Lesson 5 The Cycling of Earth’s Materials – Max learned the difference between rocks and minerals. Then he learned that crystallization occurs when particles dissolve in a liquid or when they melt, solidify, and form crystals. Next he learned the difference between extrusive and intrusive rock. When lava cools and crystallizes on Earth’s surface the igneous rock that forms is called extrusive. Igneous rock that forms as magma cools underground and is called intrusive rock. Next he learned how crystals are formed in rock. If rocks cool slowly underground large crystals form but if they cool quickly small crystals form. If lava cools too quickly to form crystals it is called volcanic glass.