Alberta Grade 9 Science PAT Practice Test

Rocks are classified by how they form. When molten rock from inside Earth cools and solidifies, its minerals crystallize and the rock becomes igneous. The cooling rate shapes the texture: slow cooling underground yields large crystals, as in granite; rapid cooling at the surface produces small crystals, as in basalt, or can create a glassy texture like obsidian. Sedimentary rocks form from deposited and cemented fragments, organic rocks come from accumulated plant or animal remains, and metamorphic rocks arise when existing rocks are reshaped by heat and pressure without melting. So the rock formed primarily by cooling and crystallization of molten rock is igneous.