Fissure

A fissure in geology is a crack in the Earth's crust. It is the first sign of the Earth's crust pulling apart. Characteristically, it is long and narrow. As the vent widens it may be called a fissure, and if there is volcanic activity, then it is a fissure vent. A very good example is the rift valley in North-Eastern Africa.

Karst is vast areas of barren land with rocky ground. It is formed by dissolving soluble carbonate rocks such as limestone, dolomite, and gypsum. There may be caves, fissures, and sinkholes. Despite rainfall, they lack vegetation, and usually have no lakes, rivers, or streams on their surface. They form when limestone, gypsum, or dolomite are dissolved by underground torrents of flowing water.

We now know that the huge karst area in Mexico known as the Yucatan was formed by the meteorite which ended the Cretaceous period. The whole of this huge area is made of porous limestone. The limestone is regularly punctuated with holes or "cenotes". The land is almost entirely covered with scrubby trees, all of a similar size.

A more familiar type of ground fissure is seen in Iceland, where the fissure vent is an active volcano. In such places the fissure may pour out molten rock. Mauna Loa is another good example. In this case it is one of a number of volcanic islands formed by a hot-spot gradually moving across the Earth's mantle.