Also called “tuff cones,” _maars_ are shallow, flat-floored craters that
scientists interpret have formed above diatremes as a result of a
violent expansion of magmatic gas or steam; deep erosion of a maar
presumably would expose a diatreme. Maars range in size from 200 to
6,500 feet across and from 30 to 650 feet deep, and most are commonly
filled with water to form natural lakes. Most maars have low rims
composed of a mixture of loose fragments of volcanic rock and rocks torn
from the walls of the diatreme.
Maars 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 6,500 feet
across and 400 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.
[Illustration: Zuni Salt Lake Maar, Catron County, New Mexico.]