With extensions that mountains extend out towards under the ground as well as over the ground, they clench different plates of the earth together like a peg. The Earth's crust consists of plates that are in constant motion. This clenching property of mountains prevents shocks to a great extent, by fixing the earth's crust, which has a very movable structure.

The Qur'an draws attention to a very important geological function of mountains.

"We placed firmly embedded mountains on the earth, so it would not move under them…" (The Qur'an, 21:31)

As we have noticed, it is stated in the verse that mountains have the function of preventing shocks in the Earth.

This fact was not known by anyone at the time the Qur'an was revealed. It was in fact brought to light only recently as a result of the findings of modern geology.

According to these findings, mountains emerge as a result of the movements and collisions of massive plates forming the Earth's crust. When two plates collide, the stronger one slides under the other, the one on the top bends and forms heights and mountains. The layer beneath proceeds under the ground and makes a deep extension downward. That means that mountains have a portion stretching downwards, as large as their visible parts on the Earth.

In a scientific text, the structure of mountains is described as follows:

Where continents are thicker, as in mountain ranges, the crust sinks deeper into the mantle. (General Science, Carolyn Sheets, Robert Gardner, Samuel F. Howe; Allyn and Bacon Inc. Newton, Massachusetts, 1985, s. 305)

In a verse, this role of the mountains is pointed out by a comparison with "pegs":

"Have We not made the earth as a bed and the mountains its pegs?" (The Qur'an, 78:6-7)

Mountains, in other words, clench the plates in the Earth's crust together by extending above and beneath the Earth's surface at the conjunction points of these plates. In this way, they fix the Earth's crust, and prevent it from drifting over the magma stratum or among its plates. Briefly, we may liken mountains to nails that keep pieces of wood together.

This fixing function of the mountains is described in scientific literature by the term "isostasy". Isostasy means the following:

Isostasy: general equilibrium in the Earth's crust maintained by a yielding flow of rock material beneath the surface under gravitational stress. (Webster's New Twentieth Century Dictionary, 2. edition "Isostasy", New York, s. 975)

This vital role of mountains, that was discovered by modern geology and seismic research, was revealed in the Qur'an centuries ago as an example of the supreme wisdom in God's creation.

"We placed firmly embedded mountains on the earth, so it would not move under them..." (The Qur'an, 21:31)