Chapter 2 Rocks: Mineral Mixtures
Section 1 The Rock Cycle
A rock is defined as a naturally occurring solid mixture of one or more minerals and organic matter
The continual process by which new rock forms from old rock material is called the rock cycle
The value of Rock
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Early humans used as tools
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Used for centuries to make buildings
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Granite, limestone, marble, sandstone, slate, etc.
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Modern buildings use concrete and plaster where rock is an important ingredient
Processes that Shape the Earth
Weathering, Erosion, and Deposition
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The process in which water, wind, ice, and heat break down rock is called weathering.
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The process by which sediment is removed from its source is called erosion
o
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Weathering breaks rocks into fragments that become the sediment that are used to make sedimentary
rocks
Water, wind, ice and gravity can erode and move sediments and cause them to collect
The process in which sediment moved by erosion is dropped and comes to rest is called deposition.
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Sediment is deposited in bodies of water and other low-lying areas and can be pressed and cemented
together by minerals dissolved in water to form sedimentary rock.
Heat and Pressure
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Sedimentary rock made of sediment can also form when buried sediment is squeezed by the weight of overlying
layers of sediment
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If the temperature and pressure are high enough at the bottom, the rock can change into metamorphic rock.
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Sometimes the rock gets hot enough to melt, creating magma that eventually cools to form igneous rock
How the Cycle Continues
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Uplift is movement within the Earth that causes rocks inside the Earth to be moved to the Earth’s surface.
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Buried rock is exposed at the Earth’s surface by a combination of uplift and erosion
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When uplifted rock reaches the Earth’s surface, weathering, erosion, and deposition begin.
Illustrating the rock cycle
Round and Round it Goes
Each type of rock can change into one of the 3 types of rock
This cycle, in which rock is changed by geological processes into different types of rock, is known as the rock cycle
The location of rock determines which natural forces will have the biggest impact on the process of change
Rock Classification: Each rock type can be further divided depending on how it was formed.
Composition: The minerals a rock contains determine the composition of that rock
Texture: The size, shape, and positions of the grains that make up a rock determine a rock’s texture
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The texture of a rock can provide clues as to how and where the rock is formed
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Section 2: Igneous Rock
Igneous rock forms when hot, liquid rock or magma, cools and solidifies
The type of igneous rock that forms depends on the composition of the magma and the amount of time it takes the
magma to cool.
Origins of igneous Rock
Igneous rocks begin as magma.
Magma forms in 3 ways:
1. When rock is heated
2. When pressure is released
3. When rock changes composition
When magma cools enough, it solidifies to form igneous rock
Liquid magma contains many melted minerals and these minerals have different melting points so magma will freeze or
become solid before other minerals do.
Composition and Texture of igneous Rock
Felsic Rocks – light colored rocks rich in elements such as aluminum, potassium, silicon, and sodium
Mafic Rocks – Dark-colored rocks rich in calcium, iron, and magnesium, and poor in silicon
The longer it takes for the magma to cool, the more time the mineral crystals have to grow and therefore the larger the
crystals are and the coarser the texture of the resulting igneous rock is.
The less time it takes for the magma to cool, the less time the crystals have to grow so the rock being formed will be fine
grained.
Igneous Rock formation
Formations are located above and below the Earth’s surface
Intrusive Igneous Rock – when magma intrudes, or pushes, into surrounding rock below Earth’s surface and cools
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Usually coarse because it is insulated and cools slowly
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The minerals that form are large visible crystals
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Plutons are large, irregular shaped intrusive bodies
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Batholiths are the largest igneous intrusions
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Stocks are intrusive bodies that are exposed over smaller areas than batholiths
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Dikes are sheet-like intrusions that cut across previous rock units
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Sills are sheet-like intrusions that are oriented parallel to previous rock units
Extrusive igneous Rock – igneous rock that forms from the magma that erupts, or extrudes, onto the Earth’s surface
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Common around volcanoes
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Cools quickly on the surface
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Contains very small or no crystals
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Sometimes lava flows from volcanoes
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Sometimes lava flows from cracks in the Earth’s surface called fissures
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Fissures in the ocean floor causes the creation of new ocean floor
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Large amounts of lava from fissures on land can cover a large area and form a plain called a lava plateau
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Pre-existing landforms are often buried by these lava flows.
Section 3: Sedimentary Rock
Through the process of erosion sediments are moved from one place to another, eventually deposited in layers; older
layers become compacted; dissolved minerals separate from water that passes through the sediment to form a natural
cement that binds the rock and minerals together into sedimentary rock.
Sedimentary rock forms at or near the Earth’s surface, without heat or pressure that is involved in metamorphic or
igneous rocks
The most noticeable feature of these rocks are the layers, or strata.
Composition of Sedimentary Rock
Sedimentary rock is classified by the way it forms
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Clastic sedimentary rock forms when rock or mineral fragments (clasts) are cemented together
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Can be coarse, medium or fine grained
Chemical Sedimentary rock forms when minerals crystallize out of a solution (like sea water) to become
rock
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Example is Halite which forms when sodium ions and chlorine ions in shallow bodies of water
become so concentrated that halite crystallizes from solution
Organic Sedimentary Rock forms from the remains of once-living plants and animals
o Most Limestone forms from the fossils of ocean animals, such as coral or mollusks
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Coal is another of this type
Sedimentary Rock Structures
Stratification is the process in which sedimentary rocks are arranged in layers
Strata differ from each other depending on the kind, size, and color of the sediment
Can record the motion of wind and water waves on lakes, oceans, rivers, and sand dunes in features called ripple marks
Structures called mud cracks form when fine grained sediments at the bottom of a shallow body of water are exposed to
the air and dry out
Section 4: Metamorphic Rocks
Metamorphic rocks are rocks in which the structure, texture, or composition of the rock have changed, or gone through
metamorphism.
Origins of Metamorphic Rock
Form at 50 to 1,000 ⁰C, but some form at temperatures that are higher!
It is the depth and pressure that the rock forms that allows it to heat up to this temperature and still remain a solid.
Two Types of Metamorphism:
1.
Contact metamorphism
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When rock is heated by nearby magma
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Some minerals in the surrounding rock are changed into other minerals by this increase in temperature
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Contact metamorphism occurs near igneous intrusions
2. Regional Metamorphism
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When pressure builds up in rock that is buried deep below other rock formations or when large pieces of
the Earth’s crust collide with each other, regional metamorphism occurs
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Occurs over thousands of cubic kilometers deep within Earth’s crust
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Are found beneath most continental rock formations
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Composition of Metamorphic Rock
Index Minerals are certain minerals that only form at certain temperatures and pressures so they are used to estimate
the temperature, depth, and pressure at which a rock undergoes metamorphism.
They include: biotite mica, chlorite, garnet, kyanite, muscovite mica, sillimanite, and staurolite
2 Textures of metamorphic Rock:
1.
Foliated
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Mineral grains are arranged in plains or bands
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Usually contains aligned grains of flat minerals such as mica or chlorite
2. Nonfoliated
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The mineral grains are not arranged in planes or bands
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Commonly only made of one or only a few minerals
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During metamorphism, the crystals of these minerals may change in size or the minerals may change in
composition in a process called recrystallization
Metamorphic Rock Structures
Deformation is a change in the shape of a rock caused by a force placed on it. (these forces may cause a rock to be
squeezed or stretched)
Folds, or bends are structures that indicate that a rock has been deformed.
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