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.

• http://en.wikipedia.org/wiki/Diatreme
• https://commons.wikimedia.org/wiki/File:Moses_Rock_Dike.jpg
• Padovanic, E. R. and Tracy, R. J. 1981, A pyrope-spinel (alkremite) xenolith from Moses Rock Dike: first known North American occurrence: Am. Mineral., v. 66, p. 741-745.