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Module 1 Part 3 - Igneous Rocks
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Figure 1 - The Rock Cycle shows the different possible fates for rocks in or on the Earth. Igneous rocks formed from molten magma can be exposed on the surface and weathered in sediment particles.

Figure 1 - M4K. The Rock Cycle shows the different possible fates for rocks in or on the Earth. Igneous rocks formed from molten magma can be exposed on the surface and weathered in sediment particles.

Figure 2 - Intrusive and extrusive igneous rocks. If a hot magma does not reach the Earth’s surface but cools slowly underground, the resulting rocks is called intrusive. If the magma reaches the surface as lava or cinders and forms volcanoes or lava flows, the rock is called extrusive.

Figure 2 - M4K. Intrusive and extrusive igneous rocks. If a hot magma does not reach the Earth’s surface but cools slowly underground, the resulting rocks is called intrusive. If the magma reaches the surface as lava or cinders and forms volcanoes or lava flows, the rock is called extrusive.

Figure 3 - Bowen’s Reaction Series shows the sequence of crystallization of the most important igneous minerals from a mafic magma as it cools. Minerals that crystallize early (at high temperatures) form mafic igneous rocks like basalt. Minerals that crystallize late (at low temperatures) form felsic igneous rocks like granite.

Figure 3 - E. Bowen’s Reaction Series shows the sequence of crystallization of the most important igneous minerals from a mafic magma as it cools. Minerals that crystallize early (at high temperatures) form mafic igneous rocks like basalt. Minerals that crystallize late (at low temperatures) form felsic igneous rocks like granite.

Figure 6 - Volcanic ash fall/flow. Extrusive igneous lavas can form lava flows, volcanoes and blankets of ash and cinders that were blown into the air. Ash flows are clouds of hot cinders, ash, and gases that race down the sides of volcanoes as Vesuvius did to ancient Roman Pompeii. Ash falls can cover large areas.

Figure 6 - ETE. Volcanic ash fall/flow. Extrusive igneous lavas can form lava flows, volcanoes and blankets of ash and cinders that were blown into the air. Ash flows are clouds of hot cinders, ash, and gases that race down the sides of volcanoes as Vesuvius did to ancient Roman Pompeii. Ash falls can cover large areas.

The Rock Cycle

We think of rocks as being permanent and unchanging (“steady as a rock”).

• In the context of over 4 billion years of Earth history, however, the atoms that compose a rock may have been in the Earth’s mantle as molten magma. They may then have risen from the mantle and cooled to form an igneous rock. For the following discussion, see Figure 1.
• When the granite rock weathers at the Earth’s surface, its individual crystals may be eroded from the rock and form a sediment, like sand or gravel which eventually becomes a sedimentary rock like sandstone.
• That sedimentary rock, in turn, might become buried in a down-going subducted lithospheric plate and the tremendous heat and pressure may cause it to form a metamorphic rock.
• As the plate is carried further down into the Earth, it may heat enough to cause the rocks in it to become molten.
• And so the atoms that started out in the mantle may arrive there again. This change of Earth materials from one rock type to another over time is called the rock cycle (Figure 1).
• There are three types of rocks in the rock cycle, igneous, sedimentary, and metamorphic and any one type can become another during the cycle. For instance, a metamorphic rock exposed at the Earth’s surface may decompose and form a sediment which becomes a sedimentary rock. We will finish this module talking about igneous rocks.

Igneous Rocks

Igneous rocks are the first part of the rock cycle mentioned above and form by cooling from molten material called magma.

• Igneous rocks = rocks that result from fire (same word root used in word “ignite”)

• Igneous rocks crystallize or freeze from liquid hot molten magma
• Igneous rocks “freeze” between 650-1100°C (1202°-2012°F)

• Magma = melted or molten rock material below Earth's surface; lava = molten rock at Earth's surface
• Intrusive igneous rock = rock that has crystallized below Earth’s surface where it “intruded” into pre-existing rock
• Extrusive igneous rock = rock that crystallized on the Earth's surface

• Includes: lava (from lava flows) or pyroclastics (rock from sprayed lava or lava bombs, particles, ash). See Figure 2.

Magma types:

• Silicic magmas: rich in SiO2 (called silica), have 70% or more silica; little magnesium (Mg) and iron (Fe); are cooler (as low as 650°C) than mafic and ultramafic lavas (over 1100°C); are most viscous or sticky; as a result, are often explosive.
• Intermediate magmas: 55-70% silica
• Mafic magmas: richer in Mg, Fe, in the form of ferromagnesian minerals; have less than 55% silica. These are runny lavas, spreading far before cooling, like those lavas on Hawaii.
• Ultramafic magmas: rich in Mg, Fe or ferromagnesian minerals; less than 40% silica.

Bowen's Reaction Series: describes the sequence of minerals that crystallize from a mafic magma. High temperature minerals crystallized first like olivine and Calcium (Ca) plagioclase, then intermediate temperature minerals like amphiboles and Ca-Na (sodium) plagioclases, then low temperature minerals like quartz and muscovite (light-colored mica). See Figure 3.

Magma movement

• Magma rises due to: decrease in pressure in fractured areas; buoyant due to liquid nature; may contain volatiles that change from a liquid to a gas as pressure decreases, causing gas bubbles to form which further decrease density.
• Silicic magmas are more viscous, often explosive; mafic magmas more fluid, pore out of cracks and spread rapidly into sheets

Extrusive igneous settings. Extrusive igneous rocks are found on the Earth’s surface and are lavas or pyroclastics that have cooled quickly. Extrusive igneous rocks are finely crystalline because the atoms in the cooling lava have little time to form big crystals.

• Volcanoes and lavas: volcanoes form from lava and ash. Lava is magma that flows on to the Earth’s surface. Runny lavas form large, broad volcanoes called shield volcanoes. Viscous or sticky lavas form volcanoes with steeper slopes. When lava is spewed into the air, the droplets cool into volcanic ash (very small glass fragments) or tephra (larger particles). A small volcano formed mainly of tephra is called a cinder cone. See Figure 4 from the U.S. Geological Survey for the types of volcanoes. Figure 5 - Capulin Volcano cinder cone.
• Ash flows and falls are glass shards from lava spewed into air. They can settle from the air in layers (pyroclastic ash falls, tuffs) or can move down the sides of volcanoes in hot clouds of particles called ash flows. Figure 6 - Ash fall.

Part 3 - Igneous Rocks - Module 1 Home | Page 1 | Page 2 | Web Sites and References

Part 1 - Plate Tectonics | Part 2 - Minerals | Part 3 - Igneous Rocks
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