Igneous rock is formed from the cooling of a magma.

The interior of the Earth is hot. Geothermal Gradient: the heat within the Earth steadily increases with depth; the heat is from radioactive decay and remnant heat of the Earth's formation.

Magma is molten rock material.  As magma cools the elements within the magma combine and crystalize into minerals that form an igneous rock.

Magma cools either below the surface or at the surface (magma that reaches the surface is called lava). 

As magma cools igneous rock is formed.  There are two types of igneous rock:

1) Extrusive: magma reaches the surface of the Earth before cooling and the lava cools rapidly.

2) Intrusive: magma cools under the Earth's surface.  The magma cools very slowly.

As magma cools minerals are formed into an interlocking arrangement producing an igneous rock.

As  magma cools it undergoes reactions that form minerals.  The rate of cooling is very important.  If the magma cools slowly then the reactions proceed for a longer period of time and the resulting mineral crystals (grains) become large.  If the magma cools rapidly then the reactions are much quicker and the mineral crystals (grains) are smaller.

Igneous rocks will have a distinctive appearance (this is referred to as texture) based on the rate of cooling (fast or slow).

Textural terms for igneous rocks:

1. Phaneritic (coarse grained): rocks are composed of mineral grains large enough to be seen with the unaided eye.  Indicitive of SLOW cooling, usually deep within the Earth.

2. Aphanitic (fine grained): rocks are composed of mineral grains too small to be seen by the unaided eye.  Indicitive of faster cooling, usually at or near the Earth's surface.

3. Glassy: magma cooled so quickly that crystal formation could not occur. Indicative of almost instant cooling at the Earth's surface.

4. Porphorytic: large mineral cyrstals surrounded by a fine grained matrix (background).  This texture forms when slow cooling begins to form large mineral crystals.  Before the magma is completely cooled the magma (and its large mineral grains) moves closer to the surface and finishes cooling at a more rapid pace.

Igneous rocks have a range of chemical composition.

1. Sialic (granitic) or felsic: rocks have highest silica, contain feldspar and ferromagnesium minerals. Rocks are light in color.

2. Intermediate (andesitic): rocks have less silica, contain feldspar and ferromagnesium minerals.  Rocks are medium gray in color.

3. Mafic (basaltic): rocks have higher amounts of ferromagnesium minerals and less silica and feldspar.  Rocks are dark in color, black or dark gray.

4. Ultramafic: highest amount of ferromagnesium minerals and least amount of silica.  Rocks are usually a dark green to green-black.

Classification of Igneous Rocks
 

Igneous rocks are classified by both texture and composition.  You will learn to identify common igneous rocks in lab.
 
 
 

 LINK TO ROCK CHART FOR LAB