Chapter 6 Rocks 6.2 Igneous Rocks

Igneous Rock Formation Form from the cooling of molten rock Extrusive (volcanic)- form at or near the surface from lava Intrusive (plutonic) – form under the surface from magma

Igneous Rock Formation Igneous rocks are classified by mineral composition (mafic or felsic) and texture Texture is the size, shape and arrangement of grains (crystals)

Texture Grain or crystal size depends on: Rate of cooling Fast = small Slow = large Dissolved gasses More = larger Order of solidification

The Starting Material Magma is Classified as Felsic, Mafic or and intermediate Felsic Light colored High silica magma with abundance of quartz and feldspar. Thick slow flowing in molten state. Most intrusive rocks. Acidic

The Starting Material Mafic Dark colored, Low silica ferromagnesian magma More fluid than felsic magma. Most extrusive igneous rocks.

Underground Magma Magma can cool slow or fast depending on where cooling occurs Intrusive rocks cool very slowly underground Result of slow cooling Coarse (large) grains

INTRUSIVE – large grains GRANITE GABBRO

At the Surface Magma pouring onto the surface is called lava Extrusive igneous rocks cool rapidly Microscopic grains or none at all

EXTRUSIVE – fine grains OBSIDIAN BASALT

At the Surface Porphyry Rocks that cool both underground and at the surface Results in large and small grains

PORPHYRY – large grains embedded in fine grains RHYOLITE PORPHYRY

Igneous families Grouped according to mineral composition Texture is determined where it cools – intrusive or extrusive Each family can have coarse grained, fine grained or glassy textures Also can have vesicles Vesicles are gas cavities within the rock

Igneous Families Granite family Gabbro Family Felsic magma Light colored Quartz and Feldspar Coarse Granite Fine Rhyolite Glassy Pumice and Obsidian Gabbro Family Mafic magma Dark colored Pyroxene and Olivine Coarse Gabbro Fine Basalt Vesicular Scoria

Igneous Families Diorite family Intermediate Coarse Fine Diorite Andesite

IGNEOUS INTRUSIONS PLUTONS – A ROCK MASS THAT FORMS WHEN MAGMA COOLS UNDER GROUND DIKES SILLS LACOLITHS VOLCANIC NECKS BATHOLITHS – LARGEST OF ALL PLUTONS, FORM THE CORES OF MOST MOUNTAINS

Laccolith Volcanic neck Stock Dike Sill Batholith

DIKE

SILL

LACOLITH

VOLCANIC NECK

BATHOLITH