Pyroclastic rock

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USGS scientist examines pumice blocks at the edge of a pyroclastic flow from Mount St. Helens Pyroclastic Flow St. Helens.jpg
USGS scientist examines pumice blocks at the edge of a pyroclastic flow from Mount St. Helens
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 or pyroclastics (derived from the Greek: πῦρ, meaning fire; and κλαστός, meaning broken) are sedimentary 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 unsieved 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. Tephra is any sized material formed by a volcanic eruption.

Contents

Pyroclastic rocks may be a 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.

Classification

Clast sizePyroclastMainly unconsolidated: TephraMainly consolidated: Pyroclastic rock
> 64 mmBlock, bombAgglomerateAgglomerate, pyroclastic breccia
< 64 mmLapillusLayer, lapilli tephraLapilli tuff, lapillistone
< 2 mmCoarse ashCoarse ashCoarse (ash) tuff
< 0.063 mmFine ashFine ashFine (ash) tuff

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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 Kīlauea 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.

See also

Related Research Articles

Tuff Rock consolidated from volcanic ash

Tuff, also known as volcanic 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 compacted into a solid rock in a process called consolidation. Tuff is sometimes erroneously called "tufa", particularly when used as construction material, but geologically tufa is a limestone precipitated from groundwater. Rock that contains greater than 50% tuff is considered tuffaceous.

Volcanic cone Landform of ejecta from a volcanic vent piled up in a conical shape

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.

Volcanic rock Volcanic rocks composing or associated with volcanoes, volcanic activity or volcanism

Volcanic rock is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and metamorphic rocks and constitute an important element of some sediments and sedimentary rocks. For these reasons, in geology, volcanics and shallow hypabyssal rocks are not always treated as distinct. In the context of Precambrian shield geology, the term "volcanic" is often applied to what are strictly metavolcanic rocks. Volcanic rocks and sediment that form from magma erupted into the air are called "volcaniclastics," and these are technically sedimentary rocks.

Lapilli Small pyroclast debris thrown in the air by a volcanic eruption

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".

Ignimbrite A variety of hardened tuff

Ignimbrite is a variety of hardened tuff. Ignimbrites are igneous rocks made up of crystal and rock fragments in a glass-shard groundmass, albeit the original texture of the groundmass might be obliterated due to high degrees of welding. The term ignimbrite is not recommended by the IUGS Subcommission on the Systematics of Igneous Rocks.

Agglomerate Coarse accumulation of large blocks of volcanic material that contains at least 75% bombs

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, or folded, chilled margins and 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.

Explosive eruption Type of volcanic eruption in which lava is violently expelled

In volcanology, an explosive eruption is a volcanic eruption of the most violent type. A notable example is the 1980 eruption of Mount St. Helens. Such eruptions result when sufficient gas has dissolved under pressure within a viscous magma such that expelled lava violently froths into volcanic ash when pressure is suddenly lowered at the vent. Sometimes a lava plug will block the conduit to the summit, and when this occurs, eruptions are more violent. Explosive eruptions can send rocks, dust, gas and pyroclastic material up to 20 km (12 mi) into the atmosphere at a rate of up to 100,000 tonnes per second, traveling at several hundred meters per second. This cloud may then collapse, creating a fast-moving pyroclastic flow of hot volcanic matter.

Fiamme Small lens-shaped inclusions in volcaniclastic rocks

Fiamme are lens-shapes, usually millimetres to centimetres in size, seen on surfaces of some volcaniclastic rocks. They can occur in welded pyroclastic fall deposits and in ignimbrites, which are the deposits of pumiceous pyroclastic density currents. The name fiamme comes from the Italian word for flames, describing their shape. The term is descriptive and non-genetic.

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:

  1. Ballistic transport of ejecta such as volcanic blocks, volcanic bombs and lapilli from volcanic explosions
  2. Deposition of material from convective clouds associated with pyroclastic flows such as coignimbrite falls
  3. 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
  4. Ejecta settling from an eruptive plume or eruption column that is displaced laterally by wind currents and is dispersed over great distances
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Mount Pleasant Caldera mountain in Canada

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Phreatomagmatic eruption Volcanic eruption involving both steam and magma

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Pahvant Butte butte in Millard County, Utah, United States

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Bridge River Vent mountain in Canada

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The Semilir eruption was a major volcanic event which took place in Indonesia during the Early Miocene. This eruption formed Semilir Formation and Nglanngran Formation. These two geological formations are in Southern Mountains of East Java. The eruption created two formations consists mostly pyroclastic rock. It has incredibly thickness. In contrast, Toba eruption only formed 600 m (2,000 ft) ignimbrite. Semilir eruption age date by isotope method is 21 million years ago.

Cerro Panizos mountain in Bolivia

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References

  1. Heiken, G. and Wohletz, K., 1985 Volcanic Ash, University of California Press;, pp. 246.