UPCAT Earth Science Practice Test

The size of solidified crystals in igneous rocks is primarily determined by the rate of cooling of the magma. When magma cools slowly, typically beneath the Earth's surface, it allows for larger crystals to form as the minerals have more time to arrange themselves into a structured lattice. This gradual cooling process leads to the development of well-formed, larger crystals, as seen in intrusive igneous rocks like granite.

In contrast, when magma cools rapidly, such as when it erupts onto the surface as lava, the crystals do not have sufficient time to grow and are often much smaller. This results in fine-grained textures characteristic of extrusive igneous rocks like basalt.

While other factors, such as mineral composition, the presence of water, and pressure during formation, can influence the overall characteristics of the rock, they do not have as direct an impact on crystal size as the cooling rate does. Thus, the cooling rate of magma is the critical factor in determining the size of the solidified crystals in igneous rocks.