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

volcanic rock Volcanic rock (often shortened to volcanics in scientific contexts) 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 ...

. 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), formerly simply known as the Geological Survey, is a scientific agency of the United States government. The scientists of the USGS study the landscape of the United States, its natural resources, ...

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 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 pyrocl ...

such as the

Bandelier Tuff The Bandelier Tuff is a geologic formation exposed in and around the Jemez Mountains of northern New Mexico. It has a radiometric age of 1.85 to 1.25 million years, corresponding to the Pleistocene epoch. The tuff was erupted in a series of at ...

;

cinder cones A cinder cone (or scoria cone) is a steep conical hill of loose pyroclastic fragments, such as volcanic clinkers, volcanic ash, or scoria that has been built around a volcanic vent. The pyroclastic fragments are formed by explosive eruptions ...

and other

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 r ...

deposits; the basal and capping

breccia Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix. The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of ...

that characterize ʻaʻā lava flows; and

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

s and debris flows 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 include

pyroclastic rock 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 pyrocl ...

and

; volcanic autoclastic, alloclastic, and epiclastic materials; and fault gouge 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 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 ...

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 extrusion of viscous lava from a volcano. Dome-building eruptions are common, particularly in convergent plate boundary settings. Around 6% of erupti ...

or spines. The characteristic basal and capping breccia of ʻaʻā 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

Tuffaceous lithic sandstone - Flickr - aspidoscelis (2)

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 The Espinaso Formation is a geologic formation in New Mexico. It has a radiometric age of 34.6 to 26.9 million years, corresponding to the late Eocene through Oligocene epochs. Description The Espinaso Formation is principally debris flows a ...

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe, New Mexico, Santa Fe , LargestCity = Albuquerque, New Mexico, Albuquerque , LargestMetro = Albuquerque metropolitan area, Tiguex , Offi ...

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 Washburn (alternatively Wasseburne, Wasseborne, Wasshebourne, Wassheborne, Washbourne, Washburne, Washborne, Washborn, Wasborn, Washbon) is a toponymic surname, probably of Old English origin, with likely Anglo-Norman and Norman-French influenc ...

of the

Yellowstone Yellowstone National Park is an American national park located in the western United States, largely in the northwest corner of Wyoming and extending into Montana and Idaho. It was established by the 42nd U.S. Congress with the Yellow ...

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