What is a makhtesh (pl. makhteshim)? Isn’t it a crater?
Are craters only found in the Negev?
Various types of depressions in the Earth’s surface are classified as craters.
Impact craters are the result of collision between two celestial bodies, volcanic activity can form a caldera (volcanic crater), and dolines or sinkholes form because of dissolution leading to collapse of upper layers.
Craters are characterized by a topographic depression surrounded by cliffs.
The erosional craters of the Negev, termed makhteshim, are different from the rest of the craters in that they are formed by prolonged weathering and erosion processes and that they are situated on geological anticlines.
The term ‘makhtesh’ is local and portrays landforms typical of the Negev and Sinai Peninsula.
There are five well-known makhteshim in Israel – Ramon, Yeruham, Hazera and the two Arif makhteshim – and some lesser known makhteshim, whose definition as a true makhtesh is equivocal (Timna, Amram, Eshet and others).
Throughout the world, there are other examples of erosional craters or cirques, but few are as complete, well preserved and accessible as those of the Negev.
How is a makhtesh formed?
Unique weathering patterns of an anticline, made of a succession of soft and hard layers, can form a makhtesh. In the past, the anticlines and synclines of the Negev were not topographical features – they were buried under other rock layers and the surface was planar.
During prolonged regional erosion, the upper parts of the anticlines were carved and truncated, exposing the softer, highly erodible, inner layers.
When the rocks overlaying the anticlines were eroded and the softer rocks within those anticlines were also removed, the hard layer formed slanted walls, abruptly terminating in cliffs that encircle an inner depression – and voila! A makhtesh is born.
The steep, high walls and circular cliff-line define the makhtesh as much as the deep, exposed layers at its center.
The shape of the makhtesh depends on the initial shape of the anticline. A domed and rounded anticline will give birth to a rounded makhtesh, for example Makhtesh Hazera. An elongated anticline will form an oval makhtesh, like Makhtesh Yeruham. The anticline that formed Makhtesh Ramon featured a small syncline between two anticline crests.
Erosion removed the higher layers, leaving the lower hard layers intact – thus forming the heartshaped makhtesh, and Har Ardon. In heavily cracked and faulted anticlines, the cliff walls are less continuous and the makhtesh will have many outlets, such as Makhtesh Ramon’s southern cliff. A steeper anticline will form a makhtesh that exposes older layers.
The Negev makhteshim exhibit soft and hard rock layers. The soft sandstone of the Kurnub Group underlays the hard limestone and dolomite the Judea Group. The soft rocks above the anticline are the marl, chalk and shales of the Har HaZofim Group. These can be found outside of the makhtesh (for example, Mount Oded, Mount Zin). The hard Judea Group erodes very slowly, ‘protecting’ the underlying soft sandstones that were removed by erosion. As the makhtesh emptied, Jurassic and Triassic rocks of the Arad and Ramon Groups were exposed, revealing a variety of phenomena spanning hundreds of millions of years.