Pyroclastic rock
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Pyroclastic rocks or pyroclastics (derived from the Greek πῦρ, meaning fire, and κλαστός, meaning broken) are debris thrown from volcanoes during an eruption. It consists of fresh lava that has risen to the surface or older volcanic material that has remelted and has been expelled into the air by the explosion.
Three modes of transport can be distinguished: pyroclastic flow, pyroclastic surge, and pyroclastic fall. During Plinian eruptions, pumice and ash are formed when silicic magma is fragmented in the volcanic conduit, because of decompression and the growth of bubbles. Pyroclasts are then entrained in a buoyant eruption plume which can rise several kilometres into the air and cause aviation hazards. Particles falling from the eruption clouds form layers on the ground (this is pyroclastic fall or tephra). Pyroclastic density currents, which are referred to as 'flows' or 'surges' depending on particle concentration and the level turbulence, are sometimes called glowing avalanches. The deposits of pumice-rich pyroclastic flows can be called ignimbrites.
A pyroclastic eruption entails spitting or "fountaining" lava, where the lava will be thrown into the air along with ash, pyroclastic materials, and other volcanic byproducts. Hawaiian eruptions such as those at Kilauea can eject clots of magma suspended into gas; this is called a 'fire fountain'. The magma clots, if hot enough may coalesce upon landing to form a lava flow.
The term "pyroclastics" can be applied to anything resulting from a volcanic eruption (ash, lava, cinder, and other volcanic debris). Pyroclasts of different sizes are classified as volcanic bombs, lapilli and volcanic ash. Ash is considered to be a "pyroclastics" because it is a fine dust made up of volcanic rock.
Pyroclastic deposits consist of pyroclasts which are not cemented together. Pyroclastic rocks (tuff) are pyroclastic deposits which have been lithified.
[edit] References
- Blatt, Harvey and Robert J. Tracy (1996) Petrology: Igneous, Sedimentary, and Metamorphic, W.H. Freeman & Company; 2nd ed., pp. 26-29; ISBN 0-7167-2438-3
