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