How do volcanic necks form

Most maars have low rims composed of a mixture of loose fragments of volcanic rock and rocks torn from the walls of the diatreme. M aars occur in the western United States, in the Eifel region of Germany, and in other geologically young volcanic regions of the world. An excellent example of a maar is Zuni Salt Lake in New Mexico, a shallow saline lake that occupies a flat-floored crater about 6, feet across and feet deep. Its low rim is composed of loose pieces of basaltic lava and wallrocks sandstone, shale, limestone of the underlying diatreme, as well as random chunks of ancient crystalline rocks blasted upward from great depths.

S ome well-exposed, nearly circular areas of intensely deformed sedimentary rocks, in which a central vent-like feature is surrounded by a ring-shaped depression, resemble volcanic structures in gross form. As no clear evidence of volcanic origin could be found in or near these structures, scientists initially described them as "cryptovolcanic," a term now rarely used.

Recent studies have shown that not all craters are of volcanic origin. Impact craters, formed by collisions with the Earth of large meteorites, asteroids, or comets, share with volcanoes the imprints of violent origin, as evidenced by severe disruption, and even local melting, of rock.

Fragments of meteorites or chemically detectable traces of extraterrestrial materials and indications of strong forces acting from above, rather than from below, distinguish impact from volcanic features.

The type of erosion will define the shape of the volcanic neck. For example, glaciers will erode away one side of the volcanic neck, but leave a long tail behind.

We have written many articles about volcanoes for Universe Today. You can also find out more information about volcanic necks from the USGS. We have also recorded an episode of Astronomy Cast dealing with volcanoes on Earth and across the Solar System.

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