Presentation on theme: "Igneous Rocks and Classifying Igneous Rocks"— Presentation transcript:
1 Igneous Rocks and Classifying Igneous Rocks Chapter 5
2 Igneous RocksIgneous rocks are formed from the crystallization of magma.
3 Igneous RocksExtrusive igneous rocks are fine-grained rocks that cool quickly on Earth’s surface.
4 Igneous rocksBasalt is an extrusive igneous rock that is very dark in color. It is the most common type of rock in the Earth's crust and it makes up most of the ocean floor.
5 Igneous RocksIntrusive igneous rocks are coarse-grained and cool slowly beneath Earth’s surface.
6 Igneous Rocks Granite The most common intrusive igneous rock Many granite deposits cross-cut into other rock formationsThis cross-cutting is evidence that granite was intruded into existing rocks
7 Igneous Rocks Review of magma: A slushy mix of molten rock, gases, and mineral crystals.Silica (SiO2) is the most abundant compound in magma and has the greatest effect on its characteristics.Basaltic: 50% silica, Andesitic: 60% silica, Rhyolitic: 70% silicaSilica affects melting temp. and viscosity
8 Igneous Rocks Factors that affect magma formation Temperature Pressure Temperature generally increases with depth in Earth’s crust.PressurePressure also increases with depthAs the pressure on a rock increases, its melting point also increases
9 Igneous Rocks Water content Mineral content As water content increases, melting pt. decreasesMineral contentDifferent minerals have different melting pointsIn general, oceanic crust melts at higher temperatures than continental crustRocks melt only under certain conditions – the right combination of temperature, pressure, and composition
10 Igneous Rocks How rocks melt Different parts of a rock may melt at different temperatures due to the different minerals present in the rockPartial melting: the process whereby some minerals melt at low temperatures while other minerals remain solid
11 Igneous Rocks How rocks melt As each group of minerals melts, different elements are added to the magma “stew,” thereby changing its compositionIf temperatures are not great enough to melt the entire rock, the resulting magma will have a different chemistry from that of the original rock.
12 Igneous Rocks How rocks melt Fractional crystallization The process wherein different minerals form at different temperaturesWhen magma cools, it crystallizes in the reverse order of partial melting (the first minerals to crystallize from magma are the last minerals to melt during partial melting)
13 Igneous Rocks Feldspars Feldspar minerals undergo a continuous change of compositionAs magma cools, the first feldspars to form are rich in calciumAs cooling continues, these feldspars react with magma, and their calcium-rich compositions change to sodium-rich compositions
14 Igneous Rocks Feldspars In come instances, as when magma cools rapidly, the calcium-rich cores are unable to react completely with the magma.The result is a zoned crystal that has sodium-rich outer layers and calcium-rich cores
15 Igneous Rocks Iron-rich minerals These minerals undergo abrupt changes during fractional crystallization.As minerals form, elements are removed from the remaining magmaSilica and oxygen are left overWhen the remaining magma finally crystallizes, quartz is formed.
16 Igneous Rocks Crystal separation Layered intrusions Crystal separation can occur when:Crystals settle to the bottom of the magma bodyLiquid magma is squeezed from the crystal mush to form two distinct bodies with different compositions.Layered intrusionsFormed when minerals form into distinct bands
18 Classifying Igneous Rocks Mineral compositionFelsicLight-colored, have high silica contentsContain quartz and feldspars orthoclase, and plagioclaseExample: Granite
19 Classifying Igneous Rocks Mineral compositionMaficDark-colored, have lower silica contents, rich in iron and magnesiumContain plagioclase, biotite, amphibole, pyroxene, and olivine.Example: Diorite
20 Classifying Igneous Rocks UltramaficLow silica content and very rich in iron and magnesiumTheory: formed by the fractional crystallization of olivine and pyroxeneThe minerals may have separated from magma and did not convert to another mineral upon reaching a particular temperature
21 Classifying Igneous Rocks Grain sizeFine-grained vs. coarse-grainedCooling ratesWhen lava cools on Earth’s surface, there is not enough time for large crystals to form.Thus, extrusive igneous rocks have no visible mineral grainsWhen magma cools beneath the surface, large crystals form.Thus, intrusive igneous rocks may have crystals larger than 1cm.
22 Classifying Igneous Rocks TexturePorphyritic texture: when a rock has grains of two different sizes.Large, well-formed crystals surrounded by finer-grained crystals of the same mineral or different minerals.Porphyritic textures indicate a complex cooling history wherein a slowly cooling magma suddenly began cooling rapidly.
23 Classifying Igneous Rocks Ore depositsVeinsThe fluid left over during fractional crystallization contains any leftover elements that were not incorporated into the common igneous mineralsThey include: gold, silver, lead, and copper.These elements are released at the end of magma crystallization in a hot, mineral-rich fluid that fills cracks and voids in the surrounding rockThis fluid solidifies to form metal-rich quartz veins.
24 Classifying Igneous Rocks PegmatitesVeins of extremely large-grained minerals are called pegmatites.Ores of rare elements such as lithium and beryllium are found in pegmatitesPegmatites can also produce beautiful crystalsBecause these veins fill cavities and fractures in rock, minerals grow into voids and retain their shapes.
25 Classifying Igneous Rocks KimberlitesRare, ultramafic rocks where minerals such as diamonds are foundKimberlites are a variety of perioditeThey likely form deep in the crust at depths of km or in the mantle because diamond and other minerals found in kimberlites can form only under very high pressures.