Chapter 5 Igneous Rocks Section 5.1

What Are Igneous Rocks? Rocks that formed from the crystallization of magma.

Looking At Magma and Lava Flow http://www.youtube.com/watch?v=NIWlA7ndAlw

Types of Igneous Rocks Extrusive igneous rocks are fined grained rocks that cool quickly on Earth’s surface. (Rhyolite, Obsidian) Intrusive igneous rocks are coarse grained igneous rocks that cool slowly beneath the Earth’s surface. (Pegmatite, Granite)

Intrusive Igneous Rock (Pegmatite)

Extrusive Igneous Rocks (Basalt)

Composition of Magma Magma mostly consist of these elements: Oxygen (O), Silicon (Si), Aluminum (Al), Iron (Fe), Magnesium (Mg), Calcium (Ca), Potassium (K), and Sodium (Na). The most abundant composition is SiO2. In table (5-1), magmas are classified by rhyolitic, andesitic, and basaltic.

Origins of Magma Most rocks must be heated to 800 Celsius and 1200 Celsius in order to melt. In nature, these temperatures are found in the upper mantle and lower crust. The main factors that affect magma formation are temperature, pressure, water content, and mineral composition.

Factors of Magma Formation Temperature: Generally increases with depth in the Earth’s crust. For example, the geothermal gradient explains why temperatures in deep mines are quite high. Pressure: Pressure also increases with depth. This is a result of the weight of the overlying rock. One thing to take note off, as the pressure increases, melting point increases. Water Content: The third factor that affects the formation of magma is water content. As water content increases, the melting point decreases. Mineral Content: Olivine, calcium feldspar, etc melt and higher temperatures than rocks containing quartz and potassium feldspar. Oceanic crust (high in iron/magnesium) melt at higher temperatures than continental crust silicon/aluminum)

Things To Remember -Temperature generally increases with depth in the Earth’s crust. -Pressure increases with depth, and increase in pressure raises the rocks melting point. -Rocks and minerals often contain small percentages of water, water lowers the melting point. -Mineral composition also affects the melting point. Different minerals have different melting points. Rocks only melt at certain conditions, the right combination of temperature, pressure, and composition must be present.

How Do Rocks Melt and Crystallize Partial Melting: Rocks have different melting points which means not all rocks melt at the same time. This explains why magma is often a slushy mix of crystals and molten rock. Some minerals melt at low temperatures while others remain solid. (Figure 5-4 is a great example from your textbook). Fractional Crystallization: When magma cools, it crystallizes in the reverse order of partial melting the first minerals to crystallize are the last ones to melt. That means minerals with high melting points will form first.

Quick Overview of Bowen’s Reaction Series “As magma cools, minerals form in predictable patterns.” The first pattern is characterized by a continuous, gradual change of mineral composition. The second part is characterized by abrupt change of mineral type in the iron-magnesium group.

Bowens Reaction Series Chart (Figure 5-6)

Feldspars In Bowen’s reaction series the right branch represents the

Conundrum? If Olivine converts to pyroxene during cooling, why is olivine found in rock? -Geologists hypothesize that under certain conditions, newly formed crystals are separated from magma, and the chemical reactions between the magma and the minerals stop.

End of Section 5.1

Classifying Igneous Rocks

Mineral Composition There are three main groups of igneous rocks, felsic, mafic, and intermediate.

Felsic Rocks Felsic rocks, such as granite, are light-colored, and have high silica contents. Minerals: Contain quartz, orthoclase, and plagioclase.

Minerals: Feldspar Orthoclase, Quartz, possible mica and other amphiboles. Granite

Mafic Rocks Mafic Rocks, such as Gabbro, are dark-colored have lower silica contents, and are rich in ron and magnesium. Mineral contents (most common): biotite, plagioclase, pyroxene, and olivine.

Minerals: Plagioclase, Olivine, Gabbro

Grain Size -What might account for the lack of risible crystals in obsidian and large crystals of gabbro? -Cooling Rates: When lava flows on Earth’s surface, it is exposed to air and moisture. This means lava cools quickly and the crystals will be smaller. Intrusive rocks are cool slowly beneath the earth’s surface. Which means intrusive rocks will have larger crystals.

Going Over A Special Case Porphyritic texture indicates a complex cooling history. These rocks have both large and small crystals. For example, a rock could have been “cooling” slowly beneath the earth’s surface. But then a rock suddenly intruded higher in the crust and remaining magma would cool quickly and form smaller crystals. (Won’t be a main question on the test, but possibly an extra credit question.)

Igneous Rocks As Resources Diamonds Diamond is found in a rare, ultramafic rock known as kimberlites. Likely form deep in the crust at depths of 150 km to 300 km. Diamonds can only form under very high pressures. Most likely intruded rapidly upwards to the Earth’s surface, where it forms long, narrow, pipelike structures.

Summary http://www.youtube.com/watch?v=deC5af9AW6w