Question

1. describe the three major compositional groups of igneous rocks. 3. apply what you know about cooling rates to explain differences in crystal sizes.

Explanation:

For question 2, igneous rocks are grouped by silica - content into felsic, intermediate, and mafic. Felsic rocks have high silica (65 - 100%), are light - colored, and rich in potassium and sodium feldspar and quartz. Intermediate rocks have 52 - 65% silica, and contain plagioclase feldspar and amphibole. Mafic rocks have 45 - 52% silica, are dark - colored, and rich in pyroxene and plagioclase feldspar.
For question 3, slow cooling rates allow atoms to move and arrange into large crystals as they have more time to organize. Fast cooling rates prevent atoms from arranging into large structures, resulting in small crystals or even glassy textures as the atoms are "frozen" in place quickly.

Answer:

1. Felsic rocks have high silica (65 - 100%), are light - colored, and rich in potassium and sodium feldspar and quartz. Intermediate rocks have 52 - 65% silica, and contain plagioclase feldspar and amphibole. Mafic rocks have 45 - 52% silica, are dark - colored, and rich in pyroxene and plagioclase feldspar.
2. Slow cooling rates lead to large crystals as atoms have more time to arrange. Fast cooling rates result in small crystals or glassy textures as atoms are "frozen" quickly.