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 the Earth's crust and makes contact with a shallow body of ground water, rapid expansion of heated water vapor and volcanic gases can cause a series of explosions. A relatively shallow crater is left (known as a "maar") and a rock-filled fracture (the actual diatreme) in the Earth's crust. Diatremes breach the Earth's surface and produce a steep inverted cone shape.
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. The word comes from Greek δία ('through') and τρῆμα ('hole, aperture').
Global distribution.
Maar-diatreme volcanoes are not uncommon, reported as the second most common type of volcanoes on continents and islands.
Igneous intrusions cause the formation of a diatreme only in the specific setting where groundwater exists. This means most igneous intrusions do not produce diatremes.
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. For instance, diatreme formation is sometimes associated with kimberlite magma, which originates in the upper mantle: when a diatreme is formed due to a kimberlite intrusion, there is a possibility that diamonds may be brought up, because diamonds are formed in the upper mantle at depths of 150-200 kilometers. Kimberlite magmas can sometimes include chunks of diamond as xenoliths, making them economically significant.