ROCK NOTES (chapter 3)
I.
Rocks
A. Rocks are any solid mass of mineral or mineral-like matter occurring naturally as part of our planet.
B. Types of rocks
1. Igneous rock is formed by the crystallization of molten magma.
2. Sedimentary rock is formed from the weathered products of preexisting rocks that have been
transported, deposited, compacted, and cemented.
3. Metamorphic rock is formed by the alteration of pre-existing rock deep within Earth (but still in
the solid state) by heat, pressure, and/or chemically active fluids.
II.
The Rock Cycle
A. Shows the interrelationships among the three rock types (igneous, sedimentary, and metamorphic)
1. Magma is molten material that forms deep beneath the Earth’s surface.
2. Lava is magma that reaches the surface.
3. Weathering is a process in which rocks are broken down by water, air, and living things
4. Sediment is weathered pieces of Earth elements.
B. Energy that drives the rock cycle
1. Processes driven by heat from the Earth’s interior are responsible for forming both igneous rock
and metamorphic rock.
2. Weathering and the movement of weathered materials are external processes powered by energy
from the sun.
3. External processes produce sedimentary rocks.
III.
Igneous Rock
A. Formation of igneous rock
1. Intrusive igneous rocks are formed when magma hardens beneath Earth’s surface.
2. Extrusive igneous rocks are formed when lava hardens.
B. Classification of igneous rock
1. Igneous rocks can be classified based on their composition and texture.
2. Texture
a. Coarse-grained texture is caused by slow cooling resulting in larger crystals.
b. Fine-grained texture is caused by rapid cooling resulting in smaller, interconnected
mineral grains.
c. Glassy texture is caused by very rapid cooling.
d. Porphyritic texture is caused by different rates of cooling resulting in varied sized
minerals.
3. Composition
a. Granitic composition rocks are made mostly of light-colored quartz and feldspar.
IV.
Sedimentary Rock
A. Formation of sedimentary rock
1. Weathering, erosion, and deposition
a. Erosion involves the weathering and the removal of rock.
b. Deposition occurs when an agent of erosion—water, wind, ice, or gravity—loses energy
and drops sediments.
2. Compaction and cementation
a. Compaction is a process that squeezes, or compacts, sediments.
b. Cementation takes place when dissolved minerals are deposited in the tiny spaces among
the sediments.
B. Classification of sedimentary rock
1. Two main groups
a. Clastic sedimentary rocks are composed of weathered bits of rocks and minerals.
i.
Classified by particle size
ii. Common rocks include
i. Shale (most abundant)
ii. Sandstone
iii. Conglomerate
b. Chemical sedimentary rocks form when dissolved substances precipitate, or separate,
from water.
i.
Common rocks include
i. limestone—most abundant chemical rock
ii. microcrystalline quartz known as chert, flint, jasper, or agate
iii. evaporites such as rock salt or gypsum
iv. coal
V.
Metamorphic Rock
A. Formation of metamorphic rock
1. Metamorphism means “to change form.”
2. Most metamorphic changes occur at elevated temperatures and pressures.
3. Conditions for formation are found a few kilometers below the Earth’s surface and extend into
the upper mantle.
4. Contact metamorphism occurs when magma moves into rock.
a. Occurs near a body of magma
b. Changes are driven by a rise in temperature
5. Regional metamorphism results in large-scale deformation and high-grade metamorphism.
a. Directed pressures and high temperatures occur during mountain building.
b. Produces the greatest volume of metamorphic rock
B. Agents of metamorphism
1. Heat
a. Provides the energy needed to drive chemical reactions
2. Pressure
a. Causes a more compact rock with greater density
3. Hydrothermal solutions
a. Hot water-based solutions escaping from the mass of magma
b. Promote recrystallization by dissolving original minerals and then depositing new ones
4. Two main categories
a. Foliated metamorphic rock
i. Has a banded or layered appearance
b. Nonfoliated metamorphic rock
i.
Does not have a banded texture