Ontario Grade 9 Geography Practice Exam

Igneous rocks are primarily formed through the cooling and solidification of magma or lava. This process occurs when magma, which is molten rock beneath the Earth's surface, rises and cools either underground, forming intrusions like granite, or on the surface after a volcanic eruption, resulting in rocks like basalt.

The speed at which the magma cools determines the texture of the igneous rock. If cooling occurs slowly beneath the surface, larger crystals develop, leading to a coarse-grained texture. Conversely, rapid cooling at the surface results in a fine-grained texture with smaller crystals.

This formation process is unique to igneous rocks, setting them apart from metamorphic rocks, which originate from the alteration of existing rocks due to heat and pressure, and sedimentary rocks, which form from the accumulation and compaction of sediments. Fossilized rocks, while containing fossils, are specifically a subset of sedimentary rocks that have preserved biological material but don’t directly involve the processes of magma cooling.