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Chapter 5 Section 1 – Reading Notes What are Igneous Rocks Rocks formed from crystallization of magma Lava is magma that flows out onto Earth’s surface Igneous comes from Latin word ignis, which means “fire”. Types of Igneous Rocks Extrusive – fine-grained rock that cools quickly on Earth’s surface o Formed “out of” (exit) the ground Intrusive – coarse-grained rock that cooled slowly “in” the ground o Granite – most common intrusive rock See Double Bubble of Intrusive/Extrusive comparison Composition of Magma Slushy mix of molten rock, gases, and mineral crystals Most common elements include: o O, Si, Al, Fe, Mg, Ca, K, Na o Compounds – SiO2 – silica is the most abundant 3 Types of Magma – based on Si content o Basaltic – low Si, darker minerals o Andesitic – intermediate Si content o Rhyolitic – high Si content Origins of Magma 800° - 1200° Celsius – melting point of rock These temps are found in upper mantle and lower crust. Heat leftover from Earth formation and radioactive decay of elements. Factors that Affect Magma Formation Temperature – increases with depth – geothermal gradient Pressure – increases with depth – higher pressure = higher melting point for a rock Water content – wet rocks melt quicker Mineral Content – different minerals melt at different temperatures o Continental crust melts before Oceanic crust because of mineral content. How Rocks Melt Rocks do not melt all at once because their minerals all have different melting points. Partial Melting Process in which some minerals melt before other minerals Chemistry of the magma changes as more minerls melt within it. Fractional Crystallization When magma cools it crystallizes in reverse order As minerals crystallize they change the chemistry of the remaining “melt” Bowen’s Reaction Series Early 1900’s, Bowen predicts the order in which minerals will crystallize in a magma Two-main branches of crystallization Continuous/Discontinuous Feldspars – continuous reaction series Ca-rich feldspars form at higher temperatures Na-rich feldspars form at lower temperatures Zoned-crystals Ca-rich cores and Na-rich outer rings Iron-rich Minerals Minerals undergo abrupt changes as magma cools. Olivine 1800 – 1557 degrees C Leftover minerals crystallize last: quartz last to form Crystal Separation As crystals form they can get removed from the melt and settle to the bottom of the magma chamber. This takes them out of the reaction and keeps them from changing into new minerals. Layered Intrusions Banded layers of minerals found in some magma bodies, due to settling of minerals as they form during the cooling process. Valuable sources of rare metals, platinum, chromium, nickel, or gold. Chapter 5 Section 2 Reading Notes Classifying Igneous Rocks Broad classification = Intrusive / Extrusive Further classification would include Chemical Composition and Texture Mineral Composition Three (3) main groups of compositions: Felsic, Intermediate, Mafic, (Ultramafic) Felsic Rocks – granite, light in color, high silica content Mafic Rocks – gabbro or basalt, dark color, low silica content, rich in Fe and magnesium Ultramafic Rocks Peridotite and dunite – low Si and High Fe, magnesium Formed through fractional crystallization with crystals being removed from magma and thus not changing into new minerals Could also represent pieces of the upper mantle that have been brought to Earth’s surface Grain Size Size of the mineral grains is also a way to identify igneous rocks Cooling Rates Lava – at surface cools quickly – may not have any mineral grains – glass to fine-grain Magma – cools slowly in Earth – mineral grains are visible – coarse grain Texture Crystals of minerals in igneous rocks interlock as they grow together as seen by thin sections of rocks Aphanitic Texture – fine grained Phaneritic Texture – coarse grained Porphyritic Texture – coarse grains in a fine grain matrix – represents complex cooling slow then fast as magma intrudes near surface or erupts at surface Igneous Rocks as Resources Interlocking grains give them strength and make them good for building materials They are also resistant to weathering Kitchen counters, tiles, pillars or columns, memorials, tombstones Ore Deposits Layered intrusions and veins can be sources for ore deposits Veins Fluid leftover during magma crystallization contains high levels of silica and water as well as any leftover elements Gold, Silver, Lead, and Copper form in veins of hot mineral-rich solutions that fill the cracks and voids around a magma chamber Pegmatites Veins of extremely large mineral crystals that grow into voids and spaces between rocks Gemstones, lithium, beryllium, etc. Kimberlites Rare ultramafic rock that contains diamonds. Kimberly, South Africa Intrusions deep in Earth where minerals that only form under high pressure are found Large pipe-like structures