Pyroclastic rock

Pyroclastic rocks or "pyroclastics" (derived from the Greek "πῦρ", meaning fire, and "κλαστός", meaning broken) are clastic rocks composed solely or primarily of volcanic materials. Where the volcanic material has been transported and reworked through mechanical action, such as by wind or water, these rocks are termed "volcaniclastic". Commonly associated with explosive volcanic activity - such as Plinian or krakatoan eruption styles, or phreatomagmatic eruptions - pyroclastic deposits are commonly formed from airborne ash, lapilli and bombs or blocks ejected from the volcano itself, mixed in with shattered country rock.

Pyroclastic rocks may be composed of a large range of clast sizes; from the largest agglomerates, to very fine ashes and tuffs. Pyroclasts of different sizes are classified as volcanic bombs, lapilli and volcanic ash. Ash is considered to be pyroclastic because it is a fine dust made up of volcanic rock. One of the most spectacular forms of pyroclastic deposit are the ignimbrites, deposits formed by the high-temperature gas and ash mix of a pyroclastic flow event.

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 kilometers 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.

Pyroclastic deposits consist of pyroclasts which are not cemented together. Pyroclastic rocks (tuff) are pyroclastic deposits which have been lithified.

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


Wikimedia Foundation. 2010.

Look at other dictionaries:

  • pyroclastic rock — noun Any rock formed by the accumulation of fragmented volcanic material (pyroclasts) ejected during explosive eruptions …   Wiktionary

  • Pyroclastic flow — Pyroclastic flows sweep down the flanks of Mayon Volcano, Philippines, in 1984 …   Wikipedia

  • Pyroclastic surge — A pyroclastic surge is a fluidized mass of turbulent gas and rock fragments which is ejected during some volcanic eruptions. It is similar to a pyroclastic flow but contains a much higher proportion of gas to rock, which makes it more turbulent… …   Wikipedia

  • Pyroclastic fall — A pyroclastic fall is a uniform deposit of material which has been ejected from an eruption or plume such as an ash fall or tuff. Pyroclastic flows occur relatively spontaneously in the geological time scale and are great indicators of time.… …   Wikipedia

  • pyroclastic Geology — [ˌpʌɪrə(ʊ) klastɪk] adjective of or denoting rock fragments or ash erupted by a volcano, especially as a hot, dense, destructive flow. noun (pyroclastics) pyroclastic material. Derivatives pyroclast noun …   English new terms dictionary

  • Rock microstructure — includes the texture of a rock and the small scale rock structures. The words texture and microstructure are interchangeable, with the latter preferred in modern geological literature. However, texture is still acceptable because it is a useful… …   Wikipedia

  • pyroclastic flow — ▪ volcanism       in a volcanic eruption, a fluidized mixture of hot rock fragments, hot gases, and entrapped air that moves at high speed in thick, gray to black, turbulent clouds that hug the ground. The temperature of the volcanic gases can… …   Universalium

  • pyroclastic flow — ALASKA VOLCANO OBSERVATORY GLOSSARY A dense, hot, chaotic avalanche of rock fragments, gas, and ash that travels rapidly away from an explosive eruption column, down the flanks of the volcano (synonymous with ash flow ). Pyroclastic flows move at …   Glossary of volcanic terms

  • Pyroclastic Peak — Infobox Mountain Name=Pyroclastic Peak Photo= South Face of Pyroclastic Peak.jpg Caption= South face of Pyroclastic Peak Elevation=Convert|2349|m|ft|0|abbr=on Location=British Columbia, Canada Range=Pacific Ranges Prominence =… …   Wikipedia

  • pyroclastic surge — ALASKA VOLCANO OBSERVATORY GLOSSARY A low density, turbulent flow of fine grained volcanic rock debris and hot gas. Pyroclastic surges differ from pyroclastic flows in that they are less dense and tend to travel as a low, ground hugging, but… …   Glossary of volcanic terms


Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.