaspect-ratio ignimbrite) to ⬎10⫺2 (high-aspect-ratio ig-
Radial sheet
burying topography
Valley-fill with
overbank / veneer
deposits
Oruanui RST
102
101
100
VTTS
Taupo
Alban Hills
Aso-4
Rabaul
10-1
10-2
10-2
Koya
Skessa
Pinatubo
MSH blast
MSH pfs.
10-3
10-4
10-5
Aspect ratio
10-6
FIGURE 2 (a) Illustration of cross-sectional patterns from
strictly valley-confined to topography-burying ignimbrites. (b)
Distribution maps of the Valley of Ten Thousand Smokes (VTTS),
Alaska, and Taupo, New Zealand, ignimbrites. Arrows indicate
flow directions; black dots are vent sites. Schematic cross section
shows flat top of ignimbrite in the lower VTTS. Inset diagram
illustrates exponential thickness decay of IVD and layer 1 in the
Taupo ignimbrite. (c) Illustration of ridge heights surmounted by
some pyroclastic currents tens of kilometers from vent. Bold
lines show ignimbrite runout length; numbers indicate ridge elevations in meters. Ridge width and steepness not to scale. (d) Plot
showing that aspect ratio is not related to volume of ignimbrites.
Taupo and VTTS emphasized as low- and high-aspect-ratio examples, respectively.
nimbrite). Extreme examples are the high-aspect-ratio
VTTS and low-aspect-ratio Taupo ignimbrites (Fig.
2d). Documented deposits have average thicknesses that
range over about 2 orders of magnitude and are not
simply related to either the volume or the welding state
of the deposit. For example, the 30-km3 Taupo ignimbrite has an average thickness of 1.5 m whereas the 150km3 Bishop Tuff is about 100–150 m thick, i.e., having
about 5 times the volume yet 100 times the thickness
of the Taupo ignimbrite. Thickness of more or less
radially distributed ignimbrite sheets diminishes with
distance from vent in an approximately exponential fashion (Fig. 2b); the thickness–decay pattern of small channelized pyroclastic flow deposits can be complex, depending on topography.
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B. Composition and Componentry
Ignimbrites are composed of juvenile pyroclastic material, represented by pumice or scoria in the block-lapilli
fraction and by glass shards and crystals in the ash fraction, and of foreign lithic fragments derived from the
conduit walls or picked up from the surface. Juvenile
pyroclasts are commonly moderately to highly vesicular
but can be poorly vesicular to dense in ignimbrites generated by phreatomagmatic eruptions. The chemical
composition of juvenile pyroclasts in small- to intermediate-volume pyroclastic flow deposits ranges across the
entire compositional spectrum (basaltic to rhyolitic) of
tholeiitic, calcalkaline, and alkaline magmatic series
whereas large ignimbrites mostly have highly evolved
compositions. Many ignimbrites are vertically zoned or
mixed in chemical composition, providing insight into
magma chamber structures. Corresponding with composition, ignimbrites occur in all geotectonic settings
on land. Ignimbrites generated on land have been emplaced in the sea and some even erupted from calderas
in shallow waters of a few hundred meters depth. We
are not aware of any ignimbrite generated in deeper
seawater (⬎1 km).
The relative proportions of components vary between
grain size fractions as well as with distance from the
vent or height within a single ignimbrite, and they vary
between ignimbrites as a function of magma composition and mode of pyroclastic flow generation. For example, pyroclastic flow deposits generated during phreatomagmatic eruptions are generally richer in wall rock
lithics than those produced by magmatic eruptions.
Highly welded ignimbrites are generally poor in lithics
probably because lithic fragments incorporated at the
vent can drastically cool the pyroclastic mixture and thus
inhibit welding. Lithic fragments in ignimbrites may be
basement rocks entrained in the magma chamber or
conduit or surface rocks entrained during passage of the
pyroclastic currents. Wall rock lithics can be useful to
constrain magma chamber depth or conduit position,
indicate the former presence of a hydrothermal system,
or mark major events of vent erosion or caldera subsidence during eruption where they are enriched in particular horizons.
The crystal content of the ignimbrite matrix is often
higher than in the juvenile material, implying that some
concentration of crystals has occurred during eruption
and transport of the currents. The corresponding glass
fraction can occur as the distal portions of the same
ignimbrite or form co-ignimbrite ash. Rounded pumice
clasts, abraded glass rims on crystals, and fine-ash contents increasing with distance from the vent suggest
additional vitric-ash formation occurred by abrasion
during transport.
Debris from vegetation engulfed by pyroclastic flows
is occasionally preserved in deposits emplaced at low
temperature; in general, however, finer plant debris is
completely destroyed and only thick branches and logs
survive transport but become charred. The charred
wood can be used to isotopically (14C method) determine
the age of eruption.
C. Grain Size, Texture, and Structure
The grain-size frequency distribution of pyroclastic flow
deposits is often polymodal and does not follow a singledistribution law (e.g., Gaussian or Rosin–Rammier),
although it is common to present distributions on lognormal graphs and employ the Inman descriptive measures that are based on Gaussian distribution (Fig. 3a).
The wide range in particle sizes, from micron to meter
scale, is reflected in high sorting coefficients (commonly
␴⌽ ⬎ 2) of ignimbrites whereas the predominance of
ash-sized particles causes the relatively small median
size Md⌽ ⫽ 0 to 2 (Figs. 3a and 3b), but local facies
can show extremely diverging grain-size characteristics.
Some ignimbrite facies have been described as ‘‘finesdepleted,’’ by which some authors mean that blocks are
enriched relative to ash (case A in Fig. 3c) whereas others
imply depletion of ash (case C). We recommend considering the relationships between at least three size fractions (e.g., F1 , F2 , and F3 in Fig. 3c) in order to be
able to identify coarse-enriched (A), coarse-depleted (B),
fines-depleted (C), and fines-enriched (D) facies. This
approach is necessary to distinguish size fractions that
are actively modified by dynamic processes from those
that are passively affected by the constant-sum effect.
The polymodality of ignimbrite grain-size distributions has several causes: (1) fragmentation in the vent,
which produces different size distributions for each
component (e.g., pumice, wall rock lithics, and crystals),
(2) segregation of certain components and size fractions
during transport and emplacement, and (3) additions to
certain component and size fractions either by fragmentation and abrasion during transport or by entrainment
of bed material (e.g., surface lithic fragments and older
tuff). Interpretation of ignimbrite grain-size distributions thus must consider variations in componentry between grain-size fractions. For example, a mode in the
250- to 500-애m size range might be attributed to a
concentration of crystals in this range. Ideally, then,
subpopulations characterized by individual modes may
(a)
Folk and Ward (1957)
Cumulative weight percent
MdΖ = (Φ16+Φ50+Φ84)/3
σΙ = (Φ84-Φ16)/4 + (Φ95-Φ5)/6.6
95
84
Inman (1952)
MdΦ = Φ50
σΦ = (Φ84-Φ16)/2
50
16
5
0.5
Φ5 Φ16 Φ50
Φ84 Φ95
-5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10
Grain diameter (Φ = -log2[d(mm)])
(b)
Sorting coefficient, σΦ
6
5
Ignimbrite
Surge
4
3
2 Fallout
1
0
-6
(c)
-5 -4
-3 -2 -1 0 1 2 3
Median grain size, MdΦ
F3: Medium lapilli to blocks,
coarser than -3Φ
A
C
Normal
Ignimbrite
D
F1: Fine ash,
smaller than 4Φ
B
4
5
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B LOCK -A ND -A SH F LOW D EPOSITS
6
A Coarse-enriched,
F3/(F1+F2) increases,
F1/F2=constant
B Coarse-depleted,
F3/(F1+F2) decreases,
F1/F2=constant
C Fines-depleted,
F1/(F2+F3) decreases,
F2/F3 decreases
D Fines-enriched,
F1/(F2+F3) increases,
F2/F3 increases
F2: Medium ash to
fine lapilli, 4Φ to -3Φ
FIGURE 3 (a) Cumulative grain size distribution and definition of the statistical parameters, median size (Md ) and sorting
(␴). Gray field includes most ‘‘normal’’ ignimbrite data. (b) Md– ␴
diagram illustrating different grain size characteristics of ignimbrites and fallout beds, while surge deposits overlap with both.
Dashed lines give probability distribution inside fields. Many ignimbrite facies are now known to have grain size characteristics well
outside the ignimbrite field and overlapping with the other fields.
This diagram should thus not be used to discriminate deposits
based on grain size alone. (c) Illustration of selective enrichment
and depletion of certain grain size fractions as discussed in the
text.
be identified in the bulk size distribution of each component. These can then be interpreted in terms of fragmentation and transport processes.
In general, with distance from source, median grain
size decreases, the content of fine ash increases, and size
sorting improves in unwelded ignimbrites. Maximum
sizes of vent-derived lithic clasts decrease exponentially
with distance from source and may range from metersized blocks in proximal outcrops to millimeter-sized
lithic particles in distal deposits. Maximum sizes of
pumice clasts show more variation in trends with distance
from vent; some exponentially decrease outward, others
remain high or increase out to an intermediate maximum, and some small-scale historic flows (Komagatake
1929; post-May 18 Mount St. Helens 1980) have carried the largest pumices to the distal tips of the deposits.
The absolute distance from vent up to which clasts
of a given size and density (i.e., weight) have been carried
is commonly much larger in ignimbrites than in pyroclastic surge deposits (Fig. 4a), suggesting a higher transport capacity of pyroclastic flows compared to surges.
On the other hand, the fractional range of clasts, i.e.,
their absolute distance from vent divided by the runout
length of the deposit, is highly variable but generally
lower in ignimbrites than in pyroclastic surge deposits
(Fig. 4b). This suggests that, with respect to runout
distance, pyroclastic flows may lose transport capacity
faster than surges.
(a)
(b)
-9
51.2
Pumice
Lithics
-8
25.6
RST
Ito
-7
RST
12.8
RST
ElCh
-6
6.4
ElCh
Taupo
Taupo
-5
MSH
3.2
Ito
Max. clast size (cm)
AND
Max. clast size (Φ)
I GNIMBRITES
Ito
1.6
-4
MSH
-3
0
20
40
60
80
Lateral clast range (km)
100 0
0.2
0.4
0.6
0.8
1
0.8
Clast range / runout length
FIGURE 4 (a) Distribution of maximum pumice and lithic
clast sizes with distance from vent in selected ignimbrites and
surge deposits: Ito pyroclastic flow deposit, Japan; Taupo ignimbrite, New Zealand; Rattlesnake Tuff (RST), Oregon; Mount St.
Helens blast deposit (MSH); El Chichon 1982 surge deposits
(ElCh), Mexico. Clasts of a given size reach farther from vent in
large ignimbrites (Taupo, Ito, and RST) than in small pyroclastic
flow or surge deposits (MSH and ElCh). Note pumice size maximum about 30 km from vent in Ito ignimbrite. (b) Data in (a)
presented with the maximum lateral range of clasts normalized
to runout length. Clasts of a given size reach greater fractional
distances in surges than in large ignimbrites.
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Platy fragments and tree logs, and fluidal deformation
textures in welded ignimbrites, as well as elongate microscopic crystals are often oriented in a systematic fashion that indicates flow direction controlled either by
local topography or by radial spreading from the vent.
As with local thickness changes, interpretation of local
flow directions needs to carefully consider that drainage
into valleys may have occurred during or immediately
after deposition.
Grading textures commonly allow subdivision of an
ignimbrite into depositional units, termed flow units (Fig.
1a). The standard ignimbrite flow unit comprises three
layers (Fig. 5), which are now envisioned to represent
different regions within a pyroclastic density current:
Layer 1 Deposit laid down at the flow front during
strong interaction with ambient air and
ground surface
Layer 2 Deposit of the main body and tail, i.e., the
major portion of the flow
Layer 3 Deposit from the overriding dilute ash cloud
The association of all three layers is rarely complete;
more often, layer 1 or 3, or both, are not present at all
or are at least regionally absent.
is strongly controlled by local environmental conditions.
Ground layers (also coined layer 1(H) or lithic-rich
ground layer) enriched in heavy components (lithics,
crystals) appear to be the most common type of layer 1
deposit. They range from meter-thick fines-depleted
lithic breccias to centimeter-thin ash beds rich in millimeter-sized lithics and are interpreted as segregation
bodies sedimented from within the head of a pyroclastic
current, which is postulated to be a highly dynamic
region where intense mixing with air occurs. Ground
surges are fines-poor, laminated and cross-bedded layer
1 deposits thought to be emplaced by surges moving
ahead of the pyroclastic current. Ground surges can
be either lithic- or pumice-rich (layer 1(H) and 1(P)
varieties). Jetted deposits rich in fine ash and pumice
(major layer 1(P) variety at Taupo, New Zealand) are
thought to be deposited from concentrated jets ejected
ahead of the current. Common to all layer 1 deposits is
their production at or in the head of pyroclastic density
currents so that they potentially provide insight into
how the currents’ fronts interacted with the atmosphere,
surface water, roughness and shape of the ground,
and vegetation.
1. Layer 1
2. Layer 2
A great number of specific varieties of layer 1 deposits
have been described, suggesting that layer 1 formation
Layer 2 is composed of an often reversely graded basal
ash layer 2a that is variably developed, with sharp contact
or gradational transition into overlying layer 2b. Layer
2a is thought to form in a lower boundary layer by
interaction of the flow with the substratum. The main
body of a flow unit, layer 2b, is characterized by reverse
coarse-tail grading and topward enrichment of pumice
clasts and normal coarse-tail grading and baseward enrichment of lithic clasts. The development of grading
and segregation structures in ignimbrites varies widely
(see Section ID) and is interpreted to reflect vertical
motion of clasts according to their density contrast to
the current.
Flow units contain lapilli pipes, i.e., more or less vertical narrow channels composed of lapilli that are strongly
enriched in lithics and crystals but depleted in vitric
ash. They form by the localized escape of gas from the
compacting deposit that blows out the fine vitric ash.
Lapilli pipes preferentially occur in the upper part of
layer 2 but also originate from local lithic concentrations
or burned logs and can be especially abundant above
wet ground. Lapilli pipes are mostly subvertical and
formed after emplacement of the flow unit; occasionally,
however, lapilli pipes are deformed to sigmoidal shapes
by shearing either during flow unit emplacement or
during differential compaction. Lapilli pipes may cut
Ignimbrite flow unit
3
Layers
PCZ
2b
Ash-cloud deposit
Lapilli pipes
Reverse
coarse-tail
grading of
pumice
Main body
LCZ
2a
Basal layer
1
Ground layer
Normal
coarse-tail
grading of
lithics
Reversely
graded ash
FIGURE 5 Idealized flow unit of ponded ignimbrite. Layer 2
can show a variety of grading structures from no grading to
extremely enriched lithic and pumice concentration zones (LCZ
and PCZ). Different layer 1 and layer 3 facies are discussed in
the text. White particles, pumice; black particles, lithics; dots, ash.
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through several flow units and in hot pyroclastic flow
deposits may form encrustations by vapor-phase mineralization (fumaroles; e.g., Bishop Tuff, and Valley of Ten
Thousand Smokes).
3. Layer 3
Layer 3 is a deposit of fine vitric ash that is commonly
massive with fall characteristics but may also show evidence for lateral transport on the ground, such as miniature flow units, stratification, or even cross-bedding.
The ash of layer 3 is believed to have settled from the
ash cloud overriding a pyroclastic flow. Layer 3 may not
be associated with each flow unit; in fact, layer 3 may
only be found on top of the entire ignimbrite if emplacement of the succession of flow units was faster than the
settling of fine ash from the overriding ash cloud. Some
ignimbrites are associated with very extensive co-ignimbrite ash layers, suggesting that the depositing ash clouds
had plinian-like dimensions.
Ignimbrites composed of a single flow unit exist but
more often they are composed of flow unit packages.
Ideally, a flow unit is envisioned as the deposit of a
single pyroclastic current. Flow units have, however,
been observed to split into two or more thinner flow
units laterally, probably due to splitting into lobes or
pulsatory motion of the pyroclastic flow. The number
of flow units in an outcrop thus need not be equal to
the number of depositing flows. In moderate- to largevolume ignimbrites, and especially in welded ignimbrites, it is often difficult to identify flow units as defined
above. This may be due to very thin layers 2a associated
with very thick, unstructured layers 2b or to welding
compaction and rheomorphic deformation obscuring
depositional boundaries. On the other hand, some thick,
gradually compositionally zoned ignimbrites do not reveal any clear internal depositional boundaries; this observation provoked the idea that at least some ignimbrites are emplaced by progressive aggradation (grain
by grain) from a maintained current rather than layer
by layer (flow unit by flow unit) from a number of shortlived flows.
The basal contact of pyroclastic flow deposits is
mostly conformable to the preexisting surface but can
also be highly unconformable where the pyroclastic currents were able to erode, e.g., close to the vent, on steep
slopes, or on soft substrate. Pyroclastic flows of the April
19–20, 1993, eruption of Lascar volcano (Chile) stripped
scree and talus from their channels and scratched striations and percussion marks into bedrock; strongest erosion occurred on steep slopes, where flows accelerated
through narrow channels and where they already con-
587
tained abundant lithic blocks. Ash of the strongly welded
ignimbrite P1 on Gran Canaria intruded into, completely filled, and welded within (a) void spaces of a
canyon boulder bed to several meters depth and (b)
tectonic cracks as narrow as a few millimeters and more
than a meter deep, indicating great fluidity of the pyroclast–gas mixture.
D. Facies Variations
Within a single ignimbrite, significant changes in facies
can occur on a regional scale, including changes with
distance from vent and over areas of different topographic character, and on a local scale, where they are
commonly controlled by local topographic features.
1. Regional Facies Variations
Many ignimbrites that are massive across intermediate
to distal regions contain variably bedded, tractionally
formed deposits close to the vent which are associated
with erosion and transitional to, or resemble, pyroclastic
surge deposits. Such deposits typically show lensoid,
wavy, discontinuous bedding, laminations or cross-bedding, with variable grain size and poor to moderate
sorting of individual beds, and gradually merge into
massive ignimbrite farther from the vent (e.g., Roccamonfina, Italy; Laacher See, Germany; Taupo, New
Zealand; Novarupta 1912, Alaska; Pinatubo 1991, Philippines). In addition, cross-bedded pyroclastic flow deposits also occur on steep flow paths, such as at Mount
St. Helens, during the 1980 eruptions. The Neapolitan
Yellow Tuff (Italy) is an ignimbrite that seems to be
characterized by such facies throughout most of its extent. These transitional facies are interpreted to have
been emplaced under intermediate-concentration conditions from fast pyroclastic density currents that were
evolving from the collapsing eruption column toward
higher concentration farther from the vent.
The range of facies developed in ignimbrites is particularly shown by the 30-km3 Taupo ignimbrite (Fig. 6a),
which is interpreted to represent a single short-lived
(ca. 400 s) catastrophic outburst of material, i.e., equivalent to a single vent-derived flow unit. These facies are
developed in three ways:
First, a spectacular development of layer 1 deposits
(as discussed above), which form about 15 vol% of the
ignimbrite (Fig. 6b). The exceptional development of
layer 1 deposits at Taupo is inferred to reflect the extreme velocities attained by the flow coupled with the
abundance of vegetation.
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B LOCK -A ND -A SH F LOW D EPOSITS
(b)
Taupo Ignimbrite facies
(c)
Layer 2 facies
100
Intermediate
material
IVD
Stranded
PCZ
VPI
Layer 1
Thickness (m)
(a)
IVD
10
1(P)
1
0.1
1(H)
0.01
0.5-3m
1-5m
1-70m
0.7->20m
1000
Layer 1
100
ML (cm)
Layer 2
Layer 1
Layer 1 facies
FDI facies of
layer 1(P)
Layer 1(P)
1(H)
1(P)
FDI
4
3
80
σI
60
Mz
2
40
1
0
F
20
-1
0
0 20 40 60 80
Distance from vent (km)
Fraction <63 µm, F (wt%)
Layer 1(P) everywhere
5
Layer 1(H)
Layer 1
Distant facies,
IVD discontinuous
Layer 1(H) everywhere
Layer 2
IVD continuous
IVD
0.1
Layer 2
VPI
VPI
1
Mz, σI (Φ units)
50 km
10
FIGURE 6 (a) Map of facies distribution in the Taupo ignimbrite. VPI, valley-ponded ignimbrite; IVD, ignimbrite veneer deposit; 1
(H) and 1 (P), lithic- and pumice-rich layer 1; FDI, fines-depleted ignimbrite, a variety of layer 1 (P). (b) Illustration of layer 2 and layer
1 facies changes across topography. (c) Variation with distance from vent of thickness and maximum lithic clast size (ML) in IVD, VPI,
and layer 1 deposits, and grain-size characteristics of layer 2 material.
Second, the reaction of the bulk of the ignimbrite (i.e.,
layer 2) to the landscape over which it was emplaced. In
the Taupo ignimbrite, layer 2 comprises a landscapemantling ignimbrite veneer deposit (IVD) and a landscapemodifying valley-ponded ignimbrite (VPI; Fig. 6b), the
two merging into each other over distances of decimeters to tens of meters. The relationships between the
IVD and VPI are seen in many ignimbrites where the
current(s) were able to surmount interfluves partly or
wholly.
Third, the Taupo ignimbrite shows spectacular variations (up to 2–3 orders of magnitude) in such parameters
as mean grain size, contents of lithics and crystals, fineash abundances, and largest clast sizes over the 80 ⫾ 10
km from source to distal limits (Fig. 6c). The scale of
the lateral variations at Taupo is again in large part
driven by the velocity of the parent current, with the
concomitant effective segregation and separation of different fractions.
Facies changes from channel-fill to overbank in the
much smaller (⬍1 km3) multiple-flow-unit ignimbrite
at Laacher See, Germany, differ from those in the Taupo
ignimbrite. Thick flow units in channels taper out to
ash layers on the overbanks that are only a few centimeters thick but flow unit structures, albeit mostly incomplete, persist up to some 300 m out of the channels
(Figs. 7a and 7b). Farther out, and with increasing distance from the vent, correlation of gradually thinner
overbank ash layers with channel-fill flow units becomes
almost impossible. Areal distribution and lateral changes
of the overbank facies suggest that the phase segregation
into lapilli- and lithic-rich channeled and lapilli- and
lithic-poor overbank flow occurred very close to the
vent during funneling through passes in the Laacher
See basin wall.
2. Local Facies Variations
Small-volume pyroclastic flow deposits derived from
valley-confined flows terminate marginally against the
valley walls. The Valley of Ten Thousand Smokes
(VTTS) ignimbrite, with its ‘‘high-energy’’ proximal
facies on high ground and perhaps 100-m thickness in
upper reaches of the valley, stops dead against a moraine
only 15 m high blocking the valley about 15 km from
the vent. Apparently, each of the many flows that con-
I GNIMBRITES
AND
tributed to the bulk deposit was too sluggish to surmount
this small barrier.
At Laacher See, pyroclastic flows funneled through
deep, narrow, rough-walled canyons (Fig. 7a) and interacted with canyon walls in a variety of ways, producing
marginal flow unit structures that change with travel
distance and shape of the valley walls (Fig. 7c). Some
flows were not wide enough to touch the canyon walls
that are 200–300 m apart. In upstream regions of the
canyons, flow units then taper out into thin ash-rich
margins, in some places fringed by a carpet of lagpumice blocks (Fig. 7c, diagram 5), whereas flow units
Northern
canyons
e
in
Rh
(a)
N
589
B LOCK -A ND -A SH F LOW D EPOSITS
farther down canyon are bounded by steep pumice-block
levees (Fig. 7c, diagram 9). Similar marginal structures
were also observed at Mount St. Helens. Such rare narrow flows were not accompanied by ash clouds large
enough to leave laterally continuous ash layers. In contrast, flows that filled the width of the canyons did have
ash clouds depositing on the canyon walls and shoulders
but these ash layers were flushed from the steep walls
by rain soon after emplacement and are now contained
in reworked fine ashes overlying the canyon-fill ignimbrite. The downstream changes (Fig. 7c) from proximal
wavy-bedded deposits through flow units containing
Scoria
Ignimbrite
cones
>8 m
<8 m
>5 cm overbank
0 1 2 3 4 5 km
Laacher
See
Basin
wall
Southern
basin
(b)
10
(c)
4
Flow-unit profiles
along valley center
2c
2b
3
pumice
levee 9
2a
8
7
1
2b
ground
layer
6
Marginal
flow-unit
sections
pumice
lense
ground layer
lag pumice
5
Laacher
See
basin
FIGURE 7 (a) Map of ignimbrite distribution around Laacher See volcano. Circle with arrow inside lake indicates vent migration
during pyroclastic flow production. (b) Transition of thin overbank ash layers into thick flow units in channels on the smooth topography
south of Laacher See. (c) Proximal to distal structural and compositional changes in flow unit sections along valley center (left) and
along valley margin (right) in the northern canyons in (a). Diagrams 2a–2c show changes from valley center toward the margin.
590
I GNIMBRITES
AND
B LOCK -A ND -A SH F LOW D EPOSITS
abundant segregation bodies and complex marginal
structures to distally massive flow units without marginal
interaction are interpreted to reflect progressive deflation.
3. Lithic Breccias
Heavy lithic clasts are most susceptible to changes in
the transport capacity of pyroclastic currents, which diminishes with distance from vent as reflected in the
decreasing size of lithics in ignimbrites. Local reductions
in transport capacity can be inferred from steps in the
lithic size vs distance patterns but are most evidently
marked by the occurrence of lithic lag breccias. These are
commonly clast-supported beds of lithic blocks with
an ash matrix that may or not be depleted in fine ash
compared to the normal ignimbrite matrix. Lithic blocks
are rounded in some breccias, yet others contain large
fragments of fragile rock types such as paleosols, lake
sediments, or diatomite. Lithic breccias occur in proximal settings, where they are dominated by vent-derived
wall rocks, or elsewhere in specific paleotopographic
settings, where locally derived lithic fragments may be
important.
Near-vent, commonly fines-depleted lithic lag breccias lack evidence of ballistic impact of large blocks but
may contain degassing structures such as lapilli pipes.
Such breccias are either separated from normal ignimbrite by erosional unconformities or laterally merge into
either lithic-rich ground layers (layer 1) or lithic-concentration zones of flow unit layer 2. Proximal lithic lag
breccias have been left behind by pyroclastic currents
that lost their excess load when dynamically adjusting
from column collapse to subhorizontal flow.
The formation of more distal local lithic breccias is
controlled by specific topographic conditions. Most
commonly, local lithic breccias are found at sudden
changes from steep to shallow ground slope, where pyroclastic currents possibly passed through stationary hydraulic jumps. Other local breccias occur inside sharp
bends of a valley, where large vortices may have formed
or where pyroclastic flows entered large bodies of water.
Local lithic breccias can also form immediately downstream of topographic settings that favor erosion and
massive entrainment of surface clasts by a pyroclastic
current, which subsequently dumps this extra load. Local lithic breccias are mostly not depleted in fine ash
and laterally merge into layer 2; they are commonly
massive and unbedded although some layering, crossbedding, or stoss-side stacking beds can be developed
close to vent and on steep slopes where the pyroclastic
currents had a high velocity.
II. Welded Ignimbrites
A. Introduction
Ignimbrites show textures and structures developed in
response to their high postemplacement temperatures
and retained volatiles, viz., welding, devitrification, and
vapor-phase alteration. The onset of welding in time and
space is determined primarily by the magma composition (including retained volatile species that lower glass
viscosities), load stresses, and temperature (both the absolute value and the time spent above any given value,
i.e., the cooling rate). Typical minimum temperatures
for welding are between 500 and 650⬚C for calcalkaline
compositions. Most welded ignimbrites, regardless of
temperature, include some material that is nonwelded
due to either rapid cooling (e.g., directly overlying a
cold substrate) or inadequate load stresses (e.g., the topmost part of a deposit).
B. Small-Scale Features
Welding is the cohesion, deformation, and eventual coalescence of pyroclasts at high temperatures under loading stress. In hand specimen, welding is manifested in
turn by (1) cohesion of clasts across point contacts where
load stresses are focused (‘‘sintering’’), (2) flattening of
pyroclasts under the load stress (Fig. 1d), and then, in
the most extreme cases, (3) flow as a coherent liquid
(the process termed rheomorphism).
In young deposits, the contrast between sintered and
nonwelded material may be very clear, but in ancient
deposits, sintering cannot easily be distinguished from
other diagnetic processes causing induration, and clear
welding is seen only in the presence of flattened clasts.
The higher the emplacement temperature and the lower
the clast viscosity, the more intense is the compaction
(i.e., elimination of pore space) under a given load and
less load is needed to achieve a certain degree of compaction. In extreme cases, vitroclastic textures can be eliminated in densely welded material.
The degree of welding is measured either by the bulk
density (Fig. 8a) or by the horizontal to vertical axis
ratio (strain ratio) of vitric shards and fiamme (Figs. 8b
and 1d), i.e., flattened pumice clasts, which can reach
values of 100 : 1 in densely welded ignimbrites. The
vertical variation of the strain ratio often parallels that
of bulk density. Strain ratios larger than 10 : 1 are commonly taken to indicate viscous flow has occurred.
I GNIMBRITES
(a)
Height above base
(c)
B LOCK -A ND -A SH F LOW D EPOSITS
(b)
Strain ratio
3
Te>Ts
Loose tuff
AND
2
Te>>Tmw
Constant-mass deformation
Pumice
60 vol% porosity
a:b = 1:1
b
1
a
Te>Tmw
Fiamme
zero porosity
a:b = 10:1
591
tiles that exsolve during welding can become trapped in
densely welded material and accumulate to form subspherical cavities filled by vapor-phase minerals, called lithophysae.
C. Large-Scale Features
a:b = 100:1
Bulk density
Dense rock
vapor-phase crystallization
devitrification
no welding
partial welding
dense welding
FIGURE 8 (a) Schematic vertical variation in bulk density of
welded ignimbrites emplaced at different temperatures Te above
that of minimum welding, Tmw (curves 1 and 2). Ignimbrites that
are almost entirely densely welded may have been emplaced at
above-solidus temperature, Ts (curve 3), especially if they are
thin. (b) Illustration of fiamme geometries. Mass remains constant,
but volume decreases during compaction as pore space is eliminated. The strain ratio 10 : 1 is achieved by keeping a constant
and collapsing b until pore space is zero. Higher strain ratios
involve flowage in addition to simple compaction. (c) Welding and
crystallization zones in a schematic, vertically highly exaggerated
section through a welded ignimbrite.
Highly stretched fiamme can be cracked, pulled apart,
and rotated (Fig. 1d).
Ignimbrites emplaced at temperatures sufficient to
weld commonly cool slowly enough for juvenile glass
to devitrify. There is a wide spectrum of effects from
microscopic crystallization preserving the original pyroclastic texture, though to total overprinting by a coarse
granophyric texture. At one extreme, thick, hot ignimbrites (e.g., caldera-fill deposits) can recrystallize to resemble an intrusive rock. However, minor posteruptive
devitrification in ancient deposits can be difficult to distinguish from long-term diagenetic devitrification.
Welding and devitrification release most or all of the
residual volatiles from the glass, and these volatiles can
cause further vapor-phase alteration of the ignimbrite
and/or deposition of further mineral species in pore
spaces. Common new minerals are alkali feldspar, silica
polymorphs, and hydrous minerals, but a wide variety of
more exotic mineral species are also known. In addition,
movement of vaporized groundwater or rainwater
through the still-hot ignimbrite can cause further transport and deposition of mobile elements. Residual vola-
In a welded ignimbrite, there are often systematic arrangements of the various zones of welding, devitrification, and vapor-phase alteration (Fig. 8c). Ignimbrites
deposited rapidly at uniform temperature show vertical
variations in density (i.e., welding intensity), with a maximum in the lower part of the ignimbrite, due to downward increasing load pressure and nonsymmetric cooling through the upper and lower boundaries (Fig. 8a).
For a given emplacement temperature, the zone of dense
welding widens with deposit thickness, whereas for a
given deposit thickness, the proportion of the deposit
that is densely welded increases with emplacement temperature (Fig. 8a). Overlapping with, but displaced vertically from, the welding zones are zones of devitrification
and vapor-phase alteration, the upward displacement
being due to the transport direction of heat and released
volatiles in the cooling rock body. Welded ignimbrites
also typically develop prismatic jointing—the spacing
of which is widely interpreted to reflect the cooling
time—oriented perpendicular to isotherms in the
rock mass.
A welded ignimbrite that accumulated so rapidly that
negligible cooling occurred (whether composed of one
or many flow units), resulting in a simple arrangement of
welding zones (Fig. 8c), is a simple cooling unit. Compound
cooling units show departures from the simple profile,
most clearly seen in irregular vertical gradients in bulk
density and strain ratio, and are widely interpreted to
reflect prolonged emplacement with the opportunity for
partial cooling to occur between successive packages of
material. It is not clear whether the complex density/
welding profiles in compound cooling units represent
significant time breaks in deposition (which need not
be long if intervening heavy rainfall occurs) or variations
in emplacement temperatures of successive flows.
D. Rheomorphism
Ignimbrites on sloping ground may flow rheomorphically as a coherent viscous liquid once dense welding has
eliminated intergranular friction. There is a complete
spectrum from (1) stretching and boudinage of flattened
I GNIMBRITES
AND
B LOCK -A ND -A SH F LOW D EPOSITS
fiamme to (2) deformation of internal bedding and welding fabrics into flow folds of tens of meters to millimeters
amplitude to (3) wholesale flow of the coherent liquid
to produce features similar to those of conventional lavas
of the same composition. Ramp structures and shear
faults then can severely disrupt the initial stratigraphy.
At high temperatures, partially liquid particles may
flatten under their own weight (cf. curve 3 in Fig. 8a)
and flow downslope as they aggregate on the ground.
Welding then is syn-depositional, controlled by viscosity and emplacement dynamics, as opposed to welding
after emplacement. Under such conditions viscous flow
is a continuation of the particulate pyroclastic current
and may follow primary flow directions rather than local
slope. Mapping of flow direction indicators and 3-D
structural analyses thus provide means to distinguish
between syn- and post-depositional welding.
III. Block-and-Ash
Flow Deposits
A. Distribution and Facies Associations
Block-and-ash flows can form by collapse of vulcanian
eruption columns but most historic examples were generated by partial to total collapse of viscous lava domes
as they oversteepen at their fronts during growth. Volcanoes producing lava domes and block-and-ash flows are
numerous; among the most recent examples are Mount
Unzen, Japan (1991), Mt. Merapi, Indonesia (1984,
1994, 1997–1998), and Montserrat (1995–1996). Dome
collapse can be gravitational, explosive, or both. At Unzen, a strong explosion was observed to immediately
precede the major block-and-ash flow of June 8, 1991.
Video recordings show the formation of abundant ash
as soon as the lava front collapsed, and small ash puffs
from cracks in the dome can be seen before dome-flank
failure, suggesting overpressure in the dome as a major
cause of fragmentation, although collapse was probably
initiated by gravitational instability.
Block-and-ash flows commonly extend up to 10 km
from their source at speeds up to 100 km/h and are
accompanied by ash cloud surges, which reach a wider
areal distribution compared to the valley-confined
block-and-ash flows (Fig. 9a). A seared zone where little
or no ash is deposited but vegetation is burned fringes
the ash cloud surge deposits. The hot ash-cloud surges
can knock down trees and destroy houses. They are the
most dangerous aspects of block-and-ash flow activity,
(a)
Wind
SZ
ACS
Dome
RF
1
2
BAF
Valley
(b)
6
Sorting coefficient, σΦ
592
5
Ignimbrite
Surge
4
BAF
<64mm
3
2 Fallout
1
3
ACS
0
-6
-5 -4
-3 -2 -1 0 1 2 3
Median grain size, MdΦ
4
5
6
FIGURE 9 (a) Distribution of block-and-ash flow (BAF), ashcloud surge (ACS; arrows give flow direction), and the seared
zone (SZ) below a partially collapsed dome with a rock-fall scree
(RF) at its base. Drawn after observations at Unzen and Merapi.
Numbers 1–3 indicate enhanced ACS production and detachment where BAF rushes down a waterfall (1), ACS surmounts
a bending valley wall that deflects BAF (2), and ACS advances
beyond the limit of BAF, coming to rest on lower plain (3).
Distribution of ACS deposit can be affected by wind. (b) Range
of median size and sorting values for ACS deposits and BAFmatrix material ⬍64 mm; poorly constrained since little data
are available.
most of all because they are less controlled by topography and can detach from their associated channelized
block-and-ash flows. The formation of ash-cloud surge
is, however, strongly influenced by topography (Fig.
9a). Increased ash-cloud production occurred at Mount
Unzen, where the June 3, 1991, block-and-ash flow cascaded down two waterfalls, causing a wider spread of
the ash-cloud surge downstream. Farther downstream,
a sharp bend in the valley was followed by the blockand-ash flow but not by the ash-cloud surge, which
rushed straight on out of the valley and killed 43 people.
B. Structure and Composition
Block-and-ash flows commonly have basaltic–andesite,
andesite, or dacite compositions but may also be rhyolitic. The deposits are usually described as monolithologic but it is commonly difficult to distinguish juvenile
and older dome fragments, because vesicularity and crystallinity vary across even single domes. Hydrothermal
alteration of clasts can be helpful to identify older dome
fragments but is ambiguous.
I GNIMBRITES
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B LOCK -A ND -A SH F LOW D EPOSITS
Block-and-ash flow deposits differ from pumiceous ignimbrites by containing little fine ash ⬍1/16 mm (usually ⬍5 wt%) but a large fraction of dense to moderately
vesicular, rarely pumiceous, blocks up to several meters
in diameter (Fig. 1c) that are derived from the juvenile
source dome and possibly from older dome remnants.
The deposits are poorly sorted but a complete grainsize distribution (including the block fraction) has not
yet been reported (Fig. 9b). Bed structure can be either
clast-supported or matrix-supported and generally massive but vague layering may locally occur. Flow unit
structures as in ignimbrites are not commonly observed
due to the absence of low-density fragments, but a finegrained (or block-free) basal layer may be present and
lapilli pipes are rare. The blocks are either uniformly
distributed through the bed or show normal or reverse
coarse-tail grading. Large blocks may be carried up to
the distal tip of the deposit but the largest block size has
been observed to reach a maximum at some intermediate
distance, dropping off toward the vent as well as to the
flow terminus. Block-and-ash flow deposits are about
1–10 m thick and thin to 1–2 m at their lobate, blunt
termini. The detailed variation pattern with distance of
both clast size and thickness strongly depends on how
the channel gradient evolves. There may be no deposition at all on steep slopes such as immediately below
the dome.
Ash cloud surge deposits are typically ⬍1 m thick, consist
of sand-sized and finer ash with median grain sizes ranging from 500 to ⬍100 microns, and are relatively well
sorted (Fig. 9b). Beds are laminated or consist of discontinuous or wavy layers, dune structures occur, and basal
layers can be depleted in fine ash while overall fine-ash
content increases toward the top. A topmost fine-ash
layer may contain accretionary lapilli. Ash cloud surge
deposits are exposed on the margin of the block-andash flow deposits but may also cover and/or underlie
them.
IV. Pyroclastic Flows That
Entered the Sea
A. Flow–Water Interaction
Pyroclastic flows generated at near-shore or ocean-island volcanoes can enter the sea and produce widespread
submarine volcaniclastic deposits. There is geologic evidence for a variety of ways in which pyroclastic flows
593
interact with the sea. Scenarios for pyroclastic flows
erupted on land or in shallow water include
1. Pyroclastic flows mix with seawater, causing coastal
steam explosions or transformation into debris
flows or turbidity currents
2. Pyroclastic density currents travel across the water,
dropping sediment into the sea
3. Pyroclastic flows remain intact and either push
aside shallow water near shore or flow under
deeper waters until they come to rest or transform
into debris flows
Evidence for submarine pyroclastic-flow-related debris flows and turbidites is abundant, suggesting that
mixing of pyroclastic flows with seawater commonly
occurs. That such mixing involved phreatic explosions
has been implied by increased crystal/lithic ratios or a
shift to finer sizes in the crystal population of the submarine deposits compared to those on land, but evidence
for coastal explosions is not commonly reported.
Welded ignimbrites in ancient near-shore environments
probably displaced the seawater without much interaction except for local breccia pipes at the base of the
ignimbrite where water in wet sediment was vaporized.
Little convincing evidence exists that primary, hot pyroclastic flow deposits were emplaced in deep waters.
There is proof, however, that some pyroclastic currents
were able to travel large distances across the sea; the
Koya flow in Japan, for example, had to cross 30 km
over the deep waters of the Japanese sea to be emplaced
on Kyushu and neighboring islands.
B. The Krakatau 1883 Example
The 1883 eruption of Krakatau volcano in Indonesia
provides an excellent opportunity to examine the effects
of pyroclastic flow interaction with the sea. It was a
moderate ignimbrite-producing event, which resulted
in the formation of a 5-km-diameter caldera that is open
to the sea. For 2 days a series of large-volume pyroclastic
flows entered the sea around Krakatau. Pyroclastic flow
deposits consist of 2- to 4-m-thick flow units and locally
have a total accumulated thickness of 60 m. The great
thickness of flow deposits directly adjacent to the sea
indicates that substantial volumes of flow material must
have been discharged into the sea during the eruption.
Outside of the caldera there was significant addition
of material to the sea floor as a result of the 1883 eruption (Fig. 10). Coring of the submarine deposits has
yielded two distinct facies in water depths less than
594
I GNIMBRITES
Telok
Betong
AND
B LOCK -A ND -A SH F LOW D EPOSITS
Sumatra
Java Sea
Loudon
10:30 am
Hurricane winds,
violent mudrain
W.H. Besse
After 10 am,
hurricane winds,
heavy ash fall,
strong smell of sulfur
Katimbang
burn fatalities
Lagoendi
Sebuku
stic current
rocla
trav
Py Sebesi
ell
ing
0
Panjang
Rakata
20 km
o
sea
the
Sunda
Straits
Merak
Sangiang
r
ve
Krakatau
Sertung
Charles Bal
10:30 am, 2 tsunamis,
strong wind, downpour
of mud, sand
Submarine
pyroclastic flow
deposits
Anjer
JAVA
Scale
FIGURE 10 Map of the Sunda Straits area showing Krakatau islands surrounded by submarine pyroclastic flow deposits (gray
shading) and extent of the pyroclastic current that traveled over the sea (light gray shading), overran the islands of Sebesi, Sebuku,
and Lagoendi, and reached the Sumatra coast, leaving deposits described in the text. Locations of ships that recorded passage of the
pyroclastic current (and tsunami waves) are indicated.
20 m. The first, and more abundant, is massive and
consists of a poorly sorted mixture of pumice and lapilli
in a gray, silty matrix of glass shards, crystals, and lithics.
The other facies is marked by better sorting, finer grain
size, and the presence of parallel and cross laminations.
Thermoremanent magnetization studies of samples
from the submarine massive facies indicate deposition
at temperatures in excess of 450⬚C. Because of their
high temperature and submarine emplacement, these
deposits most likely formed from hot flows that traveled
under water.
There is also evidence that a hot gas/particle current
traveled over the sea surface during the 1883 eruption
of Krakatau. The most compelling observation is a large
number of burn fatalities (ca. 2000) that took place 40
km from the vent across the Sunda Straits, along the
southeast coast of Sumatra (Fig. 10). Ships 65–80 km
away from Krakatau were engulfed by ash clouds. Pyro-
clastic deposits on islands provide important information about the nature of the ash currents as they traveled
to the north and northwest from Krakatau (Fig. 10).
The currents overran all of these islands and most of
the vegetation was destroyed, with total loss of life. The
deposits consist of three major types:
1. Volumetrically most important, poorly sorted massive beds of angular to subangular dacitic pumice
and lithic clasts in a matrix of fine ash with small
pieces of charcoal
2. Stratified beds containing pumice and lithic clasts
that are more rounded
3. Intercalated layers of pumice lapilli with virtually
no dense components, which are attributed to tsunami inundation of the islands
Grain-size characteristics of the massive and stratified
deposits support deposition from some type of pyroclas-
I GNIMBRITES
AND
595
B LOCK -A ND -A SH F LOW D EPOSITS
tic density current. These distal samples form a continuum in grain-size trends defined by the more proximal
subaerial and submarine pyroclastic flow deposits. With
increasing distance from Krakatau, there is a systematic
decrease in the median diameter and sorting coefficients
of the deposits. As the currents traveled across the sea
surface, they apparently became cooler and more dilute
(surgelike). Eventually, the currents became buoyant,
producing a large-scale co-ignimbrite plume rich in
moisture. From this plume an episode of mudrain deposition affected a large area of the Sunda Straits.
Welded-ignimbrite vent
Outflow
Vent fill
Shattered
country rock
Marginal vitrophyre
Country rock
Conduit
FIGURE 11 Section through a welded-ignimbrite vent showing foliations in marginal vitrophyre and central vent fill and a
zone of shattered country rock outside the vent.
V. Ignimbrite Source Vents
Ignimbrite source vents, especially of large-volume examples, are often hidden by caldera subsidence and caldera fill. Buried vents can be located by mapping regional facies variations in the ignimbrite (thickness,
grain size, maximum clast size, degree of welding), by
lithic lag breccias, and by the flow-orientation patterns
of logs, platy fragments, and crystals. Small- to intermediate-volume ignimbrites are commonly derived from
central vents that can be identified by mapping the distribution of associated fall beds. Fall isopachs of Laacher
See tephra bracketing the main pyroclastic flow deposits
show that these formed during a period of vent migration within the crater basin (Fig. 7a).
Moderate- to large-volume ignimbrites are commonly associated with caldera collapse. Evidence that
such ignimbrites erupted through ring fractures rather
than central vents has been provided in some cases. In
the Bishop ignimbrite, characteristic vent-derived lithic
rock types show that earlier fallout and ignimbrite production from a single vent was followed by eruption of
middle and upper parts of the ignimbrite from multiple
vents along a ring fracture that defined the structural
outline of the Long Valley caldera. Sectorial variations
in the mixing proportions of rhyolite and trachyte in
the composite ignimbrite P1 on Gran Canaria cannot
be reconciled with a central vent but require eruption
through a ring fissure; there is no associated plinian fall
and thus no evidence of previous eruption through a
central vent.
During fissure eruptions, activity probably focuses on
several sites along the fissure rather than occurring along
its whole length. Some ignimbrites have been found not
to be associated with any subsidence structure but rather
with linear fissures, several to tens of kilometers long,
and apparently controlled by regional tectonic fault systems, especially in regions of crustal extension.
Exposed sections through ignimbrite source vents not
destroyed by caldera collapse, such as the Gabbs Valley
vent in Nevada, are rare but share typical zonation patterns. The vent is roughly funnel-shaped, widening dramatically toward the surface, and is surrounded by a
zone of shattered country rock that may be 100 m wide
but diminishes with depth (Fig. 11). Inside the vent,
a vitrophyric, parataxitic textured zone with foliations
(flattened fiamme and flow lineations, local folding) parallel to the wall–rock contact lines the vent walls. The
vent interior is filled with welded, devitrified, eutaxitic
tuff with more shallowly dipping foliations that may
contain zones enriched in lithic lapilli. Deeper into the
vent where it narrows, the marginal zones merge to fill
the conduit; near the surface, foliation dips decrease and
merge with those in the outflow tuff (Fig. 11). Steeply
foliated marginal tuff is thought to have agglutinated
along the walls during eruption whereas the bowlshaped interior foliation reflects compaction of ventfilling tuff after eruption.
VI. Products of
Secondary Processes
A. Compaction and Reworking
The typical concave-up surface of ignimbrite emplaced
within valleys or overlying buried pre-eruption valleys
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is commonly interpreted to result from postdepositional
compaction during escape of gas trapped during emplacement and by densification of the grain framework
under load and during welding. Such a surface shape,
however, may already be present during emplacement
when a horizontal velocity gradient across the flow
causes marginal portions from an initially deeper flow
phase to lag behind and become emplaced next to deposits from a thinner flow tail in the center of the valley.
In addition, remobilization and secondary flow down
topographic gradients can change the shape of the deposit.
Secondary pyroclastic flows were generated from primary
ignimbrite for more than 4 years after the Mount Pinatubo eruption. The secondary pyroclastic flows formed
by collapse of primary, largely uncompacted ignimbrite,
leaving single or multiple headscarps, and traveled down
valleys for up to 10 km in much the same way as did
the primary flows. Deposits of secondary pyroclastic
flows are up to 10 m thick and almost indistinguishable
in structure and composition from the primary deposits,
containing basal lithic and topmost pumice segregation
layers and lapilli pipes. Discriminating features include
moderate fines-depletion and random thermoremanent
magnetic polarity of the secondary deposits, which may
also overlie epiclastic deposits (e.g., lahars) formed after
primary ignimbrite eruption.
Phreatic explosions, which may occur months to years
after emplacement, blast craters through pyroclastic
flow deposits emplaced on wet ground or trapped water
and form laminated to cross-bedded tuff rings on their
surface (e.g., at Mount St. Helens).
Heavy rainfall and ice melting on glaciated volcanoes
generate floods that easily erode and mix with unconsolidated pyroclastic-flow material to form lahars as, for
example, at Mount Redoubt, Alaska. It is often difficult
to distinguish primary pyroclastic flow deposits from
secondary, reworked material, especially at volcanoes
where block-and-ash flow deposits are abundant. Segregation pipes and fragments of charred vegetation are
known to occur in secondary, water-reworked deposits,
the former due to escape of steam or water and the
latter inherited from the primary hot deposit. However,
segregation pipes emanating from carbonized vegetation are good evidence for a primary, hot emplacement
mechanism, whereas the presence of a vesicular matrix
and/or close association with fluvial sediments is indicative of a secondary, wet laharic deposit. In some cases,
paleomagnetic techniques (uniformity of magnetization
direction, paleotemperature estimates) can also be used
to infer a primary or secondary origin.
B. Alteration and Cementation
During slow cooling of pyroclastic flow deposits, precipitation of iron oxides often creates an upper zone of
pink color in unwelded deposits, and precipitation of
various minerals from vapor phases (predominantly cristobalite, tridymite, and hydrous minerals) can partially
to totally fill interstices between particles. Localized degassing channels, or fumaroles, which can be active for
many years as vapor formed by interaction between the
hot deposit and rain- or groundwater moves up along
preferential channels, develop encrustations of vaporphase sublimates enriched in trace elements.
A common process of alteration and cementation of
pyroclastic flow deposits is zeolitization. The assemblage of zeolite minerals (mostly phillipsite, chabasite,
and, at a later stage, analcime, but many other zeolites
occur), the extent of zeolite cementation, and the distribution of zeolitized zones in the deposit apparently depend on the spatial and temporal variation of water
saturation, which, next to external factors, is controlled
by the local grain-size distribution (fine-grained, poorly
sorted ash is less permeable but, once soaked, retains
water longer), although zeolitization zones commonly
cross depositional boundaries. Geologic evidence does
not support a significant control of emplacement temperature or even magmatic composition on the degree of
zeolitization in most pyroclastic flow deposits. Zeolites
form from the alkali elements that become mobile during low (i.e., ambient) temperature hydration (Na⫹,
K⫹ } H⫹ ion exchange) of volcanic glass by meteoric
water.
VII. Generation and
Transport Processes
Having briefly discussed block-and-ash flow formation
in Section III, we here focus on the formation of ignimbrites that, based on historic observations, are believed
to form by collapse of eruption columns. Eruption modeling provides some insight into physical conditions of
pyroclastic flow formation. The high-speed hot pyroclast–gas mixture leaving the vent has a bulk density of
the order of 10 kg/m3 but becomes diluted by entrained
air that expands upon heating. Sufficient air entrainment
facilitates the formation of a buoyant plinian column of
lower than atmospheric density, but eruption conditions
such as wider vent diameter limit the efficiency of dilu-
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brite-producing episodes, which may or may not have
had a pulselike character, rather than by a single longlasting eruption.
Much of the present discussion on pyroclastic flow
transport relates to whether the presumably low bulk
density of the collapsing mixture is maintained for substantial lateral travel distance or the pyroclastic current
rapidly deflates to a high concentration mass flow. Generally speaking, deflation will be delayed if the flows are
fast and highly turbulent and the grain size is fine. The
transport mechanisms of pyroclastic currents are presently being discussed in terms of two extreme models,
dense pyroclastic mass flows and dilute pyroclastic suspension
currents, which essentially differ in the factors that control particle support and runout of the flows. None of
these models is, as yet, capable of quantitatively predicting both the regional mass distribution and the facies
variations of ignimbrites. We will now briefly introduce
the physical concepts and then discuss the significance
of geologic observations.
The runout of dense pyroclastic mass flows, similar to
that of avalanches, is controlled by the consumption of
initial energy by internal and external friction. A simple
(Mohr–Coulomb) model envisions a pyroclastic flow (or
avalanche) as a sliding block of some frictional strength
that will move on ground slopes ⬎␪ and reach a distance
L ⫽ H/tan ␪, where H is the height of origin and ␪ the
angle of internal friction of the material. In contrast
to cold avalanches, however, H/L ratios calculated for
pyroclastic flow deposits (Fig. 12) suffer from poor con-
10000
10
0.1
1:
100
10
1:
2
H
1:
:L
50
1000
5
tion by entrainment, keeping the mixture denser than
air so that it partially or totally collapses back to the
surface from the height where initial momentum became exhausted. High mass discharge and wide crosssectional vent area reduce the collapse height and eventually the pyroclastic mixture may spread sideways
shortly above the vent so that the resulting pyroclastic
current largely inherits the properties of the mixture at
the vent.
However, eruption models are simplified by assuming
that the eruption mixture is homogeneous in composition and physical properties across the vent but recent
detailed studies of near-vent deposits show that this is
not necessarily the case. Column collapse is not irreversible and renewed plinian activity can occur as shown,
for example, by multiple flow units intercalated with
fallout beds at Laacher See. Furthermore, geologic evidence for simultaneous plinian and ignimbrite-producing activity, such as in the historic 1912 Novarupta and
1991 Pinatubo eruptions and the prehistoric Taupo and
Bishop Tuff eruptions, and recent modeling results
show that plume buoyancy and collapse are not mutually
exclusive. Considering the complexity of natural eruption columns, modeling can only provide a first-order
approximation of the initial conditions of pyroclastic
currents.
Modeling approaches to pyroclastic density currents
employ either a finite-volume, single-pulse release from
source or steady-state conditions of maintained material
supply during transport. In the first case, the duration
of flow generation (or eruption) would be shorter than
the duration of lateral transport whereas in the second
case, eruption duration would exceed the time needed
by the current to reach its runout distance. Eruption
durations may range over ⬎3 orders of magnitude from
phreatomagmatic discrete explosions through complex
collapse patterns of nonsustained eruption columns to
more or less continuous long-lived activity and are not
simply related to pyroclastic-flow transport times, thus
allowing for a wide range of time relationships that must
be carefully identified for each deposit and eruption. A
pulselike character is attributed to pyroclastic surges
derived from instantaneous explosions and to small
pyroclastic flows derived from partial column collapse
during relatively weak eruptions. At the other extreme,
maximum eruption rates of ⬍1010 kg/s, limited by
viscous magma flow through reasonable conduit dimensions, require eruption durations longer than the
transport times of large (⬎1014 kg) ignimbrites reaching
about 100 km from source. Some large ignimbrites,
however, have been emplaced by a succession of ignim-
1:
AND
Height of origin, H (m)
I GNIMBRITES
1
Ignimbrites
Block-and-ash flows
other dense-clast pyr. flows
Mudflows
Cold avalanches
10
100
Runout length, L (km)
FIGURE 12 Height of origin (H) versus runout distance (L)
of various pyroclastic and other flow deposits. Mudflows have
lower H/L ratios than cold avalanches, and large ignimbrites have
lower ratios than small pyroclastic flow deposits, suggesting that
both lubrication of the granular material and the mass of flow
influence mobility. Note, however, that H values for pyroclastic
flows derived from column collapse are uncertain.
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straints on the value of H; even during observed eruptions, it is almost impossible to determine a specific
collapse height as collapse may occur over a height interval. Furthermore, the Mohr–Coulomb model has
proved insufficient to reproduce observed pyroclastic
flow velocities, and other friction terms, such as turbulent stresses at high speed, need to be included for more
successful modeling. The Mohr–Coulomb model also
does not consider flow mass, while it is evident that
runout length of ignimbrites generally increases with
their mass (Fig. 12). Large runout distances require significant reduction of internal friction. The effect of selflubrication observed in rapid granular flows, where the
bulk of material rides on a highly sheared boundary
layer, may be applicable to fast pyroclastic flows on steep
slopes. Pyroclastic flow mobility may also be enhanced
by fluidization, where an upward-moving gas phase partially supports and unlocks particles. The fluidization
concept envisions exsolution of magmatic gas from pyroclasts and entrainment of air, preferentially at the
flow front, as sources of gas. Hindered settling of grains
through trapped interstitial gas may have similar dragreducing effects. Inertial granular flows should stop dead
and be emplaced en masse when shear stresses drop,
e.g., upon meeting a flat ground slope. Some blockand-ash flows effectively meet this condition. Effectively
viscous, well-fluidized flows with a laminar velocity gradient should smear out their material over the runout
length.
The runout of dilute pyroclastic suspension currents is
controlled by the balance between initial mass flux and
pyroclast sedimentation and entrainment of air, which
ultimately dilute the current to below atmospheric density, causing it to loft into the air as a co-ignimbrite ash
plume. Sedimentation rates depend on grain size and
how well pyroclasts are kept in suspension by flow turbulence. Air entrainment depends on the strength of shear
at the flow surface and on engulfment at the current’s
head. It follows from the balance between mass flux,
sedimentation, and entrainment that (a) very slow and
coarse-grained currents rapidly sediment most of their
load near the vent whereas (b) very fast currents quickly
dilute by air entrainment and transform into ash plumes
carrying a large fraction of the erupted fine pyroclasts,
but (c) deep, fine-grained currents at intermediate speed
will reach the greatest runout distance. The deposit of
a pyroclastic suspension current will of necessity accumulate progressively but, since suspension currents invariably become stratified with higher particle concentration near their base, the depositional process is
probably complex and little can as yet be inferred about
the resulting bedforms and grain-size characteristics.
Studies of the Mount St. Helens May 18, 1980, blast
deposit and recent observations during the 1995–1996
eruptions on Montserrat promoted the concept that pyroclastic density currents develop a basal depositional
system that controls emplacement conditions and is dynamically distinct from the transport system, which controls overall distribution of the material. Deposit characteristics are then believed to reflect the depositional, but
not necessarily the transport, system.
Geologic evidence invoked in favor of one or the
other model is mostly ambiguous. For example, how
much are massive bedforms with floating pumice, commonly used to imply dense flow, influenced by late-stage
motion such as drainage into channels rather than by
overall transport conditions? How much dilution is really necessary to produce tractional bedforms, or can
dilute currents produce massive poorly sorted beds?
Most of what we know about bedform development
comes from aqueous systems but it is clear that gaseous
systems behave in a very different way because of different viscosity, density, and thermal expansion, so that the
value of analogies is fairly limited.
Pumiceous ignimbrite has a bulk density close to that
of water and little variation in gas content during flow
is required to allow for flow either over or under water.
The smaller the density contrast, the more efficient the
mixing with water should be and, therefore, dilute currents should travel greater distances across water. These
relationships have, however, not been explored quantitatively. Large topographic barriers can be surmounted
by dilute pyroclastic currents due to their low density
and great depth and by dense pyroclastic flows due to
large momentum. Similarly, radial flow patterns little
influenced by irregular topography would be compatible
with both dilute turbulent or dense high-momentum
flows. The suspension-current model predicts that a
large mass fraction of erupted material can be emplaced
as extensive co-ignimbrite ash. Some co-ignimbrite ash
layers subequal in volume to the ignimbrite itself would
be difficult to reconcile with elutriation from a dense
flow but it has also not yet been shown how these ash
layers, except for being time equivalents, physically relate to their associated ignimbrites. Many small channeled pyroclastic flows have clearly been emplaced in
an almost en-masse fashion with limited lateral shearing.
Other flow units grade laterally into thin widespread
ash layers that were clearly emplaced by currents much
deeper than the deposits. Thick massive ignimbrites that
are continuously compositionally zoned were apparently
emplaced by progressive aggradation from sustained
currents.
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599
VIII. Future Research
See Also the Following Articles
The deadly and destructive nature of pyroclastic density
currents makes their study an important and challenging
topic in volcanology. As in the past, most of the relevant
information has to be inferred from the resulting deposits. There are still major challenges in the documentation and interpretation of ignimbrites in order to be
able to reconstruct the dynamics and timings of their
parental currents and eruptions. We suggest that studies
of pyroclastic flow deposits should continue to focus on
facies transitions in response to topographic and other
environmental changes, since these changes are most
important in inferring changing transport conditions.
In particular, an emphasis on obtaining quantitative data
is required, as many of the comparisons, which could
be made between deposits, lack a suitable quantitative
framework. A number of such studies have been made,
but more are needed and should be placed in the context
of the general eruption conditions of the volcano investigated. Until quantitative data are available for a variety
of examples, modeling scenarios of pyroclastic density
currents as a basis for hazard assessment and emergency
planning during eruptions are still problematic. In part,
this arises from controversies over the different natures
and significance of the transport versus depositional processes and the ways in which these are interpreted from
deposits. One ultimate goal would be to model these
processes as a function of both topography and eruption
conditions that can be determined from observations
such as magma composition, volatile composition and
release patterns, vent configuration(s), and overall eruption dynamics. To advance these approaches, however,
a fundamental understanding of the compressible flow
dynamics of rapidly moving hot gas–particulate currents
is necessary, which requires dedicated experimentation
and theoretical approaches.
Calderas • Hazards from Pyroclastic Flows and Surges •
Lahars • Plinian and Subplinian Eruptions • Pyroclastic Fall
Deposits • Pyroclast Transport and Deposition •
Volcaniclastic Sedimentation around Island Arcs •
Vulcanian Eruptions
Further Reading
Carey, S. N. (1991). Transport and deposition of tephra by
pyroclastic flows and surges. In ‘‘Sedimentation in Volcanic
Settings,’’ SEPM Spec. Pub. 45, 39–57.
Druitt, T. H. (1998). Pyroclastic density currents. In ‘‘The
Physics of Explosive Volcanic Eruptions’’ (J. Gilbert and
R. S. J. Sparks, eds.), Geol. Soc. London Spec. Publ. 145,
145–182.
Freundt, A. and Bursik, M. I. (1998). Pyroclastic flow transport
mechanisms. In ‘‘From Magma to Tephra: Modeling Physical Processes of Explosive Volcanic Eruptions’’ (A. Freundt
and M. Rosi, eds.), Elsevier, Amsterdam/New York, Dev.
Volcanol. 4, 173–245.
Smith, R. L. (1960). Zones and zonal variations in welded ash
flows. U.S. Geol. Surv. Prof. Pap. 354-F, 149–159.
Sparks, R. S. J. (1976). Grain size variations in ignimbrites
and implications for the transport of pyroclastic flows. Sedimentology 23, 147–188.
Walker, G. P. L. (1983). Ignimbrite types and ignimbrite
problems. In ‘‘Explosive Volcanism’’ (M. F. Sheridan and
F. Barberi, eds.), J. Volcanol. Geotherm. Res. 17, 65–88.
Wilson, C. J. N. (1985). The Taupo eruption, New Zealand,
II. The Taupo ignimbrite. Philos. Trans. R. Soc. London, A
314, 229–310.
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