Download Chapter 3 Rocks

Chapter 3
Rocks
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
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
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
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
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
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
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
Dynamic Rock Cycle
Igneous Rocks
• Different kinds of
igneous rock form
when magma and
lava cool and harden
– Intrusive
– Extrusive
Intrusive Igneous Rocks
• Form when magma
hardens beneath
Earth’s surface
– Intrude into existing
rocks
– Common rock: granite
Extrusive Igneous Rocks
• Form when lava
hardens
– They are extruded
onto the surface
– Common: rhyolite
Classification of Igneous Rocks
• Two main characteristics:
– Texture
• Size
• Shape
• Interlocking crystals
– Composition
• light vs. dark minerals
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
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
Porphyritic – (large
crystals surrounded by
fine-grained minerals)
4.
–
Formed when minerals
that crystallize from
magma do not form at
same rate or same time
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
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
Sedimentary Rocks
•
Form when existing rocks broken down
into sediments
1. Weathering - any process that breaks
rocks into sediment
2. Erosion – water, wind, ice, gravity
3. Deposition – loses energy, drops
sediment
•
•
Sediments deposited according to size
Largest deposited first – smallest deposited
last
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
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
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
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)
Metamorphic Rocks
• Existing rocks are
changed by heat and
pressure
– Conditions are a few
km below surface and
extend into the upper
mantle
Two Types of Metamorphism
1. Contact
metamorphism: hot
magma moves into
rock
•
•
Low-grade
metamorphism
Marble – forms from
limestone
Two Types of Metamorphism
2. Regional
Metamorphism
•
•
During mountain
building, large areas
of rock subjected to
extreme pressures
and temps
High-grade
metamorphism
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
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
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
Foliated Metamorphic
Shale
Slate
Schist
Gneiss
Classification of Metamorphic
Rocks
2. Nonfoliated:
• Does not have
banded texture
• Most only contain
one mineral
• Limestone (made
of calcite) - marble
limestone
marble