NCCER Introduction to Earthmoving (22201) Practice Test

When molten material cools and solidifies, it becomes igneous rocks. This melt can crystallize slowly underground, forming coarse-grained rocks, or erupt at the surface and cool quickly to create fine-grained types. If cooling is very rapid, a glassy texture can form, as in obsidian, but the rock is still igneous in origin. This sets igneous rocks apart from sedimentary rocks, which form from weathering, erosion, and lithification of sediments, and metamorphic rocks, which arise from existing rocks altered by heat and pressure.