Igneous Igneous RocksRocksChapter 3, Chapter 3, section 2section 2

Characteristics of Igneous Characteristics of Igneous RockRock

Most igneous rocks are made of Most igneous rocks are made of mineral crystalsmineral crystals

Igneous rocks are classified Igneous rocks are classified according to their origin, according to their origin, texture, and mineral texture, and mineral compositioncomposition

OriginOrigin Geologists classify igneous rocks Geologists classify igneous rocks

according to where they formed: according to where they formed: Extrusive rockExtrusive rock formed from lava formed from lava

that erupted onto Earth’s surfacethat erupted onto Earth’s surface

Intrusive rockIntrusive rock is igneous rock is igneous rock that formed when magma cooled and that formed when magma cooled and hardened beneath the Earth’s surfacehardened beneath the Earth’s surface

Intrusive RockIntrusive Rock

Image from http://www.geo.vu.nl/~rondeel/Geology/3-minerals&rocks/volcanicforms.gif

Intrusive rock forms below the surface of Earth

Extrusive RockExtrusive RockExtrusive rock forms on top of the surface of Earth

Image from http://photo2.si.edu/earthquakes/krafla500.jpg

Erosion exposes an Erosion exposes an IntrusiveIntrusive Rock formationRock formation

Image fromhttp://www.geo.vu.nl/~rondeel/Geology/3-minerals&rocks/Intrusive_volcanic_neck.jpg

TextureTexture The texture of an igneous rock The texture of an igneous rock

depends on the size and shape of its depends on the size and shape of its mineral crystalsmineral crystals

Texture can be fine-grained, Texture can be fine-grained, course-grained, glassy, or porphyriticcourse-grained, glassy, or porphyritic

*Rapid cooling lava forms *Rapid cooling lava forms fine-grained rocks with small crystalsfine-grained rocks with small crystals

*Slow cooling magma forms *Slow cooling magma forms coarse-grained rock with large coarse-grained rock with large crystalscrystals

TextureTexture Extrusive rock cools quickly on Extrusive rock cools quickly on

Earth’s surface and the crystals grow Earth’s surface and the crystals grow for a short time and are smallfor a short time and are small

Intrusive rock cools slowly under Intrusive rock cools slowly under Earth’s surface and the crystals grow Earth’s surface and the crystals grow for a long time and are largefor a long time and are large

TextureTexture Glassy rock may form if extrusive rock Glassy rock may form if extrusive rock

cools very quicklycools very quickly PorphyriticPorphyritic texturetexture is a mixed texture is a mixed texture

that has large crystals scattered on a that has large crystals scattered on a background of much smaller crystalsbackground of much smaller crystals

Porphyritic texture forms if an intrusive Porphyritic texture forms if an intrusive rock cools in two stages. Some magma rock cools in two stages. Some magma cools slowly then the surrounding magma cools slowly then the surrounding magma cools quicklycools quickly

TextureTextureFine-grained basalt

Course-grained granite

Porphyritic texture

Volcanic glass

Mineral CompositionMineral Composition

SilicaSilica is a material made of oxygen is a material made of oxygen and silicon that is found in most and silicon that is found in most minerals. Silica content affects the minerals. Silica content affects the color of rockscolor of rocks

Lava that is low in silica tends to Lava that is low in silica tends to form dark-colored basalt while form dark-colored basalt while magma that is high in silica forms magma that is high in silica forms light-colored granitelight-colored granite

Porphyritic texture image from http://www.ux1.eiu.edu/~cfjps/1300/p_rhyolite.jpg

Granite image from http://www.beg.utexas.edu/mainweb/publications/graphics/granite-400.jpg

Obsidian image from http://www.es.ucl.ac.uk/schools/Glossary/obsidian.jpg

Basalt image from http://www.es.ucl.ac.uk/schools/Glossary/basalt.jpg