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

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 waterHyalotuffs

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