Download What are Igneous Rocks

Chapter 5 Section 1 – Reading Notes
What are Igneous Rocks
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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
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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
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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
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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
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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
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Rocks do not melt all at once because their minerals all have different melting points.
Partial Melting
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Process in which some minerals melt before other minerals
Chemistry of the magma changes as more minerls melt within it.
Fractional Crystallization
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When magma cools it crystallizes in reverse order
As minerals crystallize they change the chemistry of the remaining “melt”
Bowen’s Reaction Series
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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
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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
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Minerals undergo abrupt changes as magma cools.
Olivine 1800 – 1557 degrees C
Leftover minerals crystallize last: quartz last to form
Crystal Separation
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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
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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
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Broad classification = Intrusive / Extrusive
Further classification would include Chemical Composition and Texture
Mineral Composition
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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
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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
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Size of the mineral grains is also a way to identify igneous rocks
Cooling Rates
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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
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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
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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
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Layered intrusions and veins can be sources for ore deposits
Veins
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Fluid leftover during magma crystallization contains high levels of silica and water as well as any
leftover elements
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Gold, Silver, Lead, and Copper form in veins of hot mineral-rich solutions that fill the cracks and
voids around a magma chamber
Pegmatites
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Veins of extremely large mineral crystals that grow into voids and spaces between rocks
Gemstones, lithium, beryllium, etc.
Kimberlites
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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