Rocks are classified by their mode of formation. There are three major rock forming processes on Earth, producing three kinds of rocks.

Igneous Rocks Sedimentary Rocks Metamorphic Rocks

Formed when magma (molten rocks) solidifies

Environment: Hot enough to melt rock, pressure varies

All non-igneous rocks formed by processes acting on the surface of the Earth

Environment: normal for Earth’s surface

Formed by chemically and physically altering rocks under heat and pressure deep within the Earth’s crust.

Environment: High pressure, not hot enough to melt rock

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Metamorphic Rocks

The Rock Cycle

Igneous RocksIgneous rocks form from magma (a complex mix of molten rock with dissolved gases and other materials)

Magmas that cool slowly inside the Earth form plutonic igneous rocks. The mineral crystals in these rocks are usually large because they had lots of time to grow.

Magmas that erupt onto Earth’s surface are called lavas and very quickly. As they cool, volcanic igneous rocks form. If mineral crystals form, they tend to be small because they had little time to form. If cooling is fast enough, no minerals form and the lava solidifies to form a solid glass, with no crystalline structure.

Magma - complex mixture of high temperature solid, liquid, and gaseous material.

Dissolved gasses - mostly H2O, SO2, CO2

When the temperature is high enough (and the pressure* is low enough), some solid Earth materials

will begin to melt, forming magma.

*If the pressure is too high, melting will not occur – which is why magma is more commonly formed in the crust than the mantle, even though mantle temperatures are

much higher than crust temperatures.

Properties of Magma

Viscosity – resistance of a fluid to flow (low viscosity=water; high viscosity=syrup)

http://www.earth.northwestern.edu/people/seth/107/Rocks/magviscosity.htm

Bowen’s Reaction Series

felsic

Felsic igneous rocks contain minerals with low melting

points

Mafic igneous rocks contain minerals with high

melting points Intermediate igneous rocks

contain minerals with intermediate melting points

felsic

Texture - refers to the size of mineral crystals. The more slowly a magma cools, the more time crystals have to grow.

Magma that cooled slowly produces large crystals, and the resulting rock is coarse-grained (phaneritic).

In magmas that cooled quickly only small crystals form, and the resulting rock is fine-grained (aphanitic).

Crystalline Igneous Rock Texture

Texture - refers to the size of mineral crystals. The more slowly a magma cools, the more time crystals have to grow.

Porphyritic Texture

Sometimes a magma will have a complex cooling history, and may stay at a mineral’s crystallization temperature for a long time, allowing those mineral crystals to grow large.

If the magma is later cooled more quickly, only smaller crystals of the other minerals will form.

Crystalline Igneous Rock Texture

Felsic

Some magmas cooled too fast for minerals to form. The rocks are made of glass and contain no minerals, so are NOT described using the felsic-intermediate-mafic system.

vesicular – full of holes formed by bubbles in the magma.

Volcanic Igneous Rocks with no Minerals

fragmental or pyroclastic – formed from debris spewed out by the volcano.

glassy – obsidian and other volcanic glass

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