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Igneous Rocks

Rocks are classified by their mode of formation. There are three major rock forming processes on Earth, producing three kinds of rocks. Sedimentary Rocks. Metamorphic Rocks. Igneous Rocks. Formed when magma (molten rocks) solidifies Environment : Hot enough to melt rock, pressure varies.

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Igneous Rocks

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

  2. The Rock Cycle Sedimentary Rocks Igneous Rocks Metamorphic Rocks

  3. Igneous Rocks Igneous 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.

  4. When the temperature is high enough (and the pressure* is low enough), some solid Earth materials will begin to melt, forming magma. Magma- complex mixture of high temperature solid, liquid, and gaseous material. Dissolved gasses - mostly H2O, SO2, CO2 *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.

  5. 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

  6. Mafic igneous rocks contain minerals with high melting points Intermediate igneous rocks contain minerals with intermediate melting points felsic Felsic igneous rocks contain minerals with low melting points Bowen’s Reaction Series

  7. felsic

  8. 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. 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).

  9. 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.

  10. Felsic

  11. Volcanic Igneous Rocks with no Minerals 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. obsidian scoria glassy – obsidian and other volcanic glass vesicular – full of holes formed by bubbles in the magma. fragmental or pyroclastic– formed from debris spewed out by the volcano. bombs!

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