Download EPS 421 Lecture 14 Volcaniclastic sediments

Department of Petroleum Geology & Sedimentology,
Faculty of Earth Sciences,
King Abdulaziz University,
Jeddah, Saudi Arabia
EPS 421
CLASTIC SEDIMENTARY ROCKS
Lecture 14: Volcaniclastic sediments;
composition, classification and diagenesis
Prof. Dr. Mahmoud A. M. Aref
Volcaniclastic sediments
Volcaniclastic sediments: are those composed chiefly of
grains of volcanic origin, derived from contemporaneous
volcanicity.
EPS 321 Lecture 14
Problems in the study of Volcaniclastic rocks
1) It is often difficult (and dangerous) to observe modern volcanic processes
and mostly the eruptions that can be studied are relatively small-scale.
2) The techniques used for studying Recent volcaniclastics, such as sieving
for grain-size analyses are not applicable to their indurated ancient
equivalents.
3) Diagenesis is a major factor in altering volcanic glass and minerals,
destroying depositional textures and creating matrix.
4) The weathering of volcanic material is also very rapid; in a few years sandgrade ash particles can be reduced to clay.
5) There is also a problem with the preservation of the volcanoes themselves;
being topographic highs, they are easily eroded, and also hyrothermally
altered.
EPS 321 Lecture 14
Volcaniclastic sediments and rocks are produced from:
Lava delta
1. Weathering of lava (cooled magma flows).
2. Ejected pyroclastic material or tephra that can be
subdivided into different compositional
categories:
•
Mineral grains
•
Lithic fragments
•
Vitric material (volcanic glass or pumice)
EPS 321 Lecture 14
Classification of Volcaniclastic Sediments
according to mode of formation:
– Epiclastic sediments: volcanic fragments that are produced
by erosion of volcanic rocks by wind, water, and ice.
– Pyroclastic sediments: particles broken by volcanism, or the
products of explosive volcanism.
– Hyaloclastic sediments: the products of the granulation of
magma-water interactions.
– Autoclastic sediments: formed by mechanical or gravitational
movement of lava flows and/or domes
EPS 321 Lecture 14
Classification of volcaniclastic grains and
sediments, based on grain size
Grain size
Volcaniclastic grains
(tephra)
Volcaniclastic sediments
bombs—ejected fluid
agglomerate
blocks—ejected solid
volcanic breccia
lapilli
lapilli-stone
coarse ash
volcanic sandstone (tuffs)
> 64 mm
2 - 64 mm
0.06 – 2 mm
< 0.06 mm
fine ash
EPS 321 Lecture 14
volcanic mudstone (tuffs)
New classifications of Volcaniclastic sediments
ODP scheme
Grain size
Mazzullo et al.,
1987
New scheme
Hajime Naruse
ash / tuff
volcaniclastic
sandstone or
mudstone
2 ~ 64 mm
lapilli / lapillistone
volcaniclastic granule
~ cobble /
conglomerate
>64 mm
bomb / agglomerate or volcaniclastic boulder /
block / breccia
conglomerate
<2 mm
EPS 321 Lecture 14
General descriptive classification of volcaniclastic deposits
EPS 321 Lecture 14
Classification of Pyroclastic Rocks
•
Basic classification on the basis of particle
size
– Blocks (solid) and bombs (molten)
(>64mm)
• Volcanic breccia deposits
– Lapilli (2-64mm)
• Lapillistone
– Ash (<2mm)
• Tuff
•
Additional Classification on the basis of
composition
– Crystals
– Lithic
– Vitric fragments
EPS 321 Lecture 14
EPS 321 Lecture 14
Pyroclastic Rocks
• Pyroclast is applied to any material, regardless of size, ejected
from volcanoes. They are derived chiefly from the magma itself,
composed mostly of volcanic glass and crystals (if the magma
had begun to crystallize before its explosive eruption).
• Pyroclastic grains are angular to subrounded pumice, scoria,
shards and lapilli. Sorting is poor to very poor.
• Common structures are normal and reverse grading of thick
strata, with bombs.
• Tephra is a collective term for pyroclasts. Tephra includes lithic
fragments, of lava from earlier eruptions and of country rock.
• Tephra are subdivided on particle size into volcanic dust, ash,
lapilli and blocks.
EPS 321 Lecture 14
Bomb
• Volcanic bombs are lava fragments
that were ejected while viscous
(partially molten) and larger than 64
mm in diameter.
• Many acquire rounded aerodynamic
shapes during their travel through
the air.
• Volcanic bombs include ribbon
bombs, spindle bombs (with twisted
ends), and spheroidal bombs .
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• Block
A volcanic block is a solid rock fragment
greater than 64 mm in diameter that was
ejected from a volcano during an explosive
eruption. Blocks commonly consist of
solidified pieces of old lava flows that were
part of a volcano's cone. .
• Volcanic breccia
• Solid rock was shattered and the pieces
(light gray) cemented together by hot ash
(pinkish material)
EPS 321 Lecture 14
Overview of deposit of volcaniclastic breccia
EPS 321 Lecture 14
• Lapilli
• Rock fragments between 2 and
64 mm (0.08-2.5 in) in diameter
that were ejected from a volcano
during an explosive eruption are
called lapilli.
• Lapilli (singular: lapillus) means
"little stones" in Italian. Lapilli
may consist of many different
types of tephra, including scoria,
and pumice.
EPS 321 Lecture 14
• Pumice
• Many pyroclasts are in the form of
pumice, highly vesicular volcanic glass
(ejected from acidic magma), that may
have a porosity of more than 50%.
• Pumice is a light, porous volcanic rock
that forms during explosive eruptions. It
resembles a sponge because it consists
of a network of gas bubbles frozen amidst
fragile volcanic glass and minerals. All
types of magma (basalt, andesite, dacite,
and rhyolite) will form pumice .
EPS 321 Lecture 14
• Scoria
• The term scoria is used instead
of pumice, when formed from
more basic magma.
•
The black, oval features in this
photomicrograph are vesicles .Note the
acicular, white plagioclase laths throughout
and the white olivine crystal at the lower
right. View is under crossed polarizers.
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Pyroclastic flow
•
If the eruption column collapses a pyroclastic
flow will occur, wherein gas and tephra rush down
the flanks of the volcano at high speed.
•
A pyroclastic flow is a ground-hugging avalanche of
hot ash, pumice, rock fragments, and volcanic gas
that rushes down the side of a volcano as fast as
100 km/hour or more.
•
The temperature within a pyroclastic flow
may be greater than 500° C, sufficient to
burn and carbonize wood. Once
deposited, the ash, pumice, and rock
fragments may deform (flatten) and weld
together because of the intense heat and
the weight of the overlying material .
EPS 321 Lecture 14
ash fall
Clouds of gas and tephra that rise
above a volcano produce an eruption
column that can rise up to 45 km into
the atmosphere. Eventually the tephra
in the eruption column will be picked up
by the wind, carried for some distance,
and then fall back to the surface as a
tephra fall or ash fall .
EPS 321 Lecture 14
Tuff
Tuffs are explosively erupted volcanic material that is consolidated and lithified
after deposition.
Tuffs may contain lithic fragments, glass shards, and/or broken mineral grains
and have pyroclastic texture.
The photos above (crossed polarizers on left, plane polarized light on right) show lithic
crystal tuffs containing twinned, broken plagioclase clasts, and altered lithic clasts (right
side of photos), in a matrix of very fine-grained material .
EPS 321 Lecture 14
•
Photomicrograph of volcanic
glass shards seen in a smear
slide
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Thick coastal sections of ash
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Composition of Tuffs
• Crystals
– Euhedral +/- broken
– Compositional zoning
• Vitric (glassey) fragments
– Bubble wall shards
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Composition of Tuffs
• Vitric (glassy) fragments
– Bubble wall shards
– Hydroclastic shards
• Lithic fragments
– Volcanic rock fragments
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Factors controlling the nature of volcanic eruptions
• the volatile content (water and CO2 especially)
• viscosity of the magma.
At depth, the volatiles are in solution in the magma but as the
latter rises towards the Earth's surface and pressure decreases,
the gases exsolve and expand. This causes vesiculation of the
magma and produces foam-like magma, which on ejection and
solidification gives rise to the pumice/scoria.
Acid magmas contain a higher percentage of volatiles over basic
magmas and they; also more viscous, so that acid magmas
give rise to more widespread pyroclastic deposits.
EPS 321 Lecture 14
Classification of volcaniclastic sediments based
on their modes of formation.
1) Autoclastic deposits: sediment generation during lava flow
2) Pyroclastic- fall deposits: formed of tephra ejected from vent
3) Volcaniclastic-flow deposits (and type of flow)
A) ignimbrites (pyroclastic flows)
B) surge deposits (pyroclastic surges)
C) lahar deposits (volcanic mudflows)
4) Hydroclastites: fragmented lava through contact with water
A) hyaloclastites (non-explosive)
B) hyalotuffs (explosive)
5) Epiclastic deposits
volcanic material reworked by currents, waves, wind, gravity flows, etc.
EPS 321 Lecture 14
Upper breccia
1. Autoclastic deposits
Massive flowbanded
vesicular
centre
Lower breccia
•
These are volcanogenic rocks produced by autobrecciation of lavas. As a lava
flows along, it cools and the upper surface may develop a brittle crust, which
fractures and brecciates on top of the moving lava.
•
As the lava advances, the brecciated material slides off the front of the flow
and is then overriden to give a basal breccia carpet to the flow.
•
Lava itself is mixed in with the upper and lower breccias so that textures can
vary from clast-supported to matrix- (lava-) supported.
•
There are variations in the nature of the blocks, generally more vesicular and
angular in basaltic lavas and more homogeneous and oblate/rectilinear in
intermediate-acidic lavas.
•
In the more distal parts of lava flows, the whole of the deposit may be flow
breccia, whereas in more proximal areas, the breccias just occur at the bottom
and tops of the more massive lava beds.
EPS 321 Lecture 14
2. Pyroclastic-fall deposits (Particles broken by volcanism)
•
These sediments are simply formed through
the fallout of volcanic fragments ejected
from a vent or fissure as a result of a
magmatic explosion.
•
In the majority of cases, explosive
volcanoes are subaerial and the material is
deposited on land, but if there are
subaqueous environments nearby these will
also receive the pyroclastic debris.
•
Two types of subaerial fallout are eruptionplume derived fall deposits, which are
ejected explosively from a vent, producing a
plume of tephra and gas, and ash-cloud
derived fall deposits, resulting in part from
ash clouds rising off a moving pyrodastic
flow.
EPS 321 Lecture 14
2. Pyroclastic-fall deposits (Particles broken by volcanism)
EPS 321 Lecture 14
2. Pyroclastic-fall deposits (Particles broken by volcanism)
•
Pyrodastic falls also can be subaqueous,
from underwater volcanic eruptions.
•
The characteristic features of air-fall
deposits are a gradual decrease in both bed
thickness and grain size away from the site
of eruption, and a good to moderate sorting.
Blocks and bombs are deposited relatively
close to the vent, whereas ash may be
carried many tens of kilometres and dust
thousands of kilometres away from the
vent.
Proximal fall deposits
EPS 321 Lecture 14
Pyroclastic Deposits
Three types of pyroclastic deposits
™ Fall Deposits
Fallout from an eruptive column
™ Flow Deposits
Produced by pyroclastic flows
™ Surge Deposits
Often associated with flow
deposits
Associated with explosive events,
such as phreatomagmatic explosions
EPS 321 Lecture 14
Fall
Flow
Surge
Characteristics of Pyroclastic Deposits
™ Fall Deposits
™ Mantle topography
Fall
™ Parallel bedding
™ Well sorted
™ Often graded
™ Flow Deposits
Flow
™ Topographically constrained
™ Poorly sorted
™ Often graded
Surge
™ Surge Deposits
™ Partially topographically constrained
™ Cross bedding characteristic
™ Intermediate sorting
™ Often graded
EPS 321 Lecture 14
2. Pyroclastic-fall deposits
•
Individual beds of air-fall material typically show normal grading of
particles, although in some cases, inverse grading of pumice and lithic
clasts has occurred. Where deposition takes place in water quite large
fragments of low-density pumice may occur towards the top of an air-fall
bed, as a result of the pumice floating on the water surface before
Fall
deposition.
•
A further feature of air-fall deposits is the
development of mantle bedding, whereby the
tephra layer follows and blankets any original
topography, with a similar thickness over
topographic highs and lows.
Flow
Surge
EPS 321 Lecture 14
Pyroclastic Fall Deposits
™ General term: tephra
™ Types
™ Scoria (mafic , larger size)
™ Pumice (silicic, larger size)
™ Ash (fine grained, any composition)
EPS 321 Lecture 14
3. Pyroclastic-flow and -surge deposits
•
In the subaerial environment, pyroclastic-flow
deposits are the product of hot gaseous particulate
density currents. They generally form through
fluidization by magmatic gas and give rise to
deposits known as ignimbrites.
•
Pyroclastic flows typically move at speeds of over
100 kilometers/hour and reach temperatures of over
400 degrees Celsius
•
Pyroclastic-surge deposits result from highly
expanded turbulent gas-water-solid density currents
with low particle concentrations, which can move at
hurricane speeds. Both of these generally derive
from acidic magmas.
•
A further type of subaerial flow is a lahar or volcanic
mudflow.
EPS 321 Lecture 14
Fall
Flow
Surge
If the eruption column collapses a pyroclastic flow will occur, wherein gas and
tephra rush down the flanks of the volcano at high speed. This is the most
dangerous type of volcanic eruption. The deposits that are produced are called
ignimbrites if they contain pumice or pyroclastic flow deposits if they contain
non-vesicular blocks .
EPS 321 Lecture 14
3. A. Ignimbrites
•
These pyroclastic ash-flows are generated by the collapse
of eruption columns and they are hot, dense, laminar
flows of volcanic debris.
•
Fluidization (an up-ward movement of gas or water
causing the particles to behave as a fluid) is brought
about by the expansion of gases exsolved from the
magma and of air caught up in the advancing flow.
•
The flows, can travel for great distances (up to 100km),
even over flat ground. The deposits of these flows,
ignimbrites, are characterized by a homogeneous
appearance with little sorting of the finer ash particles,
but if coarse lithic clasts are present they may show
normal size grading, and large pumice fragments are
commonly reversely graded.
•
Ignimbrites do not mantle the topography, but tend to
follow valleys and low ground.
•
Pumice clasts may be concentrated on the top surface of
an ignimbrite. Fine ash in a flow is probably derived from
comminution of larger tephra
3. B. Pyroclastic-surge deposits
•
Pyroclastic surges are dilute, subaerial, fast-flowing turbulent mixtures of
volcanic particles and gas.
•
Two types occur: (a) hot, dry surges and (b) cool, wet surges.
•
The first type usually is associated with pyroclastic flows, usually forming the
basal part, so that a surge deposit may occur beneath an ignimbrite. Many of
these ground surges are generated by collapse of an eruption column.
•
The second type is a base surge and this usually forms during phreatic and
phreatomagmatic eruptions, where magma comes into contact with water.
•
Pyroclastic-surge deposits are generally thin and tine grained, and usually
they drape the topography, but with a thicker accumulation in depressions.
•
Grain size and bed thickness decrease away from the volcano and erosive
bases and channel structures are common.
•
The distinguishing feature of base-surge deposits is the presence of
stratification, and pla-nar and cross-bedding.
EPS 321 Lecture 14
3. C. Lahar deposits
• Lahars or volcanic mudflows occur on the slopes of some
subaerial volcanoes. Cold lahars are mostly produced by heavy
rain falling on unconsolidated ash. Hot lahars are formed when
a pyroclastic flow enters a lake or river or when air-fall ash is
dumped into a crater lake, which then overflows.
• Lahar deposits have textures similar to those of mud-flows on
alluvial fans and debris flows in the deep sea: a lack of sorting
and matrix-support fabric. The basal layer may be reversely
graded and large blocks may 'float' in the ash.
EPS 321 Lecture 14
4. Hydroclastites: hyaloclastites and hyalotuffs
•
When extruding lava comes into contact with water,
the rapid chilling and quenching causes
fragmentation of the lava. The surface of the lava
flow is chilled and as the flow moves forward, the
surface rind is fragmented and granulated, allowing
more magma to be chilled and fragmented. This
may be a fairly gentle process or highly explosive,
the grains produced by this magma-water
interaction are called hydroclasts.
EPS 321 Lecture 14
4. Hydroclastites: hyaloclastites and hyalotuffs
•
Volcaniclastic sediments produced through this process are generally known
as hyaloclastites, but a distinction can be made between hyaloclastites
sensu stricto, formed by non-explosive fragmentation of lava by water, and
hyalotuffs resulting from explosive magma-water interaction. The main
difference between these two types is in the shapes of the lava fragments:
less vesicular and more planar surfaces in the case of hyaloclastites, and
more vesicular with concave outer surfaces (broken-bubble walls) in the
hyalotuffs.
Typical grain shapes for
hydroclastic deposits. Left,
hyaloclastites; right, hyalotuffs.
EPS 321 Lecture 14
5. Epiclastic volcanogenic deposits {Any fragment of volcanic
(composition) origin}
•
Once deposited by volcanic processes, volcaniclastic sediments can be
reworked in the sedimentary environment in the same way as any other
sediment. In continental settings, volcanic ash is carried into river systems
and lakes by surface runoff, deflated by the wind and incorporated into soils.
In the shallow-marine environment, ash will be reworked by waves, tidal and
storm currents and mixed with non-volcanogenic material.
•
Redeposition of hyaloclastites in the deep sea by sediment gravity flows has
been noted already. Thus all the various depositional sedimentary structures
can be found in re-worked volcaniclastic deposits.
•
Being relatively soft and friable, volcanic debris is easily broken down into
finer grades and rounded by abrasion in moderate- to high-energy
environments.
EPS 321 Lecture 14
Diagenesis of volcaniclastic sediments
• Volcanic glass is metastable so that it is not preserved in rocks
older than mid-Tertiary.
• Volcanic glass is readily devitrified, altered and replaced during
weathering and diagenesis.
• Volcaniclastic sediments can be difficult to recognize as a result
of this alteration.
• The common alteration products are clay minerals and zeolites.
• The clay minerals that replace volcanic glass are mainly
smectites, in particular montmorillonite and saponite in more
basic ashes, and kaolinite in feldspathic ashes.
• Chlorite also may replace basic tephra.
• Smectite-rich clay beds derived from the alteration of volcanic
ash are known as bentonites.
• Some kaolinite-rich mudrocks called tonsteins are of volcanicash origin.
EPS 321 Lecture 14
Diagenesis of volcaniclastic sediments
• Apart from clay mineralogy, the presence of some
glass shards or their pseudomorphs, together with
euhedral or zoned phenocrysts, especially of quartz,
feldspar or pyroxene, will further confirm a volcanic
origin of mudrocks.
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