Department De partment of Pe Petro troleum leum Geolo Geology gy & Sedim Sedimentolog entology, y, Facul Fa culty ty of Ea Earth rth Science Sciences, s, King Abdulaziz University, Jeddah, Saudi Arabia
EPS 421 CLA CL A ST STIC IC SED SEDIME IMENT NTA A RY RO ROCK CKS S Lect Le ctur ure e 14: Vol Volca cani nicl cla ast stic ic se s edi dime ment nts; s; compos com posit itio ion, n, cla classi ssifi fica cati tion on and and dia di age gene nesis sis
Pr of of.. Dr Dr . Ma Mahm hmo o ud A. M. M. Are Ar ef
Vol olca cani nicl cla ast stic ic se sedi dime ment nts s
Volcaniclastic sediments: are those composed chiefly of grains of volcanic origin, derived from contemporaneous volcanicity.
Vol olca cani nicl cla ast stic ic se sedi dime ment nts s
Volcaniclastic sediments: are those composed chiefly of grains of volcanic origin, derived from contemporaneous volcanicity.
Prob roble lems ms in i n the th e stu study dy of o f Volc Volca anic nicla last stic ic rock r ocks s 1) It is is ofte often n diff diffic icul ultt (and (and dan dange gero rous us)) to obs obser erve ve mod moder ern n volc volcan anic ic pr proc oces esse ses s and mostly the eruptions that can be studied are relatively small-scale. 2) Th The e tech techni niqu ques es use used d for for stu studyi dying ng Rec Recent ent vo volc lcan anicl iclas asti tics cs,, such such as as si siev evin ing g for grain-size analyses are not applicable to their indurated ancient equivalents. 3) Dia Diagen genesi esis s is is a maj major or fac factor tor in alte alterin ring g volca volcanic nic gla glass ss and min minera erals, ls, destroying depositional textures and creating matrix. 4) Th The e wea weath ther erin ing g of vo volc lcan anic ic ma mate teri rial al is also also ve very ry ra rapi pid; d; in a few few ye year ars s san sanddgrade ash particles can be reduced to clay. 5) The There re is is also also a pro proble blem m with with the the pres preserv ervati ation on of of the the volc volcano anoes es them themsel selves ves;; being topographic highs, they are easily eroded, and also hyrothermally altered.
Volcanicla olcaniclastic stic se sediments diments and rocks ro cks are produce produc ed from: f rom: 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)
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
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
volcanic mudst one (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
General descriptive classification of volcaniclastic deposits
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
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.
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 .
• 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)
Overview of deposit of volcaniclastic breccia
• 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.
• 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 .
• 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 plagiocl ase laths throughout
and the white olivine crystal at the lower right. View is under crossed polarizers.
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 .
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 .
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 .
•
Photomicrograph of volcanic glass shards seen in a smear slide
Thick coastal sections of ash
Composition of Tuffs • Crystals – Euhedral +/- broken – Compositional zoning
• Vitric (glassey) fragments – Bubble wall shards
Composition of Tuffs • Vitric (glassy) fragments – Bubble wall shards – Hydroclastic shards • Lithic fragments – Volcanic rock fragments
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.
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.
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.
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.
2. Pyroclastic-fall deposits (Particles broken by volcanism)
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 deposit s
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
Fall
Flow
Surge
Characteristics of Pyroclastic Deposits
Fall Deposits
Mantle topography
Parallel bedding
Well sorted
Often graded
Flow Deposits
Topographically constrained
Poorly sorted
Often graded
Surge Deposits
Partially topographically constrained
Cross bedding characteristic
Intermediate sorting
Often graded
Fall
Flow
Surge
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 deposition.
•
Fall
A further feature of air-fall deposits is the development of mantle bedding, whereby the tephra layer follows and blankets any original
Flow
topography, with a similar thickness over topographic highs and lows. Surge
Pyroclastic Fall Deposits
General term: tephra
Types
Scoria (mafic , larger size)
Pumice (silicic, larger size)
Ash
(fine grained, any composition)
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
Fall
with low particle concentrations, which can move at hurricane speeds. Both of these generally derive
Flow
from acidic magmas. •
A further type of subaerial flow is a lahar or volcanic mudflow
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 .
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 gro und. 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.
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.
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.
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.
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.
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Being relatively soft and friable, volcanic debris is easily broken down into finer grades and rounded by abrasion in moderate- to high-energy environments.
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.
