In some areas of the Earth, the crust is squeezed and pushed upward. This is a Compression process.
The Compression is the term that basically introduces to a set of stress that is directed towards the center of a mass of the rock. This can also result in the folding of the rocks. This occurs just because of magnitudes of lithostatic stress that is large in the tectonic plates. The tectonic-scale deformation is always ruled to the net compressive stress. This is the highest compressive stress that can be implemented to a material before the failure happens.
In that, it directly forms the mountains.
The mountains are erupted and formed when the movements of tectonic plates occurs which create the volcanoes along the boundaries of the plates. The plates collide together which results in triggering the deformation and the thickening of the crust of Earth which leads to crustal uplift forming the mountains. The compression process leads to the deformation, and then the folding and after that, the faulting of the layers along the convergent boundaries of the plates.