3.2 Igneous Rocks

Text pp 70- 74

• Different kinds of igneous rocks form when magma – or lava – cools and hardens.

Formation of Igneous Rocks

• When magma hardens beneath the Earth’s surface, intrusive igneous rocks form. This is because they intrude into the existing rocks.

• The SLOW cooling of the magma allows larger crystals to grow in intrusive rock, such as in this sample of granite.

• Intrusive rock results when magma solidifies under the Earth’s surface. The magma can force itself into the cracks of existing rocks.

We see intrusive rock at the earth’s surface because of erosion or weathering.

• Intrusive rock crystals do not have to be the same size. A finely textured matrix may form around larger crystals which cooled more slowly.

Porphryitic andesite

• When lava hardens above the ground, the igneous rocks that form are called extrusive rocks. (They EXIT the Earth.)

• The fast cooling of the lava only allows small crystals to grow in extrusive rock.

Basalt, fresh broken surface.

• Extrusive igneous rocks form when lava flows onto the surface of the earth or floor of the ocean through deep cracks or fissures and at volcanic vents, and then cools and hardens.

Classification of Igneous Rocks

• Texture and Composition are two characteristics used to classify igneous rocks.

Texture

• Texture describes the appearance of an igneous rock based on the size, shape, and the arrangement of its interlocking crystals.

Composition• Composition classes of igneous rocks

are based on the proportions of light and dark minerals in the rock.

Coarse-Grained Texture• Magma, since it is beneath the surface, has a

chance to cool very slowly. This results in the formation of large crystals. Igneous rocks with large crystals exhibit a coarse-grained texture.

Syenite igneous rock

Fine-Grained Texture• Quick cooling of magma or lava results in

rocks with small, interconnected mineral grains. Igneous rocks with small grains are said to have a fine-grained texture.

Rhyolite

Glassy Texture• When lava pours onto Earth’s surface, there

may not be enough time for crystals to even form before the lava hardens. Such rocks have a glassy texture.

Obsidian

Porphyritic Texture• Especially deep in the Earth, a large body of

magma may not cool at the same rate throughout. Some minerals cool much more quickly than others.

The result is large crystals surrounded by a matrix of fine-grained materials. This is called a porphyritic texture.

feldspar mineral labradorite

Granitic Composition

• Granites are made up of almost entirely of the light-colored silicate minerals quartz and feldspar. In addition, most granites contain about 10% dark silicate minerals. Granites are the major rocks of the continental crust.

Basaltic Composition• Basalts contain many dark silicates and

metal minerals, such as magnesium and iron. They tend to be darker and denser than granite rocks.

• The most common basaltic is basalt, and makes up most of the oceanic plates.

Andesitic Composition• Rocks with a composition between granitic

and basaltic rocks have an andesitic composition. They contain at least 25% dark silicate minerals.

Andesite