What are the three types of explosive eruptions?
Dry magmatic (no external water), Phreatomagmatic (magma + water), and Phreatic (steam-driven, no direct magma contact).
Define ‘Pyroclast.’
A fragment formed from disintegration of bubble-rich magma; includes pumice, scoria, and glass shards.
What drives explosive eruptions?
Expansion of magmatic volatiles or interaction with external water producing rapid gas expansion.
What are phreatic eruptions?
Explosions caused by water heated by magma without direct magma contact.
What is a phreatomagmatic eruption?
An eruption resulting from the interaction between magma and external water, increasing explosivity.
What happens if the eruption column density exceeds that of the atmosphere?
The column collapses under gravity, forming pyroclastic flows and surges.
What are the two main dispersal processes for pyroclasts?
1) Fallout (settling through the air) and 2) Pyroclastic density currents (ground-hugging flows).
When does a buoyant eruption plume form?
When the eruption column is less dense than the surrounding atmosphere.
Define ‘eruption column.’
A high-velocity jet of hot gas and particles rising above a volcanic vent.
What are pyroclastic fall deposits?
Deposits formed from pyroclasts settling out of the eruption column or plume.
List characteristics of pyroclastic fall deposits.
Wide aerial distribution, mantle topography, well-sorted, may be graded, and typically unwelded.
What textures are found in fall deposits?
Accretionary lapilli, massive to graded bedding, and variations in grain size laterally.
What influences the distribution of fall deposits?
Wind direction, clast size, and convective velocity of the plume.
Define ‘spatter deposits.’
Coarse pyroclastic material (bombs, blocks, scoria) that falls near the vent, often associated with fire fountaining.
What are pyroclastic density currents?
Gravity-driven, gas-supported currents transporting hot pyroclasts laterally.
How fast can pyroclastic density currents move?
Between 40 and over 400 km/h.
What temperatures can PDCs reach?
From about 100°C up to over 700°C.
Differentiate between pyroclastic flows and surges.
Flows are dense, concentrated currents; surges are dilute, turbulent, and move over topographic highs.
Define ‘pyroclastic flow deposit.’
A poorly sorted, matrix-supported deposit with angular clasts that infills topography, from magma of any composition.
What are common textures in pyroclastic flow deposits?
Fiamme (flattened pumice), breadcrust textures, gas escape structures, and accretionary lapilli.
What is welding in pyroclastic deposits?
Sintering of hot juvenile clasts (pumice, scoria, glass shards) into a compact, solid mass.
At what temperature does rhyolitic glass begin to weld?
Above approximately 550–600°C, depending on water content and load.
What is ‘welding compaction’?
Plastic deformation of hot pyroclasts reducing pore space and increasing density through compaction.
Define ‘pyroclastic surge.’
A low-concentration, ground-hugging flow of gas and ash, often associated with phreatomagmatic eruptions.
