Volcanic products
Pyroclastic is commonly used to refer only to volcanic materials ejected from a volcanic vent
Volcaniclastic includes all volcanic particles regardless of their origin
Generic types of Volcaniclastic Particles
Pyroclastic particles (pyroclasts) form by the disintegration of magma, as gases are released by decompression and then ejected from a volcanic vent either in the air or beneath the water.
Hydroclasts form steam explosions from magma-water interactions.
Autoclastic fragments form by mechanical friction during movement of lava and breakage of cool brittle outer margins, or gravity crumbling of spines and domes.
Alloclastic fragments form by disruption of pre-existing volcanic rocks by igneous processes beneath the Earth's surface.
Epiclasts are lithic clasts and minerals released by ordinary weathering processes from pre-existing consolidated rocks. Volcanic epic lasts are clasts of volcanic composition derived from erosion of volcanoes or ancient volcanic terrains.
Pyroclastic falls (1)
Sedimentation of clasts through the atmosphere from an eruption jet, plume or laterally moving turbulent ash cloud during an explosive eruption
Heavy material (blocks, bombs) follow a balistic trajectory
Pyroclastic falls (2)
- Coarser material falls out closer to the eruption point than the finer material
- Finer material is moved upwards by expanding gases and then taken by the wind (direction!)
- Pyroclastic fall deposits: ‘TEPHRA’
- Study of tephra: ‘Tephra-chronology’
- Isopach map: information about distribution and thickness of pyroclastic fall
Pyroclastic flows
- Flow (density current) of volcanic material consisting dominantly of vesiculated, low-density pumice and glass shards and gases
- Also includes flows consisting of poorly-vesiculated, dense lava clasts
- Pyroclastic flow deposits: Predominantly massive, poorly sorted