A diatreme, sometimes known as a maar-diatreme volcano, is a volcanic pipe associated with a gaseous explosion. When

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

rises up through a crack in

Earth's crust Earth's crust is its thick outer shell of rock, referring to less than one percent of the planet's radius and volume. It is the top component of the lithosphere, a solidified division of Earth's layers that includes the crust and the upper ...

and makes contact with a shallow body of

groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available fresh water in the world is groundwater. A unit ...

, rapid expansion of heated

water vapor Water vapor, water vapour, or aqueous vapor is the gaseous phase of Properties of water, water. It is one Phase (matter), state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from th ...

and

volcanic gas Volcanic gases are gases given off by active (or, at times, by dormant) volcanoes. These include gases trapped in cavities (Vesicular texture, vesicles) in volcanic rocks, dissolved or dissociated gases in magma and lava, or gases emanating from ...

es can cause a series of explosions. A relatively shallow crater (known as a '' maar'') is left, and a rock-filled fracture (the actual diatreme) in the crust. Where diatremes breach the surface they produce a steep, inverted cone shape.

Etymology and geology

The word comes . The term diatreme has been applied more generally to any concave body of broken rock formed by explosive or hydrostatic forces, whether or not it is related to volcanism. Even within volcanology, the term has been used more generally by some than others, and in

kimberlite Kimberlite is an igneous rock and a rare variant of peridotite. It is most commonly known as the main host matrix for diamonds. It is named after the town of Kimberley, Northern Cape, Kimberley in South Africa, where the discovery of an 83.5-Car ...

terminology continues to be contentious. A current geological understanding is that diatreme describes the overall structure cut into the substrate (some have used the term “pipe” for this hence the common term volcanic pipe). In a simple diatreme, the structure narrows fairly regularly with depth and eventually terminates in the dike (dyke), or part of a dike, that fed the eruption. The transition from diatreme to dike occurs in a “root zone” that is the lowest part of the diatreme structure, immediately above the dike, which comprises coherent igneous rock. ''Maar-diatreme'' volcanoes are volcanoes produced by explosive eruptions that cut deeply into the country rock, with the maar being "''the crater cut into the ground and surrounded by an ejecta ring''".

Global distribution

Maar-diatreme volcanoes are not uncommon, reported as the second most common type of volcano on continents and islands. At the surface they may be hard to recognise if shallow and dry or eroded and can be up to wide, but are often much smaller. Igneous extrusions cause the formation of a diatreme only in the specific setting where groundwater exists; thus most igneous intrusions do not produce diatremes as they do not reach the surface so as to become extrusions, and further do not also intercept significant amount of groundwater when they become extrusions. Examples of diatremes include the Blackfoot diatreme and Cross diatreme in British Columbia, Canada.

Economic importance

Diatremes are sometimes associated with deposition of economically significant mineral deposits such as

magma, which originates in the

upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (geology), crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle (Earth), lower man ...

. When a diatreme is formed due to a kimberlite intrusion, there is a possibility that diamonds may be brought up, as diamonds are formed in the upper mantle at depths of 150-200 kilometers. Kimberlite magmas can sometimes include chunks of diamond as

xenolith A xenolith ("foreign rock") is a rock (geology), rock fragment (Country rock (geology), country rock) that becomes enveloped in a larger rock during the latter's development and solidification. In geology, the term ''xenolith'' is almost exclusi ...

s, making them economically significant.

References

External links

{{commons category, Diatremes Volcanism

Etymology and geology

Global distribution

Economic importance

References

Further reading

External links