A general name given to rocks formed from fragments of lava ejected from a volcano into the atmosphere. Examples are ignimbrites, consolidated volcanic ash (tuff), and volcanic agglomerate.
Pyroclastic rocks or pyroclastics (derived from the Greek πῦρ, meaning fire, and κλαστός, meaning broken) are debris thrown from volcanoes during an eruption.
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. 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.
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.
Pyroclastic deposits consist of pyroclasts which are not cemented together. Pyroclastic rocks (tuff) are pyroclastic deposits which have been lithified.
User Comments Add a comment…