Pyroclastic rock

Last updated January 09, 2024

Rocks from the Bishop Tuff, uncompressed with pumice on left; compressed with fiamme on right. BishopTuff.jpg — Rocks from the Bishop Tuff, uncompressed with pumice on left; compressed with fiamme on right.

Flight through a μCT-image stack of a lapillus of the volcano Katla in Iceland. Find spot: Beach near Vik at the end of road 215. Acquisition done using "CT Alpha" by "Procon X-Ray GmbH", Garbsen, Germany. Resolution 11,2μm/Voxel, width approx. 24 mm.

3D-Rendering of the above image stack, in parts transparent. Heavy particles in red.

Pyroclastic rocks 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 deposit, which are deposits made predominantly of volcanic particles.^[1]^[2] '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 with groundwater.^[3] The word pyroclastic is derived from the Greek πῦρ, meaning fire; and κλαστός, meaning broken.

Unconsolidated accumulations of pyroclasts are described as tephra. Tephra may become lithified 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 and diagenesis) 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.^[4]

One of the most spectacular types of pyroclastic deposit is an ignimbrite, 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, lapilli, or volcanic blocks (or, if they exhibit evidence of having been hot and molten during emplacement, volcanic bombs). 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.^[5]

Clast size	Pyroclast	Mainly unconsolidated (tephra)	Mainly consolidated: pyroclastic rock
> 64 mm	block (angular) bomb (if fluidal-shaped)	blocks; agglomerate	pyroclastic breccia; agglomerate
< 64 mm	lapillus	lapilli	lapillistone (lapilli-tuff is where lapilli are supported within a matrix of tuff)
< 2 mm	coarse ash	coarse ash	coarse tuff
< 0.063 mm	fine ash	fine ash	fine tuff

Pyroclasts are transported in two main ways: in atmospheric eruption plumes, from which pyroclasts settle to form topography-draping pyroclastic fall layers, and by pyroclastic density currents (PDCs) (including pyroclastic flows and pyroclastic surges),^[6] 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 magma 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 that rises several kilometers into the stratosphere and cause aviation hazards.^[7] Particles fall from atmospheric eruption plumes and accumulate as layers on the ground, which are described as fallout deposits.^[8]

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).^[9] 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.^[10] Where the density current becomes sufficiently dilute to loft, it rises into the atmosphere as a 'phoenix plume'^[11] (or 'co-PDC plume').^[12] These phoenix plumes typically deposit thin ashfall layers that may contain little pellets of aggregated fine ash.^[13]

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 ^[14] or 'fire-fountain'.^[15] 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.^[14]^[15]

Related Research Articles

A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.

Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock containing 25% to 75% ash is described as tuffaceous. Tuff composed of sandy volcanic material can be referred to as volcanic sandstone.

<span class="mw-page-title-main">Stratovolcano</span> Type of conical volcano composed of layers of lava and tephra

A stratovolcano, also known as a composite volcano, is a conical volcano built up by many layers (strata) of hardened lava and tephra. Unlike shield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and periodic intervals of explosive eruptions and effusive eruptions, although some have collapsed summit craters called calderas. The lava flowing from stratovolcanoes typically cools and hardens before spreading far, due to high viscosity. The magma forming this lava is often felsic, having high to intermediate levels of silica, with lesser amounts of less viscous mafic magma. Extensive felsic lava flows are uncommon, but have traveled as far as 15 km (9 mi).

A pyroclastic flow is a fast-moving current of hot gas and volcanic matter that flows along the ground away from a volcano at average speeds of 100 km/h but is capable of reaching speeds up to 700 km/h. The gases and tephra can reach temperatures of about 1,000 °C (1,800 °F).

Volcanic cones are among the simplest volcanic landforms. They are built by ejecta from a volcanic vent, piling up around the vent in the shape of a cone with a central crater. Volcanic cones are of different types, depending upon the nature and size of the fragments ejected during the eruption. Types of volcanic cones include stratocones, spatter cones, tuff cones, and cinder cones.

Pyroclast, Pyroclastic or Pyroclastics may refer to:

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 it more turbulent and allows it to rise over ridges and hills rather than always travel downhill as pyroclastic flows do.

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 is Latin for "little stones".

<span class="mw-page-title-main">Ignimbrite</span> Type of volcanic rock

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 surrounding atmosphere. New Zealand geologist Patrick Marshall (1869–1950) coined the term ignimbrite from the Latin igni- [fire] and imbri- [rain].

Scoria is a pyroclastic, highly vesicular, dark-colored volcanic rock formed by ejection from a volcano as a molten blob and cooled in the air to form discrete grains called clasts. It is typically dark in color, and basaltic or andesitic in composition. Scoria has relatively low density, as it is riddled with macroscopic ellipsoidal vesicles, but in contrast to pumice, scoria always has a specific gravity greater than 1 and sinks in water.

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 above the vent of the volcano. In the most explosive eruptions, the eruption column may rise over 40 km (25 mi), penetrating the stratosphere. Stratospheric injection of aerosols by volcanoes is a major cause of short-term climate change.

<span class="mw-page-title-main">Agglomerate</span> Coarse accumulation of volcanic material

Agglomerate is a coarse accumulation of large blocks of volcanic material that contains at least 75% bombs. Volcanic bombs differ from volcanic blocks in that their shape records fluidal surfaces: they may, for example, have ropy, cauliform, scoriaceous, folded, spindle, spatter, ribbon, ragged, or amoeboid shapes. Globular masses of lava may have been shot from the crater at a time when partly molten lava was exposed, and was frequently shattered by sudden outbursts of steam. These bombs were viscous at the moment of ejection and by rotation in the air acquired their shape. They are commonly 1 to 2 feet in diameter, but specimens as large as 12 feet (3.7 m) have been observed. There is less variety in their composition at any one volcanic centre than in the case of the lithic blocks, and their composition indicates the type of magma being erupted.

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, volcanic bombs and lapilli from volcanic explosions
Deposition of material from convective clouds associated with pyroclastic flows such as coignimbrite falls
Ejecta carried in gas streaming from a vent. The material under the action of gravity will settle out from an eruption plume or eruption column
Ejecta settling from an eruptive plume or eruption column that is displaced laterally by wind currents and is dispersed over great distances

Several types of volcanic eruptions—during which lava, tephra, and assorted gases are expelled from a volcanic vent or fissure—have been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior has been observed. Some volcanoes may exhibit only one characteristic type of eruption during a period of activity, while others may display an entire sequence of types all in one eruptive series.

Phreatomagmatic eruptions are volcanic eruptions resulting from interaction between magma and water. They differ from exclusively magmatic eruptions and phreatic eruptions. Unlike phreatic eruptions, the products of phreatomagmatic eruptions contain juvenile (magmatic) clasts. It is common for a large explosive eruption to have magmatic and phreatomagmatic components.

A subaqueous volcano is a volcano formed beneath freshwater and which never builds above lake level. They are commonly in the form of gently sloping tuff cones, although they can sometimes have an unvolcano-like form, such as White Horse Bluff in the Wells Gray-Clearwater volcanic field of east-central British Columbia, Canada.

Pilot Knob is the eroded core of an extinct volcano located in Austin, Texas, United States. It is near Austin-Bergstrom International Airport and McKinney Falls State Park.

A Surtseyan eruption is an explosive style of volcanic eruption that takes place in shallow seas or lakes when rapidly rising and fragmenting hot magma interacts explosively with water and with water-steam-tephra slurries. The eruption style is named after an eruption off the southern coast of Iceland in 1963 that caused the emergence of a new volcanic island, Surtsey.

Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced 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 to all explosive eruption products, including particles larger than 2 mm. Volcanic ash is formed during explosive volcanic eruptions when dissolved gases in magma expand and escape violently into the atmosphere. The force of the gases shatters the magma and propels it into the atmosphere where it solidifies into fragments of volcanic rock and glass. Ash is also produced when magma comes into contact with water during phreatomagmatic eruptions, causing the water to explosively flash to steam leading to shattering of magma. Once in the air, ash is transported by wind up to thousands of kilometres away.

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 was deposited in, or whether nonvolcanic material is mingled with the volcanic clasts. The United States Geological Survey defines volcaniclastics somewhat more narrowly, to include only rock composed of volcanic rock fragments that have been transported some distance from their place of origin.

References

↑ Fisher, Richard V. (1961). "Proposed classification of volcaniclastic sediments and rocks". Geological Society of America Bulletin. 72 (9): 1409. Bibcode:1961GSAB...72.1409F. doi:10.1130/0016-7606(1961)72[1409:PCOVSA]2.0.CO;2.
↑ Fisher, Richard V.; Schmincke, H.-U. (1984). Pyroclastic rocks. Berlin: Springer-Verlag. ISBN 3540127569.
↑ Fisher 1961, p. 1409.
↑ Schmincke, Hans-Ulrich (2003). Volcanism. Berlin: Springer. p. 138. ISBN 9783540436508.
↑ Heiken, G. and Wohletz, K., 1985 Volcanic Ash, University of California Press;, pp. 246.
↑ Philpotts, Anthony R.; Ague, Jay J. (2009). Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. p. 73. ISBN 9780521880060.
↑ Schmincke 2003, pp. 155–176.
↑ Fisher & Schmincke 1984, p. 8.
↑ Breard, Eric C.P.; Lube, Gert (January 2017). "Inside pyroclastic density currents – uncovering the enigmatic flow structure and transport behaviour in large-scale experiments". Earth and Planetary Science Letters. 458: 22–36. Bibcode:2017E&PSL.458...22B. doi:10.1016/j.epsl.2016.10.016.
↑ Schmincke 2003, pp. 177–208.
↑ Sulpizio, Roberto; Dellino, Pierfrancesco (2008). "Chapter 2 Sedimentology, Depositional Mechanisms and Pulsating Behaviour of Pyroclastic Density Currents". Developments in Volcanology. 10: 57–96. doi:10.1016/S1871-644X(07)00002-2. ISBN 9780444531650.
↑ Engwell, S.; Eychenne, J. (2016). "Contribution of Fine Ash to the Atmosphere From Plumes Associated With Pyroclastic Density Currents" (PDF). Volcanic Ash: 67–85. doi:10.1016/B978-0-08-100405-0.00007-0. ISBN 9780081004050.
↑ Colombier, Mathieu; Mueller, Sebastian B.; Kueppers, Ulrich; Scheu, Bettina; Delmelle, Pierre; Cimarelli, Corrado; Cronin, Shane J.; Brown, Richard J.; Tost, Manuela; Dingwell, Donald B. (July 2019). "Diversity of soluble salt concentrations on volcanic ash aggregates from a variety of eruption types and deposits" (PDF). Bulletin of Volcanology. 81 (7): 39. Bibcode:2019BVol...81...39C. doi:10.1007/s00445-019-1302-0. S2CID 195240304.
1 2 Macdonald, Gordon A.; Abbott, Agatin T.; Peterson, Frank L. (1983). Volcanoes in the sea : the geology of Hawaii (2nd ed.). Honolulu: University of Hawaii Press. pp. 6, 9, 96–97. ISBN 0824808320.
1 2 Allaby, Michael, ed. (2013). "Fire-fountain". A dictionary of geology and earth sciences (Fourth ed.). Oxford University Press. ISBN 9780199653065.

Pyroclastic rock

Contents

Classification

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Related Research Articles

References

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