Igneous Rocks Type of Igneous rock formed depends on the environment where it crystallizes.  Two main environments: Volcanic Molten rock cools on or near Earth’s surface which results in very small crystals. 1) Plutonic Molten rock cools beneath Earth’s surface which results in larger more uniform sized crystals. 2) Reference: Tarbuck and Lutgens Pages

Rocks become darker in color Volcanic Igneous Rocks Rock Types Composition FelsicIntermediateMaficUltramafic Volcanic RholiteAndesiteBasaltKomatotite Plutonic GraniteDioriteGabbroPeridotite Rhyolite:  Quartz and Orthoclase feldspar  Light in color ( < 15% dark minerals)  Microscopic crystals

Rocks become darker in color Volcanic Igneous Rocks Rock Types Composition FelsicIntermediateMaficUltramafic Volcanic RholiteAndesiteBasaltKomatotite Plutonic GraniteDioriteGabbroPeridotite Andesite:  Amphibole and Plagioclase feldspar  Medium colored ( < 40% dark Minerals)  Microscopic crystals

Rocks become darker in color Volcanic Igneous Rocks Rock Types Composition FelsicIntermediateMaficUltramafic Volcanic RholiteAndesiteBasaltKomatotite Plutonic GraniteDioriteGabbroPeridotite Basalt:  Pyroxene and Plagioclase feldspar  Dark in color ( > 40% dark minerals)  Microscopic crystals

Rocks become darker in color Plutonic Igneous Rocks Rock Types Composition FelsicIntermediateMaficUltramafic Volcanic RholiteAndesiteBasaltKomatotite Plutonic GraniteDioriteGabbroPeridotite Gabbro:  Pyroxene and Plagioclase feldspar  Dark in color ( > 40% dark minerals)  Visible crystals (unaided eye)

Rocks become darker in color Plutonic Igneous Rocks Rock Types Composition FelsicIntermediateMaficUltramafic Volcanic RholiteAndesiteBasaltKomatotite Plutonic GraniteDioriteGabbroPeridotite Diorite:  Amphibole and Plagioclase feldspar  Medium colored ( < 40% dark Minerals)  Visible crystals (unaided eye)

Rocks become darker in color Plutonic Igneous Rocks Rock Types Composition FelsicIntermediateMaficUltramafic Volcanic RholiteAndesiteBasaltKomatotite Plutonic GraniteDioriteGabbroPeridotite Granite:  Quartz and Orthoclase feldspar  Light in color ( < 15% dark minerals)  Visible crystals (unaided eye)

Classification of Igneous Rocks A general classification scheme based on chemical composition and texture is provided in the following diagram.

Sample problem Explain why two igneous rocks can have the same mineral composition, but have different rock names. Answer: The type of Igneous rock not only depends on its mineral composition, but also, on the environment in which the molten rock crystallized. The environment will determine the texture of the rock, which is another factor used to classify igneous rocks. Thus, two igneous rocks with the same mineral composition and have different textures, will have different names.

Sample problem Explain why two igneous rocks can have the same mineral composition, but have different rock names. Answer: For Example; Rhyolite and Granite (Felsic composition) Andesite and Diorite (Intermediate composition) Basalt and Gabbro (Mafic composition)