Understand science concepts and principles. Construct diagrams to describe and summarize data. Igneous rocks form from molten rock called magma. The rate at which cooling takes place determines the crystal size of the mineral. Intrusive rocks form from lava that cools beneath the earth's surface very slowly and develops relatively large crystals.
Extrusive rocks form from magma that reaches the Earth's surface and then cools rapidly.
The rapidly cooling magma cools so quickly that crystals do not have time to form and therefore are either not present or relatively small. Students need to be able to use a microscope and must be able to operate a stopwatch. See safety procedures for using matches. Place one drop of iodine on each microscope slide. Using a stopwatch or a classroom clock, record how long it takes for crystals to form on one microscope slide at room temperature.