What primarily characterizes igneous rock?

Igneous rock is primarily characterized by its formation from molten rock or magma. When magma rises to the Earth's surface and cools, it crystallizes to form solid rock. This process can occur either beneath the surface, where it is referred to as intrusive or plutonic igneous rock, or at the surface following a volcanic eruption, which results in extrusive or volcanic igneous rock. The cooling rate significantly affects the texture of the rock; for example, slow cooling results in larger crystals, while rapid cooling leads to smaller crystals.

The other options describe different geological processes. For instance, the formation from mineral deposits refers to sedimentary rock, which is created through the accumulation of sediments. The reference to forming from layers of sediment also pertains specifically to sedimentary rock, which differs fundamentally from the processes that create igneous rock. The alteration by heat and pressure describes metamorphic rock, which is formed from existing rocks that are transformed due to intense conditions but without melting. Thus, the defining characteristic of igneous rock remains its origin from molten rock or magma.