Pyroclastic rock
From Free net encyclopedia
Pyroclastic rocks and deposits comprise the entire range of fragmental products deposited directly by explosive or effusive volcanic eruptions. Pyroclastic deposits consist of pyroclasts which are not cemented together. Pyroclastic rocks (tuff) are pyroclastic deposits which have been lithified. The word 'pyroclast' is derived from Greek pyr, meaning fire, and klastos, meaning broken.
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.
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.
Pyroclasts of different sizes are classified as volcanic bombs, lapilli and volcanic ash