Lesson 2.3: Igneous Rocks Study Guide Extrusive Rock Vocabulary Igneous rock that forms from lava on Earth’s surface Intrusive Rock Igneous rock that forms from magma beneath Earth’s surface Key Idea: Igneous rocks are classified by their origin, texture, and mineral composition. Origin Extrusive- formed from lava on Earth’s surface o Basalt o rhyolite Intrusive- formed from magma beneath Earth’s surface o Granite o Porphyry o Pegmatite Texture Rocks can have same mineral composition, but different texture. Texture depends on size and shape of its mineral crystals. Intrusive rocks have larger grains (slow cooling) Extrusive rocks have smaller grains (rapid cooling) Mineral Composition Lava low in silica forms dark-colored rocks (basalt). Magma high in silica forms light-colored rocks (granite) Key Idea: Throughout history, igneous rocks have been used for tools and building materials. Igneous rocks are hard, dense, and durable. Tools Rocks with rough textures, like pumice, are used for cleaning and polishing. Native Americans used obsidian to make sharp tools. Perlite is mixed with soil and used for starting vegetable seeds. Building Materials Ancient Egyptians used granite to build statues. Incas of Peru build fortresses from granite and other igneous rock. In the early 1900s, the U.S. used granite to build bridges and public buildings. Thin sheets of granite are used for curbstones and flooring. Basalt can be used for cobblestones or as landscaping material.