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Chapter 3
Rocks
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Rocks
• Rock – any solid mass of mineral or mineral-like matter that occurs naturally as part of our planet– Usually solid mixtures
of minerals– Some composed of
just one mineral– Each mineral retains
its properties in mixture
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The Rock Cycle
• Three Major Types– Igneous– Sedimentary– Metamorphic
• Interactions among three things cause rocks to change from one type to another:– Water– Air– Land
• Rock cycle – continuous processes of rocks changing
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The Rock Cycle – Igneous Rocks
• Magma – molten material that forms beneath Earth’s surface
• Lava – magma that reaches the surface
• Igneous rocks– Form when magma
cools and hardens beneath surface -OR-
– Volcanic eruption
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The Rock Cycle – Sedimentary Rocks
• Weathering – rocks are physically and chemically broken down by water, air, living things.
• Sediments – weathered pieces of earth materials– Moved by water, gravity, glaciers, wind
• Sedimentary rocks formed by:– Sediments being compacted and cemented
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The Rock Cycle – Metamorphic Rock
• Sedimentary rock – buried deep within Earth– Exposed to increased pressure and temp
• Metamorphic rock formed:– Sedimentary rock exposed to extreme
pressure and temp
• When metamorphic rocks exposed to additional pressure or higher temps:– Melt to form magma -> eventually crystallizes
to form igneous rock again
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Alternate Paths
• Igneous Rock: – Some remains deeply buried: exposed to
strong forces & increased temp – becomes metamorphic rock
– If pressure & temp high enough – could melt and reform igneous
• Metamorphic and sedimentary:– Could weather to become sediments again– Then become sedimentary rocks again
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Energy
• Igneous & metamorphic rocks– Both driven by heat from Earth’s interior
• Sedimentary rocks– Weathering and movement of weathered
materials– External processes powered by energy from
the sun
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Dynamic Rock Cycle
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Igneous Rocks
• Different kinds of igneous rock form when magma and lava cool and harden– Intrusive– Extrusive
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Intrusive Igneous Rocks
• Form when magma hardens beneath Earth’s surface– Intrude into existing
rocks– Common rock: granite
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Extrusive Igneous Rocks
• Form when lava hardens– They are extruded
onto the surface– Common: rhyolite
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Classification of Igneous Rocks
• Two main characteristics:– Texture
• Size• Shape• Interlocking crystals
– Composition• light vs. dark minerals
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Igneous Rocks – Texture
1. Coarse-grained – Slow cooling results
in formation of large crystals
2. Fine-grained– Rapid cooling results
in small, interconnected mineral grains
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Igneous Rocks – Texture
3. Glassy– Formed when ions in
lava do not have enough time to arrange themselves in network of crystals
– Ex. – obsidian, pumice
4. Porphyritic – (large crystals surrounded by fine-grained minerals)
– Formed when minerals that crystallize from magma do not form at same rate or same time
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Igneous Rocks – Composition
1. Granitic Composition• Light-colored silicate minerals• Major rocks of continental crust – 70% silica• Ex – rhyolite: extrusive granitic rock
2. Basaltic Composition• Dark silicate minerals & plagioclase feldspar• Rich in iron and magnesium• Darker and denser than granitic rocks• Ex – basalt, gabbro
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Igneous Rocks - Composition
3. Other Compositional Groups• Andesitic composition –
between granitic and basaltic rocks
• common volcanic rock – andesite• 25% dark silicate minerals
• Peridotite – much of upper mantle
• Ultramafic – composed mostly of dark minerals
• Rare at Earth’s surface
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Sedimentary Rocks
• Form when existing rocks broken down into sediments
1. Weathering - any process that breaks rocks into sediment
2. Erosion – water, wind, ice, gravity3. Deposition – loses energy, drops
sediment• Sediments deposited according to size• Largest deposited first – smallest deposited
last
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Sedimentary Rocks
• Processes that change sediments into sedimentary rocks:
1. Compaction – squeezes, or compacts sediments
• Much of the water is driven out
2. Cementation – dissolved minerals deposited in tiny spaces among sediments
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Classification of Sedimentary Rocks
• Classified into two groups according to the way they form:
1. Clastic sedimentary rocks – rocks made of weathered bits of rocks and
minerals– Conglomerate – gravel-sized or larger particles make up
most of rock– Breccia – angular particles– Sandstone – sand-size grains
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Classification of Sedimentary Rocks
2. Chemical and biochemical sedimentary rocks– Dissolved minerals precipitate from water
solutions– Occurs when water evaporates or boils off, leaving
a solid product– Limestones, rock salt, chert, flint, rock gypsum
90 % of limestones formed from biochemical sediments Shells and skeletal remains of organisms on ocean floor
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Features – Sedimentary Rocks
• Can give clues to how, when, and where rocks formed
1. Each layer: records a period of sediment deposition• Oldest layers found at bottom
2. Ripple marks: rock formed along beach or stream bed
3. Mud cracks: record of a dry environment
4. Fossils: answer questions about rocks– Did rock form on land or ocean?– Was climate hot or cold?– Match rocks from different places (compare age)
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Metamorphic Rocks
• Existing rocks are changed by heat and pressure– Conditions are a few
km below surface and extend into the upper mantle
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Two Types of Metamorphism
1. Contact metamorphism: hot magma moves into rock
• Low-grade metamorphism
• Marble – forms from limestone
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Two Types of Metamorphism
2. Regional Metamorphism• During mountain
building, large areas of rock subjected to extreme pressures and temps
• High-grade metamorphism
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Agents of Metamorphism
1. Heat – most important agent• Comes from two sources: magma and change in
temp with depth• Provides energy to drive chemical reactions• Minerals are stable at different temps
2. Pressure (Stress)• Increases with depth• Pressure on rocks from within Earth comes from all
directions• Causes rocks to flow, not fracture
• Minerals flatten and elongate
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Agents of Metamorphism
3. Hydrothermal Solutions
• When solutions increase in temp, reactions among substances occur at a faster rate
• Promote recrystallization by dissolving original minerals and depositing new ones
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Classification of Metamorphic Rocks
1. Foliated• Some minerals
recrystallize at right angles to the direction of force
• Gives rock a layered or banded appearance
• Ex: shale – slate – schist – gneiss
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Foliated MetamorphicShale Slate
Schist Gneiss
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Classification of Metamorphic Rocks
2. Nonfoliated:• Does not have
banded texture• Most only contain
one mineral• Limestone (made
of calcite) - marble
limestone
marble