Igneous Rocks: Formation, Classification, and Properties - Prof. William Shields, Study notes of Geology

Download Igneous Rocks: Formation, Classification, and Properties - Prof. William Shields and more Study notes Geology in PDF only on Docsity! Igneous Rocks Mineral -Naturally formed -Solid -Formed by inorganic processes -Specific chemical composition -Characteristic crystal structure Rock – a coherent, naturally occurring solid, consisting of an aggregate of one or more minerals, or a mass of natural glass or organic matter. Basic Rock Classifications •Igneous •Sedimentary •Metamorphic Igneous Rocks -a rock that forms when hot molten rock (magma or lava) cools and freezes solid Two types of igneous rocks •Plutonic – cools underground •Volcanic – cools above ground Factors controlling Melting 1. Temperature: Melt at [800oC and 1200oC] Source of heat [radioactive decay] Partial Melting - some minerals melt at a lower temperature than others. Geothermal Gradient – the rate at which temperature increases with depth in the Earth. 2. Pressure: as pressure increases, melting temperature increases. Lowering pressure causes melting in divergent margins. 3. Water content: as water content increases, melting temperature decreases Water in rocks causes melting in convergent margins. Bowen’s Reaction Series: There in an order in which different minerals crystallize. Usually, the dark colored-mafic minerals crystallize first as a magma freezes. Likewise, they are the last to melt as a rock heats up. Types of Molten Material 1. Magma – molten material below the Earth's surface. 2. Lava – molten material above the Earth's surface. Magma Chemistry Molten material can consist of liquid rock, mineral grains and gases (H2O, CO2, SO2). Silicon and oxygen (SiO2) make up the majority of a magma. 45% SiO2 - "low" silica content 75% SiO2 - "high" silica content •Mafic – low SiO2; high iron and magnesium •Felsic/Silicic – high SiO2; low iron and magnesium Magma Viscosity Viscosity – resistance to flow honey – high viscosity water – low viscosity • Viscosity increases as silica content increases. Igneous Rock Types •Intrusive (plutonic) rock – cools and solidifies below the Earth's surface. • Extrusive (volcanic) rock – cools and solidifies above the Earth's surface. Textures of Igneous Rocks Texture – overall appearance, related to size, shape, and arrangement of minerals. Texture is related to cooling history of an igneous rock, not its chemistry. •Phaneritic Texture – Slow Cooling Large Crystals •Aphanitic Texture – Rapid Cooling Small Crystals