Igneous Rocks and Classifying Igneous Rocks Chapter 5

Igneous Rocks Igneous rocks are formed from the crystallization of magma.

Igneous Rocks Extrusive igneous rocks are fine-grained rocks that cool quickly on Earth’s surface.

Igneous rocks Basalt 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.

Igneous Rocks Intrusive igneous rocks are coarse-grained and cool slowly beneath Earth’s surface.

Igneous Rocks Granite The most common intrusive igneous rock Many granite deposits cross-cut into other rock formations This cross-cutting is evidence that granite was intruded into existing rocks

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% silica Silica affects melting temp. and viscosity

Igneous Rocks Factors that affect magma formation Temperature Pressure Temperature generally increases with depth in Earth’s crust. Pressure Pressure also increases with depth As the pressure on a rock increases, its melting point also increases

Igneous Rocks Water content Mineral content As water content increases, melting pt. decreases Mineral content Different minerals have different melting points In general, oceanic crust melts at higher temperatures than continental crust Rocks melt only under certain conditions – the right combination of temperature, pressure, and composition

Igneous Rocks How rocks melt Different parts of a rock may melt at different temperatures due to the different minerals present in the rock Partial melting: the process whereby some minerals melt at low temperatures while other minerals remain solid

Igneous Rocks How rocks melt As each group of minerals melts, different elements are added to the magma “stew,” thereby changing its composition If temperatures are not great enough to melt the entire rock, the resulting magma will have a different chemistry from that of the original rock.

Igneous Rocks How rocks melt Fractional crystallization The process wherein different minerals form at different temperatures When 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)

Igneous Rocks Feldspars Feldspar minerals undergo a continuous change of composition As magma cools, the first feldspars to form are rich in calcium As cooling continues, these feldspars react with magma, and their calcium-rich compositions change to sodium-rich compositions

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

Igneous Rocks Iron-rich minerals These minerals undergo abrupt changes during fractional crystallization. As minerals form, elements are removed from the remaining magma Silica and oxygen are left over When the remaining magma finally crystallizes, quartz is formed.

Igneous Rocks Crystal separation Layered intrusions Crystal separation can occur when: Crystals settle to the bottom of the magma body Liquid magma is squeezed from the crystal mush to form two distinct bodies with different compositions. Layered intrusions Formed when minerals form into distinct bands

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Classifying Igneous Rocks Mineral composition Felsic Light-colored, have high silica contents Contain quartz and feldspars orthoclase, and plagioclase Example: Granite

Classifying Igneous Rocks Mineral composition Mafic Dark-colored, have lower silica contents, rich in iron and magnesium Contain plagioclase, biotite, amphibole, pyroxene, and olivine. Example: Diorite

Classifying Igneous Rocks Ultramafic Low silica content and very rich in iron and magnesium Theory: formed by the fractional crystallization of olivine and pyroxene The minerals may have separated from magma and did not convert to another mineral upon reaching a particular temperature

Classifying Igneous Rocks Grain size Fine-grained vs. coarse-grained Cooling rates When lava cools on Earth’s surface, there is not enough time for large crystals to form. Thus, extrusive igneous rocks have no visible mineral grains When magma cools beneath the surface, large crystals form. Thus, intrusive igneous rocks may have crystals larger than 1cm.

Classifying Igneous Rocks Texture Porphyritic 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.

Classifying Igneous Rocks Ore deposits Veins The fluid left over during fractional crystallization contains any leftover elements that were not incorporated into the common igneous minerals They 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 rock This fluid solidifies to form metal-rich quartz veins.

Classifying Igneous Rocks Pegmatites Veins of extremely large-grained minerals are called pegmatites. Ores of rare elements such as lithium and beryllium are found in pegmatites Pegmatites can also produce beautiful crystals Because these veins fill cavities and fractures in rock, minerals grow into voids and retain their shapes.

Classifying Igneous Rocks Kimberlites Rare, ultramafic rocks where minerals such as diamonds are found Kimberlites are a variety of periodite They likely form deep in the crust at depths of 150-300km or in the mantle because diamond and other minerals found in kimberlites can form only under very high pressures.