Chapter 3, Section 2 Igneous Rocks Wednesday, December 2, 2009 Pages 78 -- 81

Objective Identify the characteristics used to classify igneous rocks.

Vocabulary Words Extrusive rock Intrusive rock Porphyritic rock

Characteristics Igneous rocks are any rocks that form from magma or lava. Comes from the latin word “fire”. Most igneous rocks made of mineral crystals. The are classified according to their origin, texture, and mineral composition.

Extrusive Rocks Formed from lava that is cooled above the surface. Most common is Basalt. Basalt forms much of the crust including the crust beneath the ocean floor.

Intrusive Rocks Formed when magma hardened beneath the Earth’s surface. Granite is the most abundant. Part of the Earth’s crust that makes up the continents. Granite forms the core of many mountain ranges.

Texture Depends on the size and shape of its mineral crystals. May be fine-grained, coarse-grained, glassy or porphyritic. Rapid cooling forms fine-grained rocks. Slow cooling forms coarse-grained rocks.

Porphyritic Texture A rock with large crystals scattered on a background of much smaller crystals. Formed when intrusive rocks are cooled in 2 stages. Change in cooling rate due to movement of the magma.

Mineral Composition Most of the Earth’s minerals contain silica. Material formed from oxygen and silicon. Amount of silica affects the type of rock they form. Low in silica: Dark-colored rocks (Basalt) High in silica: Light-colored rocks (Granite) Basalt Granite

Uses of Igneous Rocks Many igneous rocks are hard, dense, and durable. Used for tools and building materials

Homework Workbook 3.2 (due 12/4) Vocabulary quiz 3.2 (on 12/9)