IGNEOUS ROCKS

I.Igneous rocks are formed from molten material. - The term igneous is derived from Latin term meaning “from FIRE”!!

A. Extrusive igneous rocks are formed when molten rock cools above ground. Usually they are formed after the material has been erupted by a volcano. 1.This molten material cools quickly. 2.No crystals are visible to the eye. ENVIRONMENT

B.Intrusive igneous rocks are formed when molten rock cools below ground. 1.These rocks cool very slowly. 2.There is lots of time for visible crystals to grow

Cools quickly Cools slowly Cools above ground Cools below ground No visible crystals Visible crystals EXTRUSIVEINTRUSIVE

TEXTURE Coarse : –INTRUSIVE igneous rock composed of large mineral grains. GRANITE

TEXTURE Fine : –EXTRUSIVE igneous rock composed of fine-grained minerals. BASALT

TEXTURE Glassy : –HIGHLY VISCOUS, silica-rich magma cools rapidly, no crystals. OBSIDIAN

TEXTURE Vesicular : –HIGHLY VISCOUS, large amount of dissolved gases, rock full of holes. PUMICE

COMPOSITION Mafic : –Low silica –Rich in Iron and Magnesium DARK COLOR. BASALT

COMPOSITION Felsic : –High silica LIGHT COLOR. RHYOLITE

Igneous Rock Structures Intrusions vs. Extrusions : - Form Underground - Form on the surface of the Earth

INTRUSIONS Batholiths : –At least 100 km 2 of surface exposure. Means “Deep Rock” Sierra Nevada Mountains

Exposed Batholiths

INTRUSIONS Laccoliths : –Flat-bottom intrusion that pushes over- lying rock layers into an arc. Means “Lake of Rock” Black Hills –South Dakota

INTRUSIONS Stocks : –Like a Batholith, but covers less than 100 km 2. Devils Tower –Wyoming

INTRUSIONS Dikes : –Igneous intrusion that cuts across rock layers. Black Canyon –Colorado

INTRUSIONS Sills : –Hardened magma formed between parallel layers of rock. Salisbary Crags –Scotland

EXTRUSIONS Volcanic Necks : –Solidified central neck of a volcano. Shiprock –New Mexico