Will larger crystals will be formed when molten rock cools more slowly

The given statement is true.

Explanation:

The size of the mineral crystals of the igneous rocks relies upon the fact that how speedily the lava or the magma gets cooled. The magma that produces the intrusive igneous rocks remains underneath the Earth's surface. The bigger crystals are produced in intrusive rocks as the high temperatures within the Earth's surface permit magma to cool gradually, and this gradual cooling gives enough time for the formation of bigger mineral crystals. Thus, the more gradual cooling of the molten rocks takes place underneath the Earth's surface, the bigger will be the mineral crystals of the igneous rocks formed.