The Ants - 27:61
Who made the earth a dwelling place, and caused rivers to flow between them, and placed firm mountains on it, and placed a barrier between the two waters? Another god besides Allah? Verily, most of them do not know.
Why were firm mountains placed on the earth?
Luqman/31:10
He created the heavens without a pillar that you see. And He placed stabilizers on the earth that you may not be shaken, and He spread therein every kind of creature. And We rained down water from the sky, and We caused to grow therein every beautiful variety.
The Prophets/21:31
We placed mountains on the earth so that it might not shake them, and We made in it wide passages for them to find the way.