Tutorial:Volcanic Deposition

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Volcanic deposits
Volcanic deposits may consist of lava, ash and rock spewed out during eruptions. Pyroclastic deposits is the term given to all the fragmented rocks and ash from volcanic eruptions. These can be deposited as either air falls or flows. Collectively, those deposits that have travelled through the air are called tephra and can range in size from microscopic ash to large boulders, though this term usually conjures images of the ash sized particles which are sometimes used to date archaeological deposits.

The extent and depth of these deposits is determined very much by the height to which they are blasted into the air, the wind strength, rainfall patterns, and the size and density of the particles. They are typically deposited in an elongated plume stretching down wind from the volcano. The further away from the volcano the better sorted and the finer-grained the deposits will be. The finest ash particles can travel thousand of miles downwind and can be important as dating markers within stratigraphic sections.

Pyroclastic flows will often follow valleys and low-lying ground and because of their rapid speed and high temperature can be extremely destructive. The deposits formed by pyroclastic flows are called ignimbrite and usually consist of three layers; a fine grained, white or grey layer, a poorly sorted, welded layer with reverse grading (coarsening upwards), and an upper layer of unwelded fine ash.

Courtesy of the U.S. Geological Survey

Flows of molten rock (lava) are also typically associated with volcanic eruptions. The spatial extent of lavas is typically much less than that of tephra, hence the occurrence of the igneous rocks formed as the lava cools and solidifies is a much rarer event in the archaeological record. One famous instance of this is the site of Cuicuilco, Mexico which was partially buried by lava up to 10 m deep from the eruption of Xitle volcano approximately 2000 years ago.

People have been historically attracted to areas of volcanic activity by the fertile soils (Andosols) and by the presence of resources such as obsidian.

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