Volcanic Rock Units

• Pyroclastic
• Epiclastic rocks
• Lava flows
• Domes.
 Classification Methods

The following classification schemes

are taken from Williams et al . (1982)
and Fisher and Schmincke (1984). In
general, classification systems include a
rock chemistry designation, which may
be derived from either a major-element
chemical analysis or color and
phenocryst content. Some textural
classifications are based on hand
sample inspection, but in the case of
fine-grained rocks and tuffs or rocks
that have been altered during
diagenesis or metamorphism, it is Classification of pyroclastic rocks based on clast size
necessary to use in addition a textural (after Schmid, 1981;Fisher & Schminke, 1984)
analysis by petrographic or scanning
electron microscope.
 Rock-Unit Classification
Characteristics of Pyroclastic
and Epiclastic Rocks
Pyroclastic rocks can be
classified by their textural and
mineralogical characteristics
Complete descriptions include
important details about
thickness, grain size, pyroclast
types, bedding sets, grading,
clast orientation, flow features,
induration and welding, and
thermal remanent
magnetization.
 Chemical Classification

Most volcanic rocks are composed

of silicate minerals and glass;
notable exceptions are carbonatites,
which are composed of carbonate
minerals, and rare lavas that are
dominated by magnetite or sulfur.
The SiO2 content is the most
general basis for classification:
Silicic (acid) rocks have >66 wt%,
intermediate rocks range from 52 to
66 wt%, mafic (basic) rocks have
between 45 to 52 wt%, and
ultramafic (ultrabasic) rocks <45
wt%.
 Average major-element chemical compositions for common
volcanic rock types in order of increasing silica content
Table B.1. Average Chemical Compositions of Selected Common Volcanic Rocksa

Tholeiitic
Oxide Nephelinite Basanite Hawaiite Tephrite Basalt Basalt Mugearite

SiO2 42.43 45.46 48.65 50.06 50.06 52.72 52.48

TiO2 2.71 2.56 3.30 1.80 1.86 1.96 2.11

Al2 O3 14.90 14.89 16.32 17.31 15.99 14.98 16.98

Fe2 O3 5.78 4.14 4.92 4.21 3.92 3.51 5.17

FeO 6.60 8.02 7.73 5.48 7.46 8.22 6.52

MgO 6.76 8.93 5.15 4.80 6.96 7.38 2.52

CaO 12.32 10.53 8.21 9.34 9.66 10.35 6.14

Na2 O 4.97 3.58 4.15 3.77 2.97 2.44 4.87

K2 O 3.53 1.88 1.58 4.58 1.12 0.45 2.46

Oxide Andesite Phonolite Trachyte Latite Dacite Rhyodacite Rhyolite

SiO2 56.86 57.49 62.61 62.80 66.36 67.52 74.00

TiO2 0.88 0.64 0.71 0.83 0.58 0.60 0.27

Al2 O3 17.22 19.47 17.26 16.37 16.12 15.53 13.53

Fe2 O3 3.29 2.87 3.07 3.34 2.39 2.46 1.47

FeO 4.26 2.28 2.42 2.27 2.41 1.80 1.16

MgO 3.40 1.12 0.95 2.25 1.74 1.68 0.41

CaO 6.87 2.80 2.34 4.27 4.29 3.35 1.16

Na2 O 3.54 7.98 5.57 3.88 3.89 3.90 3.62

K2 O 1.67 5.38 5.08 3.98 2.22 3.16 4.38

a
From Le Maitre (1976).
 Refractive Index

Simple petrographic technique can also be used to estimate the SiO2 content of
volcanic rocks that contain glass. This technique is based on the decreasing
refractive index of nonhydrated glass with increasing SiO2 content
 Chemical Classification

Williams et al . (1982) demonstrated

that a reasonable chemical
classification can be assigned to
rocks and tephra containing
phenocrysts because these minerals
have characteristic SiO2 contents that
are a key to the bulk composition.
Williams et al . (1982) listed the
SiO2 contents of felsic and mafic
minerals as a useful guide.
Identification of the phenocryst
content also makes it possible to use
the international classification
scheme of Streckheisen (1967),

Quartz Diorite
 Phenocryst Abundances

Silicic Minerals %

quartz 100

alkali feldspars 64 to 66
Where phenocryst
oligoclase 62
abundances are significant
(>4%), the rock name can be labradorite 52 to 53

prefixed by the names of the bytownite 47

significant phenocrysts in leucite ~54

order of increasing abundance nepheline ~40 to 44

(for example, hornblende- kalsilite 39

biotite rhyodacite, pyroxene
Mafic Minerals %
andesite, and olivine basalt).
magnesian and diopsidic pyroxenes 50 to 55

augites 47 to 51

titaniferous augites 46 to 47

hornblendes 2 to 50

biotites 35 to 38

opaque oxides 0
 Contact Relationships
• Sharp erosional
• Depositional contact
• Tectonic displacement
• Collection of reworked clastic debris,
• Paleosol.

In the case of a depositional contact, one should ascertain whether the

deposits drape the underlying topography or are concentrated in channels and
valleys. If they are deposited within a valley, it is valuable to measure the size,
orientation, and slope of the valley floor.
 Color

A color chart is very helpful for maintaining consistency in

descriptions of rock-unit color and color variations.
 Thickness
Pyroclastic units show thickness
variations that are indicative of
vent location, deposit type (for
instance, fallout, flow, and surge),
and the effects of paleotopography
(Fisher and Schmincke, 1984).
Even where pyroclastic units are
not fully exposed, maximum
exposed thicknesses can be used
in constructing isopach maps. In
some cases, thicknesses are
estimated from topographic
constraints such as scarp heights
and bedding dips.
 Grain Size

Field estimates of grain size can

be made using the Fisher (1961)
classification, which parallels
the Folk (1966) classification of
clastic sediments; both of these
can be done with a scale and
charts. Actual measurements
will be done by sample sieving
or thin-section studies in the
laboratory, but visual estimates
are sufficient for measured
sections in the field.

SEM image of phreatomagmatic glass shards

> 0.064 mm (Cas and Wright, 1987).
 Grain Size

Coarser materials, including

pumice and lithic clasts, can either
be sieved in the field with coarse
(>4-cm) sieves or measured and
described at an outcrop within a
designated area outlined on the
rock surface (usually ~1 m2 ).
These observations are especially
useful in studies of lithic clasts
within pyroclastic units. Another
technique for recording the textural
variations within an eruption unit
is to measure the lengths of the
five largest lithic clasts and those
of the five largest pumices.
 Pyroclasts
Most of this detailed work will be
done within the laboratory,
however, it is helpful while in the
field to note pyroclast and lithic
clast characteristics that can be used
later to identify a specific formation
or member: color, shape,
percentage of phenocrysts,
phenocryst types, and variety of
lithic clasts. Lithic clasts include
those of lag breccias, mesobreccias,
and megabreccias (the two latter
types are related to catastrophic
collapses such as avalanches from a
sector collapse in a volcano or wall
collapse within a caldera).
 Clast Orientation

Within surge deposits and pyroclastic flows, there may be elongate clasts or
accidental debris, such as fossil tree trunks, that can be used to determine flow
directions. The orientations of the long axes of as many elongate clasts as possible
should be measured and averaged for each field location.
 Textural Classification
Textural classification can be very
detailed, especially if it is determined by
petrographic microscopic observation.
Williams et al . (1982) described and
illustrated many textural features of
volcanic rocks, but for the sake of
simplicity here, we limit lava textural
terminology to some hand-sample
features

Simple Textural Classification of Lava Hand Samples

Table B.2. Simple Textural Classification of Lava Hand Samples

Classification Phenocrysts Glass

Aphyric None None to subordinate

Porphyritic Present Minor to subordinate

Obsidian None to minor Dominant

Vitrophyre Present Present

 Textural Classification
Pyroclastic rocks in general
are called tephra where they
are unconsolidated and
pyroclastic rock where they
are consolidated. In the case of
ash-size pyroclasts The Granulometric Classification of Pyroclasts and
unconsolidated deposit is Unimodal, Well-Sorted Pyroclastic Deposits
Table B.3. Granulometric Classification of Pyroclasts and Unimodal, Well-Sorted Pyroclastic Depositsa
simply termed ash , whereas
Pyroclastic Deposit
the consolidated deposit is
denoted tuff . Fisher and
Clast Mainly Mainly Consolidated:
Schmincke (1984) based the Size Unconsolidated: Pyroclastic Rock
textural classification of well- (mm) Pyroclast Tephra
sorted pyroclasts on their 64 Block, bomb Agglomerate, bed of Agglomerate, pyroclastic
blocks breccia
granulometric character. or bomb, block tephra
Lapillus Layer, bed of lapilli Lapillistone
or lapilli tephra

2 Coarse ash grain Coarse ash Coarse (ash) tuff

1/16
Fine ash grain Fine ash (dust) Fine (ash) tuff (dust
(dust grain) tuff)
a
From Schmid (1981).
 Textural Classification
Because pyroclastic rocks are composed of
various proportions of vitric, crystal, and
lithic constituents of juvenile, cognate, or
accidental origin, the classification should
also be made according to the proportion of
these constituents in a sample

Classification scheme for pyroclastic samples

composed of a mixture of fragment sizes;
(Adapted from Schmid, 1981.)

Classification scheme for pyroclastic samples

composed of a mixture of constituents.
(Adapted from Cook, 1965.)
 Pyroclast Textures

SEM photographs illustrating four

common pyroclast textures. (a) V
esicularity is well
developed for this pyroclast sampl
ed at Surtsey. (b) Grain angularity
is prominent in a hydroclastic
sample from Surtsey(c) Grain rou
nding indicates transport abrasion
in this poorly vesicular
pyroclast from Kilbourne Hole m
aar in New Mexico. (d) Surface
alteration coats this pyroclast from
the Coliseum Diatreme maar in
Arizona
 Density

Volcanic rocks show a range of densities With increasing water content, magma
from <1.0 Mg/m3 for silicic pumice to ~2.9 densities generally decrease—as do their
Mg/m3 for basalt. Because of the degree of vesiculated volcanic equivalents. The
vesiculation, crystallization, fragmentation, densities for intrusive equivalents exhibit
and postemplacement compaction, it is clear maximum ranges for a given composition,
that after eruption, volcanic rock densities whereas those for volcanic glasses fall in the
change from those of their parental magma. minimum ranges.
Average Densities for Common Igneous Rocks
Table B.5. Average Densities for Common Igneous Rocksa

Range of Density Mean Density

Rock Type (Mg/m3 ) (Mg/m3 )

Silicic

Rhyolitic pumice 0.500–1.500 1.000

Rhyolitic tuff 1.000–1.800 1.400

Rhyolitic welded tuff 1.800–2.400 2.100 Because tuffs have pore

Rhyolitic obsidian 2.330–2.413 2.370 space as a result of
Rhyolite 2.51 vesicles and intergranular
Granite 2.516–2.809 2.667
voids, their densities for
Intermediate
silicic rocks are
Trachytic obsidian 2.435–2.467 2.450
commonly 40 to 60% of
Trachyte 2.57
those for their glassy lava
Andesitic glass 2.40–2.537 2.474

equivalents. Vesicles may

Andesite 2.65

Syenite 2.630–2.899 2.757

make up as much as 80%
Granodiorite 2.668–2.785 2.716
of the volume of
Quartz diorite 2.680–2.960 2.806
pumices, for example.
Mafic

Leucitic tephritic glass 2.52–2.58 2.55

Basaltic glass 2.704–2.851 2.772

 Porosity and Permeability
Although there is no direct
relationship between porosity and
permeability, both of these rock
properties are extremely important
when assessing the reservoir potential
of a given rock type. Porosity in
volcanic rocks is mainly defined by
the abundance of vesicles. In the
case of pyroclastic rocks, grain size
distribution and sorting determine the
packing density of clasts. The porosity
of a pyroclastic rock generally imparts
a primary permeability; if subjected to
hydrothermal fluid circulation, this
permeability may change as a result
of the dissolution of glass and the
growth of secondary minerals.
Porosity Of Pyroclastic

The porosity of pyroclastic

rocks may reach 80%, but for
fresh, nonaltered pyroclastic
rock, porosity is generally in
the range of 40 to 60%. Lavas,
on the other hand, exhibit
porosity only if they are
brecciated during emplacement
or contain vesicles and other
gas cavities such as
lithophysae; in these cases,
lava porosity is generally
<20%.
 Permeability Of Pyroclastic

The bulk permeability of volcanic

rocks is a function of primary and
secondary permeability. Primary
permeability (sometimes called
formation permeability ), as
discussed above, develops from the
original texture of the rock (for
example, interconnected pores and
vesicles and grain boundaries). In
contrast, secondary permeability
(sometimes called fracture
permeability ) is promoted by rock
fracture and foliation, and where it
occurs, it is generally the dominant
type of permeability.
 Permeability Of Pyroclastic
Range of Permeabilities for Common Rock Types

Volcanic rock fracture has numerous

origins, such as tectonic movement
and proximity to faults, differential
compaction that causes stress
fractures, cooling contractions,
thermal spallation, and
eruptive/emplacement brecciation.
Typical permeabilities for all rock
types range from 10-20 m2 (0.01
µDarcy) to 10-7 m2 (0.1 Mdarcy)
The permeabilities of unaltered
pyroclastic rocks should be similar
to those of silty and clean sand—in
the range of 10-14 to 10-10 m2 (0.01
to 100.0 Darcy).
 Bedding
Bedform identification is helpful for
interpreting the origin of a pyroclastic
deposits. Fisher and Schmincke (1984)
discussed various bedforms that can be
related to different types of eruptions (such
as Plinian, hydroclastic, Strombolian), as
well as the emplacement mechanism.
Where a pyroclastic deposit shows a
sequence of bedforms as a coherent unit
(bedding set), the sequence can be used
with other observations to identify a
mappable unit in the field. For example, a
specific member might consist of a fine-
grained ash fallout bed overlain by a surge
bed, two pyroclastic flow deposits, and a
volcanic mudflow breccia. Although the
thicknesses and degree of compaction and
welding within the pyroclastic flow
deposits might vary, if the sequence appears
to be unique, it can be helpful for
correlating units.
 Grading

The character of grading in pyroclastic deposits is also indicative of

origin. The field geologist should determine whether a bed is massive,
normally graded, or reversely graded.
 Environment of deposition
Finally, where the environment of
deposition or mode of emplacement
can be determined, classification may
include such a designation. For
example, tuff deposited in a marine
environment is called submarine tuff ,
which distinguishes it from subaerial
or lacustrine tuff. Tuff deposited by
fallout is denoted fallout tuff, but tuff
emplaced by pyroclastic flow is
generally termed ash-flow tuff.
Reworked tuff may be aeolian tuff
where wind-reworked or fluvial tuff
where deposited by a river or a stream.
Geosciences and Environmental Change Science Center
Combining these classification Fluvial boulder gravel and coarse sand of the
schemes produces terms such as North Park Formation, lying immediately above
crystal-lithic lapilli tuff, lithic the Thunder Mountain rhyolite ash-flow tuff
tuffaceous breccia , or lithic-vitric
fallout agglomerate .
 Flow Features

Many surge deposits are

characterized by dunes or
antidunes. Measurements of
implied current directions,
descriptions of types of cross-
bedding, and estimates of the
magnitude of the cross-beds are all
useful for evaluating eruption
types and processes and for
locating vent areas. In pyroclastic
flow deposits, flow features should
be noted, including thickening in
paleovalleys and shadow areas
behind paleotopographic high
areas where the flow is relatively
thin.
 Welded Tuff
• Degree of welding
– Non-welded
– Partially welded
– Densely welded
• Density is a good index
• Welding (density) profiles
 Induration and Welding

To establish whether a rock is

welded, partly welded, or
nonwelded, bulk sample density
can be compared to that of a
nonvesicular lava of similar
composition; welded tuffs have
densities similar to those of
equivalent lavas, nonwelded tuffs
have densities less than half of
those for equivalent lavas, and
nonwelded tuffs have
intermediate densities.
 Induration and Welding

To determine if the rock has been

indurated or cemented by post-
depositional processes, one should
look for vapor-phase alteration
within pyroclastic flow deposits,
matrix cementation by diagenesis or
weathering, and secondary clays
from hydrothermal activity. Other
evidence of induration might be
found in the form of fossil fumaroles
(pipe-like zones cemented with
vapor phase minerals) and
compaction features such as vertical
concentrations of small lithic clasts
(segregation pipes).
 Welding Profiles

• Density plotted vs. elevation

• ρ = 1.0 at base and top
• ρ = maximum value near center
• Erosion easily removes upper part
• Welding = f(temperature, Pressure)
Secondary Mineralization
• Vitrophyre
• Devitrification
• Lithophysae
• Vapor-phase crystallization
• Zeolitization
 Compositional Zoning

• Initial eruptions
– Crystal-poor rhyolites
– Crystal-rich latites or dacites
• Related to zoned magma chambers
– Highly-evolved upper parts
– More primitive lower parts
• Evidence in banded pumice
 Thermal Remanent Magnetization
Picture of lava flow courtesy of
Daniel Staudigel. b) While the
lava is still well above the Curie
temperature, crystals start to
form, but are non-magnetic. c)
Below the Curie temperature but
above the blocking temperature,
certain minerals become
magnetic, but their moments
continually flip among the easy
axes with a statistical preference
for the applied magnetic field.
As the lava cools down, the
moments become fixed,
preserving a thermal remanence.
[Figure from Tauxe and
Yamazaki, 2007.]
 Heat Capacities and Thermal Conductivities
of Selected Volcanic Rocks
Table B.10. Heat Capacities and Thermal Conductivities of Selected Volcanic Rocks a

Heat Capacity Conductivity Conductivity

Rock Type (kJ/kg-K) Range (W/m-K) Mean (W/m-K)
Rhyolitic Tuffb 0.20–0.40 0.3

Rhyolite 1.06c 1.58–4.33 3.0

Obsidian

T = 273 K 1.34

T = 373 K 1.46

T = 473 K 1.56

T = 573 K 1.67

T = 673 K 1.78

T = 773 K 1.89

Altered Rhyolite 3.1–3.7 3.44

Dacited 1.17 0.54–0.97 0.69

c
Andesite 1.04 1.35–4.86 3.7
Lavae 2.6–3.6 3.10

Lavae 2.7–3.3 3.01

Lavaf 1.7–2.8 2.10

Basalt 1.05c 1.12–2.38 1.8

Diabasic Basalt

T = 303 K 1.69

T = 348 K 1.73

a
From Clark (1966) and Nathenson et al . (1982).
 Alteration
• Deuteric alteration
– Occurs as materials cool after emplacement

• Hydrous minerals may decompose

– Due to reduction in pressure
– Fe-Ti dusty rims on reddish pseudomorphs
 Alteration
• Hydrothermal alteration
– Forms due to circulation of hot ground water

• Propylitic alteration
– Formation of hydrous minerals
– Chlorite, amphibole, epidote, phrenite
– Associated with some ore bodies
 Sampling

For each distinct unit (but not

necessarily from all measured
stratigraphic sections),
field geologists collect a sample
that is representative of that unit.
If the tephra are unconsolidated
and coarse grained, they are
sieved, the size fractions are
weighed, and chunks of the
pumice are collected (in addition
to a split of the <1-mm fraction
that is kept for laboratory
sieving). The various lithic clasts
are described and samples of each
lithic type are collected for thin-
section study.
 Sampling

If it is appropriate, samples are

chosen for radiometric dating
and chemical analysis:
pyroclastic flows often show
subtle compositional
stratification that can be related
to magma chamber evolution;
fallout layers provide
widespread time-stratigraphic
units; organic matter such as
buried tree trunks are very
helpful in dating young
Labanieh et al. (2010).
pyroclastic deposits—these
types of samples are always
particularly valuable.

Relative contribution of oceanic sediments over time, based on K-Ar dating of

individual volcanic rock samples. After 5.1 Ma, the end-member composition
changed, possibly due to the subduction of an aseismic ridge on the Atlantic Plate
 Lava Flows and Domes

For lava flows and domes, descriptions should include

• Observations of texture,
• Flow layering,
• Jointing.
• Petrology,
• Overall lava flow type,
• Thermal effects
• Thickness.
 Texture

Textural variations that might be

found within flows or domes include
differences in vesicularity (size,
shape, and orientation), phenocryst
content and size, brecciation, and
flow foliation or layering. Coarsely
pumiceous zones, which have risen
diapirically and may have been
broken or folded by flow movement
(Fink and Manley, 1987), are often
distinct features that can be mapped.
Variations in relief and vesicularity
may be visible on aerial photographs
of silicic lava flows and can be used
to map flow structures
 Flow Layering

Especially in lavas of
intermediate to silicic
compositions, layering is
common and ranges from
submicrometer shears to
macroscopic bands of dense
glass and slightly vesicular
glass. Layering attitudes,
measured vertically and over
the entire lava flow, can
provide information about
vent locations and the
physical properties of the
flow.

The inside of a stratovolcano - alternating layers of tuffs and lava

flows, cut by dikes. Santorini volcano, Greece (Photo: Tom Pfeiffer
 Jointing

Most lavas are broken into blocks

by thermal stresses during cooling.
The open fractures or joints are
often columnar and form at right
angles to the flow surface and base
(normal to the isotherms or cooling
surfaces). Fracture surfaces are
striated; Ryan and Sammis (1978)
concluded that striae provide a
record of incremental crack
advance during stress buildup in the
cooling lava
Columnar joints can range from a
few tens of centimeters to >1 m
wide and can be up to 30 m long in Ideal model for columnar jointing. (Nishiwaki, 2009).
some thick plateau basalt flows.
 Jointing
The columns can have as few as
three or as many as seven sides;
most appear to have five or six
sides (Williams and McBirney,
1979). Maps of column orientations
can sometimes help determine lava
flow boundaries, and this is
especially useful where outcrops
are poor. For example, within a
valley-filling lava flow, columns in
the center of the valley would be
vertical; however, along the valley
walls, they would be oriented at an
angle and would be perpendicular
to the walls, which had acted as
heat sinks during cooling of the
lava flow. Similar columnar volcano.oregonstate.edu
jointing can also be found in dikes,
plugs, and lava lakes.
 Estimating Permeability

Diagram showing scaling relationships for identical intrusive geometries cooling by conduction or convection
(modified after Cathles. The particular geometry of the intrusions is indicated in the insert diagram. The solid
lines show convective cooling times for different permeabilities (0.1, 1.0 and 10 millidarcies).
Examples of maps and useful observations of silicic lava flows

(a) Sketch map of Little

glass Mountain in California, made quickly from a
n aerial photograph. This is a young rhyolitic
obsidian flow for which flow lobes and the directi
on of flow can be observed by mapping
the ridged and furrowed flow surface; from this in
formation and topographic profiles,
it is possible to locate the vent area. The flow lobe
s also can be identified through
textural changes; in this example, zones of coarsel
y vesicular pumice can be mapped.
(Adapted from Fink and Manley, 1987.) (b) Map o
f Little Glass Mountain that shows
zones of coarsely vesicular pumice (dark areas). (
Adapted from Fink and Manley, 1987).

(c) Map of the Watchman dacite flow at Crater Lake in Oregon. Flow patterns
were identified by measuring the attitudes of flow foliation. This method is particularly
useful if no aerial photographs are available. (Adapted from Williams, 1942). (d) Cross
section along the long axis of a silicic lava flow illustrates textural variations, including
coarse rubble scattered over the flow surface, along the flow front, and at the base.
Ragged spines or slabs quite often extend out from the flow or dome.
 Petrology

For field identification of

lava type, geologists use the
petrographic classification
with which they are most,
but consistency is crucial.
The field descriptions should
be the best possible, but it is
likely that these will change
after thin sections have been
examined petrographically,
especially in the case of
finely crystalline rocks.
 Lava Flow Type

If possible, descriptions of the type of lava flow should include its overall texture and
morphology. Most basaltic lavas can be identified by the terms pahoehoe, aa , or block lava
 Thermal Effects

To ascertain whether there has been thermal alteration of rocks underlying the lava flow, field
geologists look for oxidation of soil layers or older rocks, formation of pipe vesicles during
heating of water in soil or bogs, and desiccation of clastic sedimentary rocks
 Thickness

In measuring thicknesses, all mappable sub-divisions (eruption unit, member,

or formation) and all textural subunits are noted Thickness is measured from
the base of a unit to the level of some significant textural change.
Cross section of a generic basaltic lava flow, showing some of the basic structural features
that should be described when mapping flows. Flow surfaces, if preserved, present
a variety of textures that range from smooth, ropy pahoehoe to spiny, rubbly aa lavas.
Flow interiors exhibit variations in structure such as different types of columnar joints,
vesicle concentrations, and lava tubes; the presence of these features often depends
on flow thickness and viscosity. Pipe vesicles are formed within flows as they cross
wet ground; rising steam leaves vesicle trains or small tubes that are bent by flow
(a good indicator of flow direction). Lava blocks spalled or extruded from the toe of
an aa lava flow leave lava rubble beneath and in front of the flow
 Sampling

Lava samples are critical for

developing a sound understanding
of the time-stratigraphy for a field
area. In addition to providing
documentation of the petrogenetic
evolution of a volcanic field,
carefully selected samples can
provide important radiometric
dates. To obtain dates and chemical
analyses that are reliable, it is
important to assess the evident
weathering and diagenetic effects
as well as phenocryst content of
samples.
 Correlation of Volcanic Rock Units

The ability to identify and correlate

eruption units becomes much more
significant if the units are large,
extensive, and within a tectonically
complex area. If a pyroclastic unit
(either fallout deposit or ignimbrite) is
to be traced to determine either its
volume or its utility as a stratigraphic
marker across complex terrain, then
correlation criteria must be
established. Pyroclastic deposits
exposed around the margins be
correlated with thick caldera-fill
deposits; these tuffs are from the same
eruption but may have substantially
different textures.
 Ash-flow Sheets

• Flow units

• Cooling units

• Welded tuffs
 Morphology
• Controlled by topography
• Fill depressions
• Even upper surface
• Valley ponded deposits
• Veneer deposits
• Multiple lobes and fans
• Lateral levees
 Flow Unit Standard Section

• Layer 1 (ground layer or surge)

• Layer 2 (flow unit)

• Layer 2a (fine-grained basal)

• Layer 2b (main body of flow)

• Layer 3 (ash cloud)

 Correlation of Volcanic Rock Units

An entire branch of volcanology,

tephrochronology , has been
developed to answer the need for
correlating volcanic ash deposits
To correlate ash beds, it is
necessary to identify the mineral
phases, glass compositions, and
particle shapes (such as shard
types and pumice
characteristics) that are
characteristic of each deposit.
 Trace Elements

If the ash is petrographically

unique, it is possible to identify
it with a hand lens plus a
reference sample of the known
deposit. If there are several ash
beds of similar composition or
appearance, it may be
necessary to use chemical
analyses of the glass pyroclasts,
including trace elements, for
correlation. Ideally, radiometric
age dates are employed, but
they are expensive

Sr-Nd isotope diagram for Plio-Quaternary mafic volcanic rocks

 Alteration and Hydration

Bulk chemical analyses are known

to be a poor basis for correlation:
with increasing distance from the
source, the gravitational
segregation of mineral phases from
a glass-shard-laden eruption plume
can change the bulk chemical
composition. The refractive indices
of glass shards used at one time for
correlation, are not always
accurate because glasses change as
a result of alteration and hydration
in different depositional
environments.
SEM of microcrystalline alteration materials coating a vesicul
ar pyroclast from Surtsey.The mineralogy of these materials
can constrain the alteration environment
 Correlation of Ignimbrites

Correlation of ignimbrites can be

difficult because of facies
variations, the degree of welding,
postdepositional alteration, and
chemical zonation of large-
volume eruption units. For
example, it is not easy to quickly
correlate a nonwelded ignimbrite
on the outer slopes of a volcano
and a densely welded,
hydrothermally altered ignimbrite
from the same eruption within the
thick caldera fill.

2006 by Geological Society of America

 Techniques For Correlating Ignimbrites

Hildreth and Mahood (1986) have

reviewed techniques for correlating
ignimbrites and conclude that the
following observations are the most
reliable:
· careful mapping of the whole unit;
· stratigraphic position;
· thermal remnant magnetic directions
within welded tuffs and high-
precision potassium-argon ages;
· a distinctive suite of lithic clasts; and
· petrographic characteristics within
pumice clasts, pyroclast shapes, and
unusual phenocrysts
careful geological mapping of
lithologic units, measurements
of many stratigraphic sections,
and age-dates for those units.
However, rock units within
volcanic fields show much more
lateral and vertical variation than
do units in most sedimentary
basins (Fisher and Smith, 1991).
They can fill caldera depressions
or deep valleys, which means
that one might find younger
volcanic rocks at a lower levels
than older rocks, even if no
folding or faulting has occurred.
Pyroclastic rocks are formed
quickly—initially with abundant
kinetic and thermal energy—and
are deposited as ashfalls that
drape topography, surges that
cross topographic highs,
pyroclastic flows that follow the
valleys, as well as wet surges of
cohesive ash that defy the laws of
original horizontality when
plastered onto vertical surfaces.
The possibilities of facies
variations within single
depositional units must be
considered when mapping
volcanic rocks
For example, surges and
pyroclastic flows can grade
outward into volcanic
mudflows because of
cooling and condensation of
steam within the flow some
distance from the source.
The degree of welding of
pyroclasts in the flow units
can vary with the unit
thickness; dense rocks are
found near the vent or in the
center of valley fills
Field and laboratory observations
must be adapted to fit the volcanic
field of interest. For example, the
approach used for a large basaltic
lava plateau would differ
considerably from that used to
study a group of small tuff rings.
Stratigraphic analysis of volcanoes
provides the necessary foundation
for all other studies, including
petrology, geochemistry, thermal
state, and structural framework;
without this foundation, sample
analysis is nothing more than rock
collecting.
Schematic cross sections illu
strate (a) facies changes betw
een volcanic units and (b)
time correlative sedimentary
units These deposits are
grouped into map units that a
re linked to the eruption
or sedimentary processes
responsible for the deposit.
(Adapted from Smith, 1987.)
Stratigraphy in Volcanic Fields

From Fisher and Schmincke (1984).

 Eruption Unit

Fisher and Schmincke also

defined the concept of an
eruption unit , which is a
deposit from a single eruptive
pulse, eruptive phase, or an
eruption. A sequence of several
eruption units can be treated as
a mappable unit or formation.
Eruption units can refer to
pyroclastic fallout deposits,
pyroclast flow deposits,
volcanic mudflows, lava flows,
and any other deposit from a
single eruptive pulse.

A map of the volcanic deposits in the Katmai

cluster. Hildreth and Fierstein (2000)
 The Rock Units

Geosciences and Environmental Change Science Center

Fasies Volkanik

Boogie & Mc Enzie

Fasies Volkanik

Sentral

Proksimal

Medial

Distal
Geology.com
Fasies Volkanik

Sentral

Proksimal

Medial

Distal
Bentuk Bentang Alam Volcanic

Sutikno Bronto,2006
(Muda ; 1, 2, 4, 8, 9) & (Tua; 3, 7, 11, 13, 16, 18)
Fasies Volcanic
Fasies Volcanic
Proksimal
Lava/Breksi Autoklastik

Breksi Piroklastik
Proksimal
Medial
Medial
Distal
Distal
Volcaniclastic Map
Proksimal
FLOW ANALYSIS
Strike & Dip Volcanic Rock

Sutikno Bronto,2006
MODEL FASIES MONOGENETIC VOLKANIK
MODEL FASIES POLYGENETIC VOLKANIK
PYROCLASTIC
SEQUENCE
Volcanic Rock Units

• Pyroclastic
• Epiclastic rocks
• Lava flows
• Domes.
Lateral Distribution Pattern of Volcanic Rocks
 Main Lithofacies Combination Pattern
of The Volcanic Edifices

I. Volcanic Conduit Facies, I 1 Volcanic neck subfacies, I 2 Subvolcanic subfacies, I 3 Cryptoexplosive breccia
subfacies, II. Explosive Facies, II 1 Airfall subfacies, II 2 Hot base surge subfacies, II 3 hot debris flow sub facies, III.
Eruptive-effusive facies, III1 Lower subfacies, III2 Middle subfacies, III 3 Upper subfacies, IV.Extrusive Facies, IV1
Intrazone subfacies, IV2 Mesozone subfacies, IV 13Outerzone subfacies, V. Volcanic Sediment facies, V 1Coal
contained tuff subfacies, V 2Re transportation subfacies, V 3Extraclast contained subfacies
 DEPOSITIONAL SEQUENCE

TURBIDIT /
BOUMA SEQUENCE
PYROCLASTIC SEQUENCE

Subset

Subset Set
Coset
Subset
 Some Structural Features of Pyroclastics Deposits
Feature Deposits in Which They Are Characteristics
Overall Geometry of Deposits Areal Distribution
1 Fan-shape or Lobate Fall Out & Pyroclastic Flow
2 Valley Fill (Shoestring Shape) Pyroclastic Flow

Vertical (Cross Sectional Distribution)

1 Wedge in direction of transport Fall Out & Pyroclastic Flow
2 Lensoid perpendicular to transport Fall Out & Pyroclastic Flow
3 Valley Profile Shape Pyroclastic Flow

Primary Stratification Relations of Upper and Lower Surfaces

(Individual Beds) 1 Flat Top, Base follows underlying surfacePyroclastic Flow
2 Top pararel to Base Fall Out & Plateau Pyroclastic Flow

Basal Relationship
1 Drapping over or against obstacle Fall Out
2 Structures in the lee of obstacle Pyroclastic Flow

Internal Structures
1 Graded Bedding Fall Out & Pyroclastic Flow
2 Cross Bedding Pyroclastic Surge
3 Massive Beds Pyroclastic Flow
4 Aligment and orientation bedding Pyroclastic Flow

Bed Forms
1 Plane Bed Pyroclastic Surge
2 Anti Dune Pyroclastic Surge
3 Chute and Pool Pyroclastic Surge

Post Depositional Structure Upper Surface Structure

1 Bedding Sags Base Surge
2 Convolute Bedding Base Surge
3 Load Cast and Bedding Sags Base Surge
4 Mudcracks Base Surge
5 Rills Base Surge
The Volcanic Reservoirs Connecting Mode