Igneous Rocks and Classifying Igneous Rocks Chapter 5

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

• 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• 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– 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

Intermission – Part II next class

Classifying Igneous Rocks

• Mineral composition– Felsic

• Light-colored, have high silica contents• Contain quartz and feldspars orthoclase, and

plagioclase• Example: Granite


• Mineral composition– Mafic

• Dark-colored, have lower silica contents, rich in iron and magnesium

• Contain plagioclase, biotite, amphibole, pyroxene, and olivine.

• Example: Diorite


– 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


• 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.


• 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.


• 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.


• 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.


• 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.

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Igneous Rocks and Classifying Igneous Rocks Chapter 5