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Pyroclastic Rocks: Explosive Volcanism
Mount St Helens
Pyroclasts
By TypeJuvenile fragments – samples of quenched glassy/devitrified magma, Crystals – phenocrysts from the magmaLithic fragments – clasts of pre-existing rock, from the walls of the conduit.
By Sizeblocks or bombs (>64 mm), lapilli (64-2mm)ash (>2mm).
Juvenile Pyroclasts
Breadcrust bomb
Juvenile Pyroclasts
Acid/intermediate/mixed - Pumice Basic/alkaline - Scoria
Often rounded by abrasion in vent
Juvenile Pyroclasts
Achneliths (glassy droplets) – (Pele’s Tears)Achneliths and scoria can “fuse” when emplaced hot to form splatter. This forms cones and ramparts.
Juvenile Pyroclasts
Juvenile Shards In a vitric tuff/ash
Juvenile Pyroclasts
Juvenile Shards
Juvenile Pyroclasts
Accretionary lapilli
Kileaua lapilli layer
Phreatomagmatic – water vapour causes grains to accrete into concentric layers.
Types of Pyroclastic Eruption
Hydrovolcanic
Eruption types are based on height of the column and the degree of fragmentation
Hawaiian Activity
Dominated by basaltic lava fountains and flows. Typical of
shield volcanoes and fissures
Types of Pyroclastic Eruption
Hydrovolcanic
Eruption types are based on height of the column and the degree of fragmentation
Strombolian Activity
Strombolian eruptions are characterized by the intermittent explosion or fountaining of basaltic lava from a single vent or
crater. Eruptions are often rhythmic explosions.
Stromboli
Strombolian Activity
Explosions caused by slugs of gas reaching the surface.
Scoria Cones
Sunset Crater, Arizona
Splatter layers with reconstituted flow
Cones can be monogeneticor polygenetic
Scoria Cones
Splatter layers with reconstituted flow
Cones can be monogeneticor polygenetic
Types of Pyroclastic Eruption
Hydrovolcanic
Eruption types are based on height of the column and the degree of fragmentation
Plinian Eruptions
Plinian (sub to ultra) eruptions result in the formation of a sustained eruption column which may exceed 50 km in height.
They are typical of intermediate and acidic magmas.
Sakurajima, 1985
Plinian Eruptions
Sakurajima, 1985
Magma droplets heat the surrounding gas. The gas + magma mixture becomes less dense than the surrounding air and rises.
Pyroclastic Deposits
Pyroclastic Deposits: Air Fall (Tephra)
Ballistic ejecta
Air Fall
Pyroclastic air fall deposits (tephra) are poorly sorted (except at large distances i.e. distal deposits)
Pyroclastic Deposits: Air Fall (Tephra)
Ballistic ejecta
Air Fall
Thickness and grainsize of air fall decrease away from vent.
Agglomerate close to vent, through lapilli to ash.
Pyroclastic Deposits: Air Fall (Tephra)
Ballistic ejecta
Air Fall
Bomb sags in bedded ash/lapilli.
Pyroclastic Deposits: Air Fall (Tephra)
Ballistic ejecta
Air Fall
Stratification due to pulsing of an eruption observed closer to the vent
Reverse grading occurs due to increasing vent diameter due to erosion
Increase in lithics
Pyroclastic Deposits: Air Fall (Tephra)
Airfall gets finer-grained away from the vent
Pyroclastic Deposits: Air Fall (Tephra)
Vent gets larger due to erosion of the walls
Velocity and Mass Flux increases
Pyroclastic Deposits: Air Fall (Tephra)
Vent gets larger due to erosion of the walls
Velocity and Mass Flux increases
Pyroclastic Deposits: Air Fall (Tephra)
Walls collapse to block vent
Finer-grained material settles out of plume
Pyroclastic Deposits: Air Fall (Tephra)
Blockage is removed
Closer to vent lithic fragments are concentrated
Pyroclastic Deposits: Air Fall (Tephra)
In more distal units layers may represent individual discrete eruptions.
Pyroclastic Deposits: Air Fall (Tephra)
Some air fall ashes can be emplaced hot and become welded (these resemble ignimbrites)
Pyroclastic Flows
• Pyroclastic flows are gravity-driven surface flows of debris which travel as a high particle density solid-gas dispersion. • They can be thought of as a slurry with gas instead of liquid water.
Pyroclastic Flows
• Emplaced hot (not usually molten).
• Restricted to topographic lows.
Pyroclastic Flows
• Emplaced hot (not usually molten).
• Restricted to topographic lows.
pumice
lithics
Pumice flows = ignimbrites
Pyroclastic Flows
ground surge
Pyroclastic Flows: Evidence for Heating
Fossil fumaroleCarbonised wood
Welded Pyroclastic Flows
Dark fiamme make up the eutaxitic texture
Pyroclastic Flows
Pyroclastic Flows
Vent erosion causes increase in mass of plume
Pyroclastic flows often found at the top of the sequence prior to eruption of lavas
Pyroclastic Deposits: Air Fall (Tephra)
Density of plume = Density of atmosphere
Pyroclastic Deposits: Air Fall (Tephra)
Density of plume = Density of atmosphere
Density of plume increases with vent widening
Pyroclastic Deposits: Air Fall (Tephra)
Density of plume = Density of atmosphere
Density of part of plume becomes greater than atmosphere
Pyroclastic Deposits: Air Fall (Tephra)
Density of plume = Density of atmosphere
Dense plume fragment falls under gravity
Pyroclastic Deposits: Air Fall (Tephra)
Density of plume = Density of atmosphere
Fragment becomes pyroclastic flow
Pyroclastic Flows
Sudden release of pressure on magma causes explosive loss of volatiles
Collapse of lava dome often produces welded ignimbrites
Pyroclastic Flows
Crater Lake, Oregon
Pyroclastic Flows
Pyroclastic Flows
Caldera collapse associated with large volume pyroclastic flows.
Pyroclastic Flows
Pyroclastic Flows
Caldera produced ignimbrites are extensive (e.g. Santorini 1470 BC, Taupo 186 AD)
Pyroclastic Surges
Base surge
Low particle density particle/gas suspension flows
Pyroclastic Surges
Base surge
Climbing dune forms
Pyroclastic Surges
Base surge
Climbing dune forms
Cross bedding
Epiclastic Deposits
Poorly consolidated volcaniclastic deposits are rapidly reworked by runoff to form epiclastics.
Flood plain
Epiclastic Deposits
Volcaniclastic deposits are often reworked to become epiclastic sediments.
Lahar Deposits
Mt St Helens, 2003Lahar deposits caused by melting of ice and snow in 1981 eruption.
Pyroclastic Rocks
Pyroclastic Rocks
Crystal Fragments
Vitric shards
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