Igneous Rocks

an example of granite, an intrusive igneous rock Devil’s Tower, Wyoming an example of granite, an intrusive igneous rock

El Capitan, Yosemite Valley, California an example of granite, an intrusive igneous rock

extrusive igneous rock Examples of basaltic formations

Igneous rocks form as magma cools and crystallizes. Igneous rocks that are formed inside the Earth are called plutonic or intrusive rocks. Rocks formed on the surface are formed from lava (a material similar to magma, but without gas) are called volcanic or extrusive rocks.

Igneous Rocks When magma cools, its ions crystallize into orderly patterns. The size of the crystal is determined by the rate of cooling. Slow cooling rates form large crystals. Fast cooling rates form microscopic crystals. Very fast cooling rates form glass.

Igneous Rocks Classification of igneous rocks is based on the rock’s texture and mineral constituents. Texture refers to the size and arrangement of the crystals in the rock. Types of igneous rock textures: Coarse-grained – slow rate of cooling Fine-grained – fast rate of cooling granite andesite

Igneous Rocks Classification of igneous rocks is based on the rock’s texture and mineral constituents. Texture refers to the size and arrangement of the crystals in the rock. Types of igneous rock textures: Glassy – very fast rate of cooling Porphyritic (two crystal sizes) – two rates of cooling obsidian pumice porphyritic granite

Figure 3.5

Figure 3.5 B

Figure 3.5 A

Glassy (obsidian) Figure 3.7

Figure 3.8 Glassy (pumice)

Figure 3.5 D

Igneous Rocks We learned that there are two criteria for classifying igneous rocks: texture (grain size) and mineral composition. Mineral composition is, of course, the types of minerals that make up the rock. The crystallization of magma is explained by Bowen’s Reaction Series, which shows the order of mineral crystallization. This is influenced by crystal-settling in the magma. Bowen’s Reaction Series is shown on the next slide.

Bowen’s reaction series Figure 3.13

Bowen’s Reaction Series Magma, with its diverse chemistry, crystallizes over a temperature range of at least 450oC. Therefore, as magma cools, certain minerals crystallize first, at relatively high temperatures. At successively lower temperatures, other minerals crystallize. This arrangement of minerals became known as Bowen’s Reaction Series.

Bowen’s Reaction Series The first mineral to crystallize from a mass of magma is olivine. Further cooling results in the formation of pyroxene, as well as plagioclase feldspar. At intermediate temperatures, the minerals amphibole and biotite begin to crystallize.

Bowen’s Reaction Series During the last stage of crystallization, after most of the magma has solidified, the minerals muscovite and potassium feldspar may form. Finally, quartz crystallizes from any remaining temperature.

Bowen’s Reaction Series In nature, we find that minerals that form in the same general temperature range on Bowen’s chart are found together in the same igneous rocks. For example, the minerals quartz, potassium feldspar, and muscovite (or hornblende) are typically found together as major components of the igneous rock granite.

Bowen’s Reaction Series Figure 3.13

What is the theme of Bowen’s Reaction Series? Knowledge Check What is the theme of Bowen’s Reaction Series? Different minerals crystallize at different temperatures as magma cools.

Classification of Igneous Rocks Figure 3.9

Common Igneous Rocks

Granitic Igneous Rocks Granitic igneous rocks are composed almost entirely of light-colored silicate minerals - quartz and feldspar. They are also referred to as felsic rocks, feldspar and silica (quartz.) Granitic rocks have a high silica content, about 70%. Obviously, the most common rock granitic rock is granite.

Granite Figure 3.11

Andesitic Igneous Rocks Andesitic igneous rocks are composed of slightly darker-colored silicates, amphiboles and feldspar. They are also referred to as intermediate rocks. Obviously, the most common rock andesitic rock is andesite.

Andesite

Basaltic Igneous Rocks Basaltic igneous rocks contain substantial amounts of dark silicate minerals and calcium-rich plagioclase feldspar. Basaltic igneous rock are also referred to as mafic; they contain magnesium and iron. The most common basaltic rock is basalt.

Basalt

Ultramafic Igneous Rocks Ultramafic igneous rocks contain substantial pyroxene minerals containing a high percentage of magnesium and iron. The most common ultramafic rock is olivine.

olivine

1/3 The Launch Pad Friday, 2/24/17 What causes the differences between the various types of igneous rocks? The rate of cooling of the magma (or lava.) Very fast cooling above ground (extrusive ) causes microscopic (glassy) grain Slow cooling underground (intrusive) causes coarse grain Fast cooling above ground (extrusive) causes fine grain

2/3 The Launch Pad Friday, 2/24/17 What causes the differences between the various types of igneous rocks? The mineral content of the magma. Pyroxene and feldspar dominant Quartz and feldspar dominant Amphibole and feldspar dominant

3/3 The Launch Pad Friday, 2/24/17 Why do granitic igneous rocks differ in their chemical composition from ultramafic igneous rocks? The minerals that comprise ultramafic rocks crystallize at much higher temperatures than those that make up granitic rocks.