II Igneous Rock
- igneous rock – rock that forms when magma cools and
solidifies
- most can be identified as crystalline (made of crystals)
- chemical composition and texture determine the identity
of the igneous rock
A. Formation of Magma
1. three factors that affect melting
a. temperature – determined by the chemical
composition of the minerals in the rock
b. pressure – when excess pressure is removed from
rock that is at a temperature above the melting
point
c. presence of fluids – addition of fluids generally
decreases the melting point of certain minerals
2. Partial Melting
a. different minerals have different melting points
b. minerals that have lower melting points are the
first to melt
c. partial melting – process by which different
minerals in rock melt at different temperatures
3. Fractional Crystallization
a. cooling process (freezing) is the reverse of the
process of partial melting
b. minerals that have the highest freezing points
crystallize first
c. fractional crystallization – the crystallization and
removal of different minerals from the cooling
magma
1c. minerals that form first tend to settle at the
bottom of the magma chamber or stick to the
ceiling and walls of the magma chamber
2c. crystals that form early are commonly the
Largest
- Why?
- they have the longest to grow
B. Textures of Igneous Rock
- classified according to where magma cools and
hardens
- intrusive igneous rock – rock formed from the
cooling and solidification of magma beneath Earth’s
surface
- extrusive igneous rock – rock the forms from the
cooling and solidification of lava at Earth’s surface
- texture of igneous rock is determined by the size of
the crystals in the rock
1. Coarse-Grained Texture
a. intrusive igneous rocks commonly have large
mineral crystals
b. slow loss of heat allows the minerals to form large,
well-developed grains
c. coarse-grained texture – igneous rock that are
composed of large mineral grains (ex. granite)
2. Fine-Grained Texture
a. many extrusive igneous rocks are composed of
small mineral grains that cannot be seen by the
unaided eye
b. magma (lava) cools rapidly
c. large crystals are unable to form
d. fine-grained texture – igneous rocks that are
composed of small crystals (ex. basalt and rhyolite
3. Other Igneous Rock Textures
a. porphyritic texture – igneous rock that has a
mixture of large and small crystals
b. glass texture – highly viscous magma cools
quickly that contains a very small percentage of
dissolved gases (obsidian)
c. vesicular texture
1c. igneous rock that contains vesicles
2c. vesicles – holes that are formed in igneous
rock from highly viscous magma that has a
high percentage of dissolved gases that
become trapped as bubbles in rock
C. Composition of Igneous Rock
- the mineral composition is determined by the
chemical composition of the lava/magma
1. Felsic Rock
a. felsic – describes magma or igneous rock that is
rich in feldspars and silica and is generally light
in color
b. commonly also contains plagioclase feldspar,
biotite mica and muscovite mica
c. examples include granite, rhyolite, obsidian and
pumice
2. Mafic Rock
a. mafic – magma or igneous rock that is rich in
magnesium and iron and that is generally darker
in color
b. main mineral components are plagioclase
feldspar and pyroxene minerals
c. ferromagnesian minerals – minerals that are rich
in iron (Fe) and magnesium (Mg)
d. examples include basalt and gabbro
3. Intermediate Rocks
a. made up of minerals plagioclase feldspar,
hornblende, pyroxene and biotite mica
b. contain lower portions of silica than felsic
c. contain higher portions of silica than mafic
d. examples include diorite and andesite
D. Intrusive Igneous Rock Structures
- intrusions – rock masses that form underground
- forms when magma intrudes or enters into other rock
masses and then cools deep inside Earth’s crust
1. Batholiths and Stocks
a. batholith
1a. largest of all intrusions
2a. spread over 100 km2
3a. means “deep rock”
4a. form the cores of many mountain ranges,
such as the Sierra Nevadas
b. stock – similar to batholiths but cover areas less
than 100 km2
2. Laccoliths
a. pushes overlying rocks into a dome
b. means “lake of rock”
c. commonly occur in groups
d. ex. Black Hills of South Dakota
3. Sills and Dikes
a. sill
1a. magma flows between layers of rock and
hardens
2a. lie parallel to the layers of rock that
surrounds it
3a. vary in thickness from a few centimeters
to hundreds of meters
b. dike
1b. forms when magma follows existing vertical
fractures or creates new ones and solidifies
2b. cut across rock layers rather than lying
parallel
3b. common in areas of volcanic activity
E. Extrusive Igneous Rock Structures
1. rock masses that form on Earth’s surface
2. volcano – a vent or fissure in Earth’s surface
through which magma and gases are expelled
3. volcanic neck – solidified central vent of a volcano
4. lava flow – flat masses of rock
5. lava plateau – series of lava flows that cover a vast
area with thick rock
6. tuff – volcanic ash deposits that can be hundred of
meters thick and cover areas of several hundred km