It says...

It says, in effect: Although you imagine the mountains to be solid and without inward movement and growth, this is not the case. The mountains that seem stagnant and stationary to you are inwardly growing and changing, just like the clouds the motion of which is visible to you. The firm structure and development of all things are ensured by that same motion, the law of motion which is of God's creation and making.

It prevails over all the particles and phenomena of nature, and it ensures their order and stability. The choice of this particular wording in the Qur’an goes back, no doubt, to the fact that mountains are a symbol of bulkiness and stability, and it enables the verse to lay particular stress on the ability of the Creator to do all things.

Five, not more than three centuries have passed since Galileo presented to the world of science the theory of the motion of the earth, in a clear fashion and accompanied by adequate proof. In an age not too far removed from us, when geocentricity and the immobility of the earth were regarded as indubitable scientific principles, his theory met with a wave of furious opposition.

By contrast, in the dark atmosphere of the Age of Ignorance, the Qur’an had already referred to certain aspects of the earth's motion and the mysterious qualities of mountains. This was an exposition of complex scientific truths, taking place already in that age.

Thus the Qur’an says: "Have We not made the earth as a cradle and the mountains like pegs?"(78:6-7) "God has placed mountains on the earth to prevent its uneven motion."(31:10) The Qur’an compares the earth to a cradle because a cradle is a place of rest that is engaged in motion.

In another verse, a different comparison is offered: "I have created the earth for you like a tamed camel that with its gentle and smooth motion does not vex its rider."(67:15) The Qur’an referred to the motion of the earth at a time when the Ptolemaic theory of geocentricity and the immobility of the earth had been ruling for centuries over the minds of the learned. It was the heavenly book of Islam that refuted that fantastic view of the world, almost a thousand years before Galileo.

In one of the verses just quoted, the mountains have been subtly and delicately compared to pegs that hold the earth in place and prevent it from becoming scattered.