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Chapter 5: Igneous Rocks

Ch. 5.1 What Are Igneous Rocks?

Igneous Rock Formation

Magma is molten rock below Earth's surface. Lava- magma that flows out onto Earth's surface. Igneous rocks- form when lava or magma cools and crystallizes.

In lab, most rocks must be heated to temperatures of 800º C to 1200º C to melt. These temperatures are present in the upper mantle and lower crust.

Composition of Magma

Magma often a mix of molten rock, dissolved gases, and mineral crystals. Silica is the most abundant element present. Magma is classified as basaltic, andesitic, or rhyolitic based on the amount of silica it contains.

Silica content affects melting temperature and impacts magma viscosity (resistance to flow). Rhyolitic magma has a higher viscosity than basaltic magma.

On surface, dissolved gases are able to escape so chemical composition changes slightly.

Magma Formation

Formed two ways: melting of crust, or melting within the mantle.

Four factors in formation of magma:

  1. Temperature- generally increases with depth in crust; called geothermal gradient
  2. Pressure- increases with depth; rock melting point rises with depth
  3. Water content- as water content increases, melting point decreases
  4. Mineral content

Mineral Content

Minerals have different melting points. Rocks rich in iron and magnesium melt at higher temperatures than rocks that contain higher levels of silicon.

Partial Melting

Not all parts of a rock melt at the same temperature. As a result, magma is often a slushy mix of crystals and molten rock. Partial melting- process in which some minerals melt at relatively low temperatures while other minerals remain solid.

Bowen's Reaction Series

Geologist N. L. Bowen demonstrated that as magma cools and crystallizes, minerals form in predictable patterns; the Bowen's reaction series. Two main branches of crystallization: right-hand branch is characterized by a continuous, gradual change of mineral compositions in the feldspar group. Left-branch is characterized by an abrupt change of mineral type in the iron-magnesium groups.

Iron-rich minerals

The left branch of Bowen's reaction series. Order as cools: olivine-pyroxene-amphibole-biotite mica.


The right branch of Bowen's reaction series. Plagioclase feldspars undergo a continuous change of composition that are rich in calcium. These then change to sodium-rich compositions.

Fractional Crystallization

When magma cools, it crystallizes in the reverse order of partial melting, meaning, those minerals that melt last are the first to crystallize. However, first crystals are removed from the magma and cannot react with it. As they are removed, magma becomes concentrated with silica.

The last two minerals to to crystallize out of a cooling magma is quartz and potassium feldspar. Quartz often occurs in veins because it crystallizes while the last liquid portion of magma is squeezed into rock fractures.

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Ch. 5.2 Classification of Igneous Rocks

Mineral Composition of Igneous Rocks

Classified into two groups: Intrusive rocks- magma is injected into the surrounding rock; crystals are generally large enough to see without magnification. Extrusive rocks- magma that cools and crystallizes on Earth's surface; crystals are small and difficult to see without magnification.

Igneous rocks classified by mineral content.

Basaltic (mafic)- dark-colored with lower silica contents, contain mostly plagioclase and pyroxene. Granitic (felsic)- light-colored, have high silica contents, contain mostly quartz and feldspar. Intermediate rocks are in between the two. Ultramafic contains olivine and pyroxene and are always dark.


Texture- the size, shape, and distribution of the crystals or grains that make up the rock. Course vs. fine.

Crystal Size and Cooling Rates- Rapid cooling rates leads to small crystals or no crystals (glassy like obsidian). When magma cools slowly beneath the surface, large crystals can form.

Porphyritic rocks- has large, well-formed crystals surrounded by finer-grained crystals of the same mineral or different minerals. Cools slow so crystals start to form and then cools quickly so surrounded by small/absent crystal rock.

Vesicular Rocks- has holes from dissolved gases that escapes. If rock is thick, forms vesicles (holes) this creates vesicular texture.

Thin Sections

Used to observe mineral grains. 2 cm X 4 cm and 0.03 mm thick so light can pass through. Viewed under a petrographic microscope, mineral grains exhibit distinct properties.

Igneous Rocks as Resources

Veins- Igneous intrusions may contain ore. Bowen's reaction series explains how liquid left during crystallization contains high levels of silica and water. The fluid also contains leftover elements that were not incorporated into the common igneous minerals; gold, silver, lead, and copper.

Pegmatites- nonmetal, igneous rocks with extremely large-grained minerals and usually found as intrusions or veins. Slow cooling leads to large crystals.

Kimberlites- form under extremely high pressure, are rare ultrabasic rocks that contain rare minerals like diamonds. Kimberlite magma forms long, narrow, pipelike structures (kimberlite pipes).

Igneous rocks in construction- igneous rocks resist weathering due to interlocking grain textures and are very strong so are useful in construction.

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Page last updated August 16, 2017.