Pyroclastic Rocks: Explosive Volcanism Mount St Helens

Pyroclasts By Type Juvenile fragments – samples of quenched glassy/devitrified magma, Crystals – phenocrysts from the magma Lithic fragments – clasts of pre-existing rock, from the walls of the conduit. By Size blocks 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 monogenetic or polygenetic

Scoria Cones Splatter layers with reconstituted flow Cones can be monogenetic or 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 fumarole Carbonised wood

Welded Pyroclastic Flows Dark fiamme make up the eutaxitic texture

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

Caldera collapse associated with large volume 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, 2003 Lahar deposits caused by melting of ice and snow in 1981 eruption.

Pyroclastic Rocks

Crystal Fragments Vitric shards