Volcanic pipes or volcanic conduits are subterranean geological structures formed by the violent,

supersonic Supersonic speed is the speed of an object that exceeds the speed of sound ( Mach 1). For objects traveling in dry air of a temperature of 20 °C (68 °F) at sea level, this speed is approximately . Speeds greater than five times ...

eruption of deep-origin

volcano 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. On Earth, volcanoes are most often found where tectonic plates ...

es. They are considered to be a type of ''

diatreme A diatreme, sometimes known as a maar-diatreme volcano, is a volcanic pipe formed by a gaseous explosion. When magma rises up through a crack in Earth's crust and makes contact with a shallow body of groundwater, rapid expansion of heated wate ...

''. Volcanic pipes are composed of a deep, narrow cone of solidified

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 natura ...

(described as "carrot-shaped"), and are usually largely composed of one of two characteristic rock types —

kimberlite Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known to be the main host matrix for diamonds. It is named after the town of Kimberley in South Africa, where the discovery of an diamond called the Star of S ...

lamproite Lamproite is an ultrapotassic mantle-derived volcanic or subvolcanic rock. It has low CaO, Al2O3, Na2O, high K2O/Al2O3, a relatively high MgO content and extreme enrichment in incompatible elements. Lamproites are geographically widespread ...

. These rocks reflect the composition of the volcanoes' deep magma sources, where the Earth is rich in

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ...

. Volcanic pipes are relatively rare. They are well known as the primary source of

diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, b ...

s, and are mined for this purpose.

Formation

Volcanic pipes form as the result of violent eruptions of deep-origin volcanoes. These volcanoes originate at least three times as deep as most other volcanoes, and the resulting magma that is pushed toward the surface is high in magnesium and volatile compounds such as

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...

and

carbon dioxide Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...

. As the body of magma rises toward the surface, the volatile compounds transform to

gaseous phase Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or co ...

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country a ...

is reduced with decreasing depth. This sudden expansion propels the magma upward at rapid speeds, resulting in a supersonic Plinian eruption. Crater-Hypipamee

Kimberlite pipes

pipes, the eruption ejects a column of overlying material directly over the magma column, and does not form a large above-ground elevation as typical volcanoes do; instead, a low ring of

ejecta Ejecta (from the Latin: "things thrown out", singular ejectum) are particles ejected from an area. In volcanology, in particular, the term refers to particles including pyroclastic materials (tephra) that came out of a volcanic explosion and magma ...

known as a ''

tuff ring 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 cont ...

'' forms around a bowl-shaped depression over the subterranean column of magma. Over time, the tuff ring may erode back into the bowl, leveling out the depression by filling it with washed-back ejecta. Kimberlite pipes are the source of most of the world's commercial diamond production, and also contain other precious

gemstone A gemstone (also called a fine gem, jewel, precious stone, or semiprecious stone) is a piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks (such as lapis lazuli, opal, ...

s and semi-precious stones, such as

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different ...

spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...

s, and

peridot Peridot ( /ˈpɛr.ɪˌdɒt, -ˌdoʊ/ ''PERR-ih-dot, -⁠doh''), sometimes called chrysolite, is a deep yellowish-green transparent variety of olivine. Peridot is one of the few gemstones that only occurs in one color. Peridot can be found in ...

Lamproite pipes

Lamproite pipes operate similarly to kimberlite pipes, except that the boiling water and volatile compounds contained in the magma act corrosively on the overlying rock, resulting in a broader cone of eviscerated rock (the ejection of this rock also forms a tuff ring, like kimberlite eruptions). This broad cone is then filled with volcanic ash and materials. Finally, the degassed magma is pushed upward, filling the cone. The result is a funnel shaped deposit of volcanic material (both solidified magma, and ejecta) which appears mostly flat from the surface.

References

* American Museum of Natural History
"The Nature of Diamonds"
Retrieved March 17, 2005
Archived version
* Tilling (1985)

United States Geological Survey: Special Interest Publication. Retrieved March 17, 2005. {{Authority control Diatremes

Formation

Kimberlite pipes

Lamproite pipes

See also

References