Pyroclastic rocks (derived from the el, πῦρ, links=no, meaning fire; and , meaning broken) are

clastic rock Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...

s composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroclasts. Pyroclastic rocks are a type of

volcaniclastic Volcaniclastics are geologic materials composed of broken fragments ( clasts) of volcanic rock. These encompass all clastic volcanic materials, regardless of what process fragmented the rock, how it was subsequently transported, what environment it ...

deposit, which are deposits made predominantly of volcanic particles. 'Phreatic' pyroclastic deposits are a variety of pyroclastic rock that forms from volcanic steam explosions and they are entirely made of accidental clasts. 'Phreatomagmatic' pyroclastic deposits are formed from explosive interaction of

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural s ...

with groundwater. Unconsolidated accumulations of pyroclasts are described as

tephra Tephra is fragmental material produced by a volcanic eruption regardless of composition, fragment size, or emplacement mechanism. Volcanologists also refer to airborne fragments as pyroclasts. Once clasts have fallen to the ground, they re ...

. Tephra may become

lithified Lithification (from the Ancient Greek word ''lithos'' meaning 'rock' and the Latin-derived suffix ''-ific'') is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Essentially, lithificati ...

to a pyroclastic rock by cementation or chemical reactions as the result of the passage of hot gases (

fumarolic A fumarole (or fumerole) is a vent in the surface of the Earth or other rocky planet from which hot volcanic gases and vapors are emitted, without any accompanying liquids or solids. Fumaroles are characteristic of the late stages of volcani ...

alteration) or groundwater (e.g.

hydrothermal alteration Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical com ...

and

diagenesis Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...

) and burial, or, if it is emplaced at temperatures so hot that the soft glassy pyroclasts stick together at point contacts, and deform: this is known as

welding Welding is a fabrication process that joins materials, usually metals or thermoplastics, by using high heat to melt the parts together and allowing them to cool, causing fusion. Welding is distinct from lower temperature techniques such as bra ...

. One of the most spectacular types of pyroclastic deposit is an

ignimbrite Ignimbrite is a type of volcanic rock, consisting of hardened tuff. Ignimbrites form from the deposits of pyroclastic flows, which are a hot suspension of particles and gases flowing rapidly from a volcano, driven by being denser than the surrou ...

, which is the deposit of a ground-hugging pumiceous pyroclastic density current (a rapidly flowing hot suspension of pyroclasts in gas). Ignimbrites may be loose deposits or solid rock, and they can bury entire landscapes. An individual ignimbrite can exceed 1000 km³ in volume, can cover 20,000 km² of land, and may exceed 1 km in thickness, for example where it is ponded within a volcanic caldera.

Classification

Pyroclasts include juvenile pyroclasts derived from chilled magma, mixed with accidental pyroclasts, which are fragments of

country rock Country rock is a genre of music which fuses rock and country. It was developed by rock musicians who began to record country-flavored records in the late 1960s and early 1970s. These musicians recorded rock records using country themes, vocal ...

. Pyroclasts of different sizes are classified (from smallest to largest) as

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, created during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to refer ...

lapilli Lapilli is a size classification of tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. ''Lapilli'' (singular: ''lapillus'') is Latin for "little stones". By definition lapilli range ...

, or

volcanic block A volcanic block is a fragment of rock that measures more than in diameter and is erupted in a solid condition. Blocks are formed from material from previous eruptions or from country rock and are therefore mostly accessory or accidental in ori ...

s (or, if they exhibit evidence of having been hot and molten during emplacement,

volcanic bomb A volcanic bomb or lava bomb is a mass of partially molten rock (tephra) larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. Because volcanic bombs cool after they l ...

s). All are considered to be pyroclastic because they were formed (fragmented) by volcanic explosivity, for example during explosive decompression, shear, thermal decrepitation, or by attrition and abrasion in a volcanic conduit, volcanic jet, or pyroclastic density current. Pyroclasts are transported in two main ways: in atmospheric eruption plumes, from which pyroclasts settle to form topography-draping

pyroclastic fall A pyroclastic fall is a uniform deposit of material which has been ejected from a volcanic eruption or plume such as an ash fall or tuff. Pyroclastic air fall deposits are a result of: # Ballistic transport of ejecta such as volcanic blocks, volca ...

layers, and by pyroclastic density currents (PDCs) (including

pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of bu ...

s and

pyroclastic surge A pyroclastic surge is a fluidised mass of turbulent gas and rock fragments that is ejected during some volcanic eruptions. It is similar to a pyroclastic flow but it has a lower density or contains a much higher ratio of gas to rock, which makes i ...

s), from which pyroclasts are deposited as pyroclastic density current deposits, which tend to thicken and coarsen in valleys, and thin and fine over topographic highs. During Plinian eruptions,

pumice Pumice (), called pumicite in its powdered or dust form, is a volcanic rock that consists of highly vesicular rough-textured volcanic glass, which may or may not contain crystals. It is typically light-colored. Scoria is another vesicular vol ...

and

ash Ash or ashes are the solid remnants of fires. Specifically, ''ash'' refers to all non-aqueous, non-gaseous residues that remain after something burns. In analytical chemistry, to analyse the mineral and metal content of chemical samples, ash ...

are formed when foaming

silicic Silicic is an adjective to describe magma or igneous rock rich in silica. The amount of silica that constitutes a silicic rock is usually defined as at least 63 percent. Granite and rhyolite are the most common silicic rocks. Silicic is the group ...

is fragmented in the volcanic conduit, because of rapid shear driven by decompression and the growth of microscopic bubbles. The pyroclasts are then entrained with hot gases to form a supersonic jet that exits the volcano, admixes and heats cold atmospheric air to form a vigorously buoyant

eruption column An eruption column or eruption plume is a cloud of super-heated ash and tephra suspended in gases emitted during an explosive volcanic eruption. The volcanic materials form a vertical column or plume that may rise many kilometers into the air ...

that rises several kilometers into the stratosphere and cause aviation hazards. Particles fall from atmospheric eruption plumes and accumulate as layers on the ground, which are described as fallout deposits. Pyroclastic density currents arise when the mixture of hot pyroclasts and gases is denser than the atmosphere and so, instead of rising buoyantly, it spreads out across the landscape. They are one of the greatest hazards at a volcano, and may be either 'fully dilute' (dilute, turbulent ash clouds, right down to their lower levels) or 'granular fluid based' (the lower levels of which comprise a concentrated dispersion of interacting pyroclasts and partly trapped gas). The former type are sometimes called ''pyroclastic surges'' (even though they may be sustained rather than "surging") and lower parts of the latter are sometimes termed ''pyroclastic flows'' (these, also, can be sustained and quasi steady or surging). As they travel, pyroclastic density currents deposit particles on the ground, and they entrain cold atmospheric air, which is then heated and thermally expands. Where the density current becomes sufficiently dilute to loft, it rises into the atmosphere as a 'phoenix plume' (or 'co-PDC plume'). These phoenix plumes typically deposit thin ashfall layers that may contain little pellets of aggregated fine ash. Hawaiian eruptions such as those at

Kīlauea Kīlauea ( , ) is an active shield volcano in the Hawaiian Islands. Located along the southeastern shore of the Big Island of Hawaii, the volcano is between 210,000 and 280,000 years old and emerged above sea level about 100,000 years ago. Hi ...

produce an upward-directed jet of hot droplets and clots of magma suspended in gas; this is called a

lava fountain Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...

or 'fire-fountain'. If sufficiently hot and liquid when they land, the hot droplets and clots of magma may agglutinate to form 'spatter' ('agglutinate'), or fully coalesce to form a clastogenic

lava flow Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...

Classification

See also

References

Other reading