Section 6.2 Igneous Rock
Objectives • Summarize the three factors that affect whether a rock melts. • Describe how the cooling rate of magma and lava affects the texture of igneous rocks. • Classify igneous rocks according to their composition and texture. • Describe intrusive and extrusive igneous rock structures.
Crystalline • Most igneous rocks are made of mineral crystals that form when magma cools. Their chemical composition and the rock’s texturedetermine the identity of the rock.
Magma Formation 3 factors affect rock melting: Temperature: Minerals melt at different temperatures. Pressure: Less pressure enables minerals to melt. Fluids: Lowers the melting temperature of minerals.
Partial Melting • The reverse of Bowen’s reaction series. Minerals with low melting temperatures (quartz) melt before those with higher melting points (Ca-rich plagioclase)
Magma Composition • Different minerals melt at different temperatures, and so the composition of the magma can change over time.
Fractional Crystallization • The reverse of partial melting (really Bowen’s reaction series). Minerals with the highest freezing points crystallize first, removing elements from the magma, allowing new minerals to form.
Changing Magma Composition • Early formed crystals can settle out or stick to the walls of a magma chamber, and are usually larger (longer time to grow). Zoned crystals are the result of the magma’s composition changing while the crystal is growing. Zoned Plagioclase Ca-rich core Na-rich rim
Igneous Rock • Classified according to where the molten rock cools and hardens
Igneous Rock Textures • The way an igneous rock looks, which depends on the size of the crystals in the rock. This is determined by the cooling rate of the magma.
Intrusive Igneous Rock: • Magma that is "forced in" • Slowly cooling: • grains have time to form • crystals are large • Coarse-grained texture (phaneritic).
Extrusive Igneous Rock: • Magma that is "pushed out" • Rapidly cooling • grains do not have time to form • crystals are not visible • Fine-grainedtexture (aphanitic).
Other Igneous Rock Textures • Depends on the magma type, cooling history, and the amount of dissolved gases.
Porphyritic (POHR fuh RIT ik) Texture • Slow cooling (large crystals) followed by quick cooling (small crystals), giving a mix of large and small crystals.
Glassy Texture • A viscous (thick, sticky) magma with little dissolved gas. Cools FAST so no crystals grow. Usually called volcanic glass, or obsidion.
Vesicular Texture • Magma has lots of gas, but cools quickly. The gas cannot escape and forms bubbles (vesicles) in the rock.
Composition of Igneous Rocks • Each type of igneous rock has a specific mineral composition, determined by the chemical composition of the magma from which the rock formed. Halfdome Yosemitie Natl. Park, Calif.
Felsic Rock • High in silica • Light coloring • It is composed of orthoclasefeldspar and quartz, and small amount of plagioclase feldspar, hornblende and muscovite mica
Intermediate Rock • Medium-colored rocks • Contain minerals of plagioclase feldspar, hornblende, pyroxene, and biotitemica • Includes little or no quartz
Mafic Rock • Low in silica but rich in iron and magnesium • Dark coloring • The main mineral components are plagioclasefeldspar and pyroxenes and may also include olivine, biotitemica and hornblende.
Underground rock masses made up of intrusive igneous rocks are called intrusions. These may be later exposed at the surface by erosion. Surface rock masses made up of extrusive igneous rock are called extrusions. Igneous Rock Structures -Two Main Types We will discuss intrusions first.
Batholiths and Stocks • Batholiths (means “deep rock”) are very large masses of igneous rock that cover over 100 square kilometers of the earth’s surface. • Stocks are smaller intrusions that cover less than 100 square kilometers.
The Sierra Nevada Range The core of this mountain range is the Sierra Nevada batholith. The light colored rocks at the top of themountains are granites of the batholith.
Laccolith (“Lake of Rock”) When magma flows between rock layers and spreads upward, it can push the overlying rock layers into an arc, with the bottom of the intrusion being parallel to the rock layer beneath it.
Laccoliths Frequently Found in Groups Laccoliths can sometimes be identified by the small dome-shaped mountains they push up on the earth’s surface. Crown Butte, Montana
Sills A sill is formed when magma flows between layers of rock. The sill lies parallel to the rock layers surrounding it, even if the layers are tilted. Sills vary in thickness and lateral extent.
Dikes A dike differs from a sill in that they cut across rock layers rather than lying parallel to the layers.
Common in areas of volcanic activity. A basalt dike cutting across sedimentary rock layers in the Grand Canyon.
Extrusive Igneous Rock StructuresExtrusions – Form on the Earth’s Surface Volcano: A vent through which magma, gases, or volcanic ash is expelled. When an eruption stops, the magma in the volcano forms rock.
Volcanic Necks If a volcano erodes away the harder rock of the vent can remain. This is called a volcanic neck. Narrow dikes sometimes radiate out from the neck. Shiprock, New Mexico
Lava Flows and Lava Plateaus Many extrusions are simply flat masses of rock called lava flows. Some lava flows develop from lava that flows out of long cracks in the earth’s surfaceand spread over a large area. They are very thick and are known as a lava plateau.
Columbia River Flood Basalts For the Columbia River plateau, over 300 separate lava flows combined to form basalts up to 3500 meters thick. This is enough lava to cover the entire continental U.S. to a depth of 12 meters (39.4 feet).
Tuff • Volcanic ash deposits, can be large and thick.
Assignment • Directed Reading • 6.2 Key Terms