USGS scientist examines pumice blocks at the edge of a pyroclastic flow from Mount St. Helens

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. Mount St Helens is an active Stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States Greek (el ελληνική γλώσσα or simply el ελληνικά — "Hellenic" is an Indo-European language, spoken today by 15-22 million people mainly Clastic rocks are composed of fragments or clasts, of pre-existing rock. Plate tectonics and hotspots Divergent plate boundaries At the 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. Plinian eruptions are Volcanic eruptions marked by their similarity to the eruption of Mt Krakatoa ( Indonesian: Krakatau) also spelled Krakatao or Krakatowa, is a volcanic island in the Sunda Strait Phreatomagmatic eruptions are defined as juvenile forming eruptions as a result of interaction between water and Magma. Volcanic ash consists of small Tephra, which are bits of pulverized rock and glass created by volcanic eruptions less than in diameter Lapilli is a size classification term for Tephra, which is material that falls out of the air during a volcanic eruption. A volcanic bomb is a globe of molten rock ( Tephra) larger than 65 mm (2 Volcanic blocks are fragments of rock which measure more than 64mm in size and are erupted in a solid condition Country rock is a geological term meaning the rock native to an area

Pyroclastic rocks may be composed of a large range of clast sizes; from the largest agglomerates, to very fine ashes and tuffs. Clastic rocks are composed of fragments or clasts, of pre-existing rock. Agglomerates (from the Latin 'agglomerare' meaning 'to form into a ball' are coarse accumulations of large blocks of volcanic material that contain at least Tuff (from the Italian "tufo" is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption Pyroclasts of different sizes are classified as volcanic bombs, lapilli and volcanic ash. A volcanic bomb is a globe of molten rock ( Tephra) larger than 65 mm (2 Lapilli is a size classification term for Tephra, which is material that falls out of the air during a volcanic eruption. Volcanic ash consists of small Tephra, which are bits of pulverized rock and glass created by volcanic eruptions less than in diameter 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. Ignimbrite is a Volcanic Pyroclastic rock often of Dacitic or Rhyolitic composition A pyroclastic flow (also known as a pyroclastic density current) is a common and devastating result of some volcanic eruptions.

Three modes of transport can be distinguished: pyroclastic flow, pyroclastic surge, and pyroclastic fall. A pyroclastic flow (also known as a pyroclastic density current) is a common and devastating result of some volcanic eruptions. 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 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. 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. Plinian eruptions are Volcanic eruptions marked by their similarity to the eruption of Mt Pumice is a textural term for a volcanic rock that is a solidified frothy lava composed of highly microvesicular glass Pyroclastic with very thin translucent Volcanic ash consists of small Tephra, which are bits of pulverized rock and glass created by volcanic eruptions less than in diameter The Chemical compound silicon dioxide, also known as silica or silox (from the Latin " Silex " is an Oxide Magma (Plurals magmas and magmata) is molten rock that sometimes forms beneath the surface of the Earth (or any other Terrestrial planet 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). Tephra is air-fall material produced by a volcanic eruption regardless of composition or fragment size 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. Ignimbrite is a Volcanic Pyroclastic rock often of Dacitic or Rhyolitic composition

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'. Kīlauea (kiːlauea is an active Volcano in the Hawaiian Islands, one of five Shield volcanoes that together form the Island of Hawai{{okina}}i The magma clots, if hot enough may coalesce upon landing to form a lava flow. Lava is molten rock expelled by a Volcano during an eruption When first expelled from a volcanic vent it is a Liquid at Temperatures

Pyroclastic deposits consist of pyroclasts which are not cemented together. Pyroclastic rocks (tuff) are pyroclastic deposits which have been lithified. Tuff (from the Italian "tufo" is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption Lithification (from the Greek word lithos meaning 'rock' and the Latin -derived suffix -ific) is the process in which Sediments compact

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