Igneous Rocks Chapter 3, section 2

Characteristics of Igneous Rock Most igneous rocks are made of mineral crystals Igneous rocks are classified according to their origin, texture, and mineral composition

Origin Geologists classify igneous rocks according to where they formed: Extrusive rock formed from lava that erupted onto Earth’s surface Intrusive rock is igneous rock that formed when magma cooled and hardened beneath the Earth’s surface

Intrusive Rock Intrusive rock forms below the surface of Earth Image from http://www.geo.vu.nl/~rondeel/Geology/3-minerals&rocks/volcanicforms.gif

Extrusive Rock Extrusive rock forms on top of the surface of Earth Image from http://photo2.si.edu/earthquakes/krafla500.jpg

Erosion exposes an Intrusive Rock formation Image fromhttp://www.geo.vu.nl/~rondeel/Geology/3-minerals&rocks/Intrusive_volcanic_neck.jpg

Texture The texture of an igneous rock depends on the size and shape of its mineral crystals Texture can be fine-grained, course-grained, glassy, or porphyritic *Rapid cooling lava forms fine-grained rocks with small crystals *Slow cooling magma forms coarse-grained rock with large crystals

Texture Extrusive rock cools quickly on Earth’s surface and the crystals grow for a short time and are small Intrusive rock cools slowly under Earth’s surface and the crystals grow for a long time and are large

Texture Glassy rock may form if extrusive rock cools very quickly Porphyritic texture is a mixed texture that has large crystals scattered on a background of much smaller crystals Porphyritic texture forms if an intrusive rock cools in two stages. Some magma cools slowly then the surrounding magma cools quickly

Texture Course-grained granite Fine-grained basalt Volcanic glass Porphyritic texture

Mineral Composition Silica is a material made of oxygen and silicon that is found in most minerals. Silica content affects the color of rocks Lava that is low in silica tends to form dark-colored basalt while magma that is high in silica forms light-colored granite

Basalt image from http://www.es.ucl.ac.uk/schools/Glossary/basalt.jpg Obsidian image from http://www.es.ucl.ac.uk/schools/Glossary/obsidian.jpg Granite image from http://www.beg.utexas.edu/mainweb/publications/graphics/granite-400.jpg Porphyritic texture image from http://www.ux1.eiu.edu/~cfjps/1300/p_rhyolite.jpg