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Igneous Rocks
Ch. 5
I. What Are Igneous Rocks?
A. Definition: Igneous Rocks form from the
crystallization of magma.
1. “Igneous” comes from ignis, which means fire because
it was associated with fiery lava flows.
2. Lava: Magma that flows out onto the Earth’s surface.
B. Types of Igneous Rocks
1. Extrusive Igneous Rocks are fine-grained igneous rocks
that cool quickly on Earth’s surface.
2. Intrusive Igneous Rocks are coarse-grained igneous
rocks that cool slowly beneath Earth’s surface.
a. Granite is the most common intrusive igneous rock.
C. Composition of Magma
1. Magma is often a slushy mix of molten rock, gasses,
and mineral crystals.
a. Elements of magma:
i.
Oxygen, silicon, aluminum, iron, magnesium, calcium, potassium, and
sodium.
b. Of all the compounds in magma, silicon (SiO2) is the most
abundant and has the greatest effect on magma
characteristics.
i.
Types of Magma
-Basaltic (50% SiO2)
-Andesitic (60% SiO2)
-Rhyolitic (70% SiO2)
ii. Silica content affect melting temperature and how the magma flows.
D. Origins of Magma:
1. Magma is found in the upper mantle and lower crust
where temps. reach 800 Celsius to 1200 Celsius.
a. This heat is theorized to come from the remaining energy from
Earth’s molten formation and the heat generated from the decay
of radioactive elements.
2. Factors that Affect Magma Formation
a. Depth
i.
Generally, the deeper you go, the higher the temperature.
b. Pressure
i.
c.
Generally increases with depth as well. This is a result of the weight
of overlying rock. As pressure increases, melting point also
increases.
Water Content
i.
As water content in a rock increases, the melting point decreases.
d. Mineral Content
i. Different minerals have different melting points.
3. How Rocks Melt
a. Partial Melting:
i. Because different minerals have different melting points, not all parts
of a rock melt at the same time.
ii. If temps. do not melt the entire rock, the resulting magma will have
a different chemistry from that of the original rock.
- This is one way in which different types of igneous rocks form.
- This is why magma is often a slushy mix of crystals and molten
rock.
b. Fractional Crystallization:
i.
ii.
iii.
Magma crystallizes in the reverse order of partial melting when
cooling.
The first minerals to crystallize were the last to melt.
Fractional Crystallization: The process wherein different minerals
form at different temperatures.
4. Bowen’s Reaction Series
a. Definition: A Diagram that demonstrates that as magma
cools, minerals form in predictable patterns.
i.
2 Major Patterns of Crystallization:
-Feldspar Group- characterized by a continuous, gradual change of
mineral compositions.
-Iron-Magnesium Groups- characterized by an abrupt change of
mineral type.
b. Feldspars:
i.
ii.
When magma cools quickly, a feldspar crystal may not have time to
react completely with magma and it retains a calcium-rich core.
The result is a crystal with distinct zones.
c. Iron-Rich Minerals:
i. Olivine begins to form around 1800 °C. When it cools to
1557 °C, pyroxene begins to form. All the olivine that
previously formed reacts and is converted to pyroxene.
ii. All that’s left in the end is silica and oxygen, which
combine to form quartz. This forms in veins, as it is shoved
into rock fractures.
d. Crystal Separation
i. Sometimes, under certain conditions, newly formed crystals are
separated from magma, and the chemical reactions between the magma
and these minerals stop.
e. Layered Intrusions:
i.
ii.
Sometimes, the minerals form in distinct bands within a magma
body.
These can be valuable sources of rare metals, like platinum, nickel,
and gold.
II. Classifying Igneous Rocks
A. Mineral Composition:
1. The 3 main groups of igneous rocks are classified
according to mineral compositions.
a. Felsic Rocks are light colored, have high silica contents, and
contain quartz and the feldspars orthoclase and plagioclase.
i. Ex: Granite
b. Mafic Rocks are dark-colored, have lower silica contents, and
are rich in iron and magnesium
i. Ex: Gabbro
c. Ultramafic Rocks have low amounts of silica and very
high levels of iron and magnesium. These tend to
have a greenish tint to them.
i. Ex. Dunite and peridotite.
B. Grain Size
1. Igneous Rocks differ in the sizes of their grains.
2. Cooling rates affect crystal size!
a. Rocks that cool on the surface cool quickly, resulting in small
crystals.
C. Texture
1. Geologists can see mineral texture when a section of
rock is placed under a microscope.
a. As mineral grains crystallize, they grow together and form
irregular edges.
b. Well-shaped crystals form only under certain conditions.
a.
During fractional crystallization, the minerals that form early in the
process have space to grow distinct crystal shapes.
2. Porphyritic Texture
a. Rocks with porphyritic texture have grains of 2 different sizes.
i.
Large, well-formed crystals are surrounded by finer-grained crystals.
b. Porphyritic textures indicate a complex cooling history.
D. Igneous Rocks as Resources
1. Interlocking grain textures help give igneous rocks
strength.
2. Many minerals in igneous rocks are resistant to
weathering.
E. Ore Deposits
1. Valuable ore deposits are often associated with igneous
intrusions.
a. Some are found within the rock (layered intrusions)
b. Sometimes, minerals are found in rocks surrounding
intrusions.
i. May occur as veins.
2. Veins
a. Recall from Bowen’s Reaction Series that the fluid left at the
end of crystallization contains a lot of silica and water.
i.
ii.
This fluid contains leftover elements not incorporated into the
common igneous material.
These are released at the end of crystallization and fills cracks to form
metal-rich quartz veins.
3. Pegmatites
a. Vein deposits may contain large-grained minerals known as
pegmatites.
i.
Can produce beautiful crystals.
4. Kimberlites
a. Rare, ultramafic rocks known as kimberlites contain
diamonds.
b. They form deep in the crust (150-300 km deep)
c. They intrude rapidly upwards to the surface.