Igneous Rocks Ch. 5

I. What Are Igneous Rocks? Definition: Igneous Rocks form from the crystallization of magma. “Igneous” comes from ignis, which means fire because it was associated with fiery lava flows. Lava: Magma that flows out onto the Earth’s surface.

Types of Igneous Rocks 1. Extrusive Igneous Rocks are fine-grained igneous rocks that cool quickly on Earth’s surface. Intrusive Igneous Rocks are coarse-grained igneous rocks that cool slowly beneath Earth’s surface. a. Granite is the most common intrusive igneous rock.

Composition of Magma Magma is often a slushy mix of molten rock, gasses, and mineral crystals. Elements of magma: Oxygen, silicon, aluminum, iron, magnesium, calcium, potassium, and sodium. Of all the compounds in magma, silicon (SiO2) is the most abundant and has the greatest effect on magma characteristics.

Types of Magma -Basaltic (50% SiO2) -Andesitic (60% SiO2) -Rhyolitic (70% SiO2) ii. Silica content affect melting temperature and how the magma flows.

Origins of Magma: Magma is found in the upper mantle and lower crust where temps. reach 800 Celsius to 1200 Celsius. a. This heat is theorized to come from the remaining energy from Earth’s molten formation and the heat generated from the decay of radioactive elements.

Factors that Affect Magma Formation Depth Generally, the deeper you go, the higher the temperature. Pressure Generally increases with depth as well. This is a result of the weight of overlying rock. As pressure increases, melting point also increases. Water Content As water content in a rock increases, the melting point decreases. Mineral Content i. Different minerals have different melting points.

How Rocks Melt Partial Melting: i. Because different minerals have different melting points, not all parts of a rock melt at the same time. If temps. do not melt the entire rock, the resulting magma will have a different chemistry from that of the original rock. - This is one way in which different types of igneous rocks form. This is why magma is often a slushy mix of crystals and molten rock.

Fractional Crystallization: Magma crystallizes in the reverse order of partial melting when cooling. The first minerals to crystallize were the last to melt. Fractional Crystallization: The process wherein different minerals form at different temperatures.

Bowen’s Reaction Series Definition: A Diagram that demonstrates that as magma cools, minerals form in predictable patterns. 2 Major Patterns of Crystallization: -Feldspar Group- characterized by a continuous, gradual change of mineral compositions. -Iron-Magnesium Groups- characterized by an abrupt change of mineral type.

Feldspars: When magma cools quickly, a feldspar crystal may not have time to react completely with magma and it retains a calcium-rich core. The result is a crystal with distinct zones.

c. Iron-Rich Minerals: i. Olivine begins to form around 1800 °C c. Iron-Rich Minerals: i. Olivine begins to form around 1800 °C. When it cools to 1557 °C, pyroxene begins to form. All the olivine that previously formed reacts and is converted to pyroxene. ii. All that’s left in the end is silica and oxygen, which combine to form quartz. This forms in veins, as it is shoved into rock fractures.

Crystal Separation i. Sometimes, under certain conditions, newly formed crystals are separated from magma, and the chemical reactions between the magma and these minerals stop.

Layered Intrusions: Sometimes, the minerals form in distinct bands within a magma body. These can be valuable sources of rare metals, like platinum, nickel, and gold.

II. Classifying Igneous Rocks Mineral Composition: The 3 main groups of igneous rocks are classified according to mineral compositions. Felsic Rocks are light colored, have high silica contents, and contain quartz and the feldspars orthoclase and plagioclase. i. Ex: Granite

Mafic Rocks are dark-colored, have lower silica contents, and are rich in iron and magnesium i. Ex: Gabbro

Ultramafic Rocks have low amounts of silica and very high levels of iron and magnesium. These tend to have a greenish tint to them. i. Ex. Dunite and peridotite.

Grain Size Igneous Rocks differ in the sizes of their grains. Cooling rates affect crystal size! a. Rocks that cool on the surface cool quickly, resulting in small crystals.

Texture Geologists can see mineral texture when a section of rock is placed under a microscope. As mineral grains crystallize, they grow together and form irregular edges. Well-shaped crystals form only under certain conditions. During fractional crystallization, the minerals that form early in the process have space to grow distinct crystal shapes.

Porphyritic Texture Rocks with porphyritic texture have grains of 2 different sizes. Large, well-formed crystals are surrounded by finer-grained crystals. Porphyritic textures indicate a complex cooling history.

Igneous Rocks as Resources Interlocking grain textures help give igneous rocks strength. Many minerals in igneous rocks are resistant to weathering.

Ore Deposits Valuable ore deposits are often associated with igneous intrusions. Some are found within the rock (layered intrusions) Sometimes, minerals are found in rocks surrounding intrusions. i. May occur as veins.

2. Veins Recall from Bowen’s Reaction Series that the fluid left at the end of crystallization contains a lot of silica and water. This fluid contains leftover elements not incorporated into the common igneous material. These are released at the end of crystallization and fills cracks to form metal-rich quartz veins.

Pegmatites Vein deposits may contain large-grained minerals known as pegmatites. Can produce beautiful crystals.

Kimberlites Rare, ultramafic rocks known as kimberlites contain diamonds. They form deep in the crust (150-300 km deep) They intrude rapidly upwards to the surface.