Volcaniclastics are geologic materials composed of broken fragments ( clasts) of

volcanic rock Volcanic rocks (often shortened to volcanics in scientific contexts) are rocks formed from lava erupted from a volcano. Like all rock types, the concept of volcanic rock is artificial, and in nature volcanic rocks grade into hypabyssal and me ...

. 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 The United States Geological Survey (USGS), founded as the Geological Survey, is an agency of the U.S. Department of the Interior whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on Mar ...

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. In the broad sense of the term, volcaniclastics includes pyroclastic rocks such as the Bandelier Tuff; cinder cones and other

tephra Tephra is fragmental material produced by a Volcano, 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, ...

deposits; the basal and capping

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

that characterize

ʻaʻā 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 unde ...

lava flows; and

lahar A lahar (, from ) is a violent type of mudflow or debris flow composed of a slurry of Pyroclastic rock, pyroclastic material, rocky debris and water. The material flows down from a volcano, typically along a valley, river valley. Lahars are o ...

s and

debris flow Debris flows are geological phenomena in which water-laden masses of soil and fragmented Rock (geology), rock flow down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. ...

s of volcanic origin.Vincent 2000, pp.27-28 Volcaniclastics make up more of the volume of many volcanoes than do lava flows. Volcaniclastics may have contributed as much as a third of all sedimentation in the geologic record.

Classification of volcaniclastics

Volcaniclastics are composed of a range of pyroclastic detritus mixed with epiclastic sediments and formed in variable depositional environments.Olavsd�ottir, J., M. S. Andersen, and L. O. Boldreel, 2015, Reservoir quality of intrabasalt volcaniclastic units onshore Faroe Islands, North Atlantic Igneous Province, northeast Atlantic: AAPG Bulletin, v. 99, no. 3, p. 467– 497, Volcaniclastics include

pyroclastic rock 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 deposit ...

and

; volcanic autoclastic, alloclastic, and epiclastic materials; and

fault gouge Fault gouge is a type of fault rock best defined by its grain size. It is found as incohesive fault rock (rock which can be broken into its component granules at the present outcrop, only aided with fingers/pen-knife), with less than 30% clasts ...

where faults displace volcanic rock. All are defined below. These can be divided into primary volcaniclastics and secondary volcaniclastics (epivolcaniclastics).Vincent 2000, pp.17-19

Pyroclastic

Pyroclastic material is composed of rock fragments produced by explosive volcanism and erupted from the vent as individual particles, without reference to the particle origin or the nature of the eruption. These may include particles of

country rock Country rock is a music genre that 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 sty ...

entrained within the vent.Vincent 2000, p.22 Accumulations of pyroclastic material that have not been consolidated are described as

, while those that have undergone significant consolidation are described as

. Hydroclastic material is a special case of pyroclastic material produced by a variety of processes at magma-water interfaces.

Autoclastic

Autoclastic volcanic material is produced by processes active during movement of solid or semisolid lava. These include rock fragments that are produced within volcanic vents but not extruded, rock fragments produced by motion or gas explosions within volcanic flows, or rock fragments produced by gravitational collapse of

lava domes In volcanology, a lava dome is a circular, mound-shaped protrusion resulting from the slow Extrusive rock, extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings. Around 6 ...

or spines. The characteristic basal and capping breccia of

lava flows are autoclastic volcaniclastics.

Other kinds of volcaniclastic material

Alloclastic volcanic material is formed by fragmentation of existing igneous rock by subsurface igneous activity that may or may not involve magma intrusions. Fault gouge produced by motion along a fault in volcanic rock is also a type of volcaniclastic material.

Epivolcaniclastics

Volcanic epiclastic material (epivolcaniclasticsVincent 2000, p.19) contains a substantial fraction of epiclasts (rock fragments produced by weathering and erosion) derived from volcanic rock.

Mixed pyroclastic-epiclastic rocks

Deposits containing pyroclastic material that has been reworked in stream or lake environments or mingled with epiclastic material (whether volcanic or nonvolcanic) pose a special difficulty and are among the materials most usefully described simply as volcaniclastic.Fisher and Schminke 1984, p.90 A more specific classification is problematic for these cases.Fisher and Schminke 1984, pp. 89-90 The Espinaso Formation of

New Mexico New Mexico is a state in the Southwestern United States, Southwestern region of the United States. It is one of the Mountain States of the southern Rocky Mountains, sharing the Four Corners region with Utah, Colorado, and Arizona. It also ...

is an example of a rock unit that is composed of a complex mixture of pyroclastic and volcanic epiclastic material and so is described simply as volcaniclastic. Another is the Washburn Group of the

Yellowstone Yellowstone National Park is a List of national parks of the United States, national park of the United States located in the northwest corner of Wyoming, with small portions extending into Montana and Idaho. It was established by the 42nd U ...

area, which includes debris flows of reworked volcanic ash and volcanic epiclastic rock. Mixed pyroclastic-epiclastic deposits may be classified by average clast size and percentage of pyroclastic material.

References

{{Commons category, Volcaniclastic rocks Petrology Volcanology Volcanic rocks Sedimentary rocks