Unit 3: Lesson 2: Igneous Rocks Assessment, p. 74

Compare and contrast the formation of intrusive and extrusive igneous rocks.

Both types of rocks form when molten material cools and solidifies. Intrusive igneous rocks form when magma cools and solidifies within Earth. Extrusive igneous rocks form when lava cools and hardens at the surface.

How to coarse-grained igneous rocks form?

Coarse-grained igneous rocks form when magma cools slowly within Earth.

How do fine-grained igneous rocks form?

Fine-grained igneous rocks form then lava cools quickly at Earth's surface.

Contrast basalt and granite in terms of how each forms, the texture of each rock, the color of each rock, and each rock's composition.

Granite forms as magma slowly cools below the surface. This slow rate of cooling produces large mineral grains. Most of these minerals are quartz and feldspar, thus granite is light-colored, with granitic composition. Basalt forms when lava cools quickly at the surface. This quick cooling rate results in very small mineral grains. The major minerals in basalt are dark-colored silicates that give basalt its dark color. A basalt has a basaltic composition.

How are igneous rocks classified according to composition?

Igneous rocks can be classified by composition based on the major minerals in the rocks. Light-colored rocks have granitic compositions. Dark colored rocks have basaltic compositions. Dark-colored rocks that contain only olivine and pyroxene are ultramafic rocks.

How do igneous rocks with glassy textures form?

Igneous rocks with glassy textures form when lava cools very quickly.

The extrusive igneous rock pumice contains many small holes. Hypothesize how these holes might form.

Lava is magma that reaches the surface. As it rises, reduced pressure on the magma causes come its gases to come out of solution. These gases form bubbles or holes as the molten material cools.