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

with

groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidated ...

. 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, lithificatio ...

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

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 surro ...

, 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. 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, or volcanic blocks (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, vol ...

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

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 and ash are formed when foaming silicic

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 Volcanic ash, ash and tephra suspended in volcanic gas, gases emitted during an explosive volcanic eruption. The volcanic materials form a vertical column or Plume (fluid dynamics), ...

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

Classification

See also

References

Other reading