Volcanic Facies

Volcanic Facies-Defined

• Lateral and/or vertical changes in physical

aspects of rock bodies deposited within a

specific interval of geologic time.

• Volcanic Facies are usually named according to:

– closeness to source (central, proximal, distal)

– Depositional environment (subaerial, shallow

subaqueous, sub-glacial, etc.)

– Within deposit type (pyroclastic flow facies, hyalotuff

facies, dome facies, etc.)

Subaerial- Stratovolcanoes:

Central Facies • Range- central vent out to about 2km

• Recognized by:

Lava domes and thick, banded lavas

Abundant dykes and sills

circular to elongate stocks

breccia pipes

coarsely stratified, poorly sorted air fall deposits (blocks and/or bombs present)

hydrothermally altered rocks

Interlayering of coarse grained tephra and lava flows

Lag-fall breccias of pyroclastic flows

No single feature- combination needed

Proximal Facies (2-15km)

• Rocks around a volcanic center deposited

from pyf’s, lava flows, debris

flows/avalanches, fallout processes and

their erosional products.

• As distance from source increases within

this facies, there is an increase in amount

of resedimented epiclastic and pyroclastic

debris

Proximal Facies

• Lahars- angular-sub angular blocks, poorly sorted, massive, reverse grading at base

• Tephra layers with good bedding and sorting, grain size coarse ash to lapilli

• Pyroclastic flow units (main body) underlain by surge deposits (lense-like) and overlain by fine-bedded ash deposits

• Broad, thick (15m) lava flows

• Block and ash flows from dome collapse-monolithic, massive, poorly sorted.

• Clastic debris reworked by water

• Debris avalanche deposits-mounds (block facies) and more normal laharic material.

Distal facies (>7-15km)

• Base of volcano and beyond

• Rocks here characterized by a much

greater lateral continuity than those of the

proximal and central facies.

Distal Facies

• Finely bedded tephra composed dominantly of fine-coarse ash, outward increasing ratio of glass to crystals

• Lahars with blocks that rarely exceed 1 meter in diameter and have rounded or subrounded particles in the matrix. Lahars may be interlayered with shallow water sediments.

• Pyroclastic flows will be thinner than in proximal areas, no surge deposits, ash fall common above flows. May find distinct layering caused by concentration of pumice at tops of flow units.

• Interlayered shallow water sediments

• Rare lava flows-restricted to isolated vents

• Debris avalance mounds

Tuff Rings and Cones

• Central-Small Lava dome or flows-shallow

water-subaerial

• D/F deposits, reworked volcanic material

(from ring-cone walls)

• Outer-massive-thin bedded deposits,

closer to vent-cross-beds, dunes

• Pepperites and dikes-sills

Characteristics of Deposits

• Thin beds in tuff cones and rings

– Thickness: few mm to several cm, most <

1cm

– Form by a large number of short eruptive

pulses- Surtsey > 60 per minute

– Characterized by fine ash with the sporadic

occurrence of lapilli and blocks

Massive Beds

• Composed of coarse to fine ash with variable lithic and pumice content.

• Beds range from about 1 to 50-60 m thick and as such may be difficult to id.

• Thick beds may be seperated by thin, fissile, laminae of fine ash which is a few mm to 1-2cm thick.

• Cross-bedding and soft sediment deformational features are rare

Shield Volcano

• Central Facies (fissure-2km):

– Dykes, sills, small intrusions-mafic

– Thick flows (lava lakes)

– Cinder cones and agglomerates

– Hydrothermal alteration

– Thick sequence of aa and/or pahoehoe flows

Proximal (2-15km)

• Thinner AA and Pahohoe flows

• Bedded coarse and fine ash

• Landslide breccias

• Shore-shallow water– Hyalotuffs

– Pillow lavas

– Self peperites and minor peperite

– Hyaloclastites

– Debris flows

– Sediments (limestone, iron formation, volcanic seds)

Distal (>15km)

• Pillow lavas and hyaloclastite

• Self-peperite

• Debris flows

• Wackes-mudstones

• Water lain ash fall deposits

Caldera Facies

• Three types:

– Intra-caldera: within caldera, near source

– Caldera margin: adjacent to ring fractures and

walls

– Outflow- outside the caldera, away from

source

Intra-caldera

• Plinian air fall (popcorn pumice) overlain

by pyroclastic flows that may be 100’s of

meters thick.

• Welded pyroclastic flows

• Meso- and mega breccias

• With resurgence may get cap of lava

flows, cinder or pumice cones, sediments,

debris flow material

Caldera margin

• Domes, dome breccias, block and ash

flows

• Dykes and sills

• Sediments (moat)

Outflow

• Pyroclastic flows (thin) which may extend

for ten’s of kms outside caldera

• Coarse to fine ash fall deposits

Subaqueous-Felsic to Intermediate

• Poorly understood, poorly known

• Mostly from volcanic reconstructions in

older volcanic terrains

• Relatively shallow water (explosive

eruptions)

• Central, Proximal, Distal

Shallow Water-Central (0-2km)• Dome-hyaloclastite complexes, Syn-

depositional peperites

• Pyroclastic flow deposits- massive-thickly bedded, base possibly lithic rich, well vesiculated pumice, heat retention features?

• Hydrothermal alteration and exhalites

• Hyalotuffs-thick-thin beds, bombs-blocks-ash

• Intrusive peperites, fluidal shapes

• Dykes and sills, multiple intrusions and dike swarms,small stocks-circular

Proximal (2-5km)• Well bedded pyroclastic flows (massive basal,

bedded ash top, repetitive), pumice and ash rich,

• Block and ash flows from domes

• Syndepositional peperites

• Debris flows separated by turbidites

• Finely bedded hyalotuffs, dominantly fine ash

• Sediments-wackes-siltstones, intrusive peperites

• Ash-fall, doubly graded?

• Dikes and sills, intrusive peperites

• Hydrothermal alteration

Distal

• Finely bedded and graded pyroclastic

flows (ash turbidites)

• Debris flows

• Volcanic siltstones, wackes, mudstones

Mafic-Subaqueous

• Sheet flows-massive lavas giving way to pillow

lavas

• Dykes and sills-multiple intrusions, dike

swarms

• Hydrothermal alteration

• Syn-deposional peperites?

• Hyalotuffs

Proximal

• Pillow lavas and hyaloclastite

• Lots of syn-depositional peperite and

peperite

• Minor massive lavas

• Reworked hyaloclastite

• Mafic sediments

• Debris flows,

• synsedimentary deformation-convolute

bedding

Distal

• Debris flows and associated sediments

• Lots of sedimentary material

• Minor pillows and hyaloclastite

Subaqueous Calderas

Intra

• Bedded pyroclastic flow deposits: >100m thick,

numerous flow units.

• Massive basal, bedded tops

• Or massive pyf’s with thick ash tops

• Pumice-rich-possibly variable vesicularity-40->70%

• Locally have heat-retention features- spherulites

• Hydrothermal alteration widespread

• Meso-Mega Breccias

• Debris flows

• Capped by volcanic sedimentary rocks, debris flows

Outflow

• Thin pyroclastic flow deposits

• Thin basal and ash beds

• Pyro-turbidites (Bouma A and E beds)

• Debris flows

• Greywackes

• Little alteration (metamorphism,

diagenetic)

Shallow Vs Deep Water

• Deep water-Deposit Types

– Pillows lavas and self peperites

– Massive lavas (sheet flows, ponded)

– Hyaloclastites

– Dome-hyaloclastite complexes, lobe lavas

– Blocky Domes

– Distal facies of pyroclastic flows (pyroturbidites)

– Debris Flows (turbidites)

– Pelagic sediments Mud, silt, Clay), chemical

sediments

Shallow Water-Deposit types

• Pillow lavas and self peperites

• Massive lavas (sheet flows, ponded)

• Hyaloclastites

• Scoria and ash beds (Hyalotuffs)

• Dome-hyaloclastite complexes, lobe lavas, blocky lavas

• Pumice-rich hyalotuffs

• Pyroclastic flow and fall deposits

• Debris flows

• Clastic and chemical sediments

• Pillow-hyaloclastite deltas

Deep Water-Characterisitics

• Marked predominance of flows, both mafic and

felsic, over pyroclastic material

• Poorly vesiculated flows and hyaloclastite,

absence of abundant scoriaceous flow tops

• Absence of Hyalotuffs

• Little evidence of sedimentary re-working, few

clastic sediments

• Minor debris flow deposits (only turbidites)

• Pyroturbidites

Shallow Water-Characteristics

• Predominance of pyroclastic deposits. These include products of both magmatic and phreatomagmatic explosions. Deposits include pyroclastic flows (with well vesiculated pumice), hyalotuffs (with bombs), waterlain tuffs

• Scoria-pumice-rich beds (hyalotuffs)

• Vesicular and scoriaceous pillow and massive lavas, associated hyaloclastite-large vesicles, gas cavities, multiple rinds, vesicles around entire pillow

• Debris flows

• Reworked pyroclastic material