Rocks

1
Rocks

• Chapter 10

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Rocks the Rock Cycle

• Magma is the parent material for all rocks
• There are 3 major types
• Any type can be changed into any of the other
  types through a series of changes called the rock
  cycle

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3 Major Rock Types

• Three major types
• Igneous magma cools hardens
• Sedimentary sediment is deposited and hardens
  after being compressed cemented.
• Metamorphic forces such as extreme heat and
  pressure and chemical processes change the form
  of existing rock.

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The Rock CycleOverview

• Hardened magma cools -gt igneous rock
• Igneous rock breaks down into sediments -gt
  sedimentary rock
• Sedimentary rock subjected to extreme
  heat/pressure -gt metamorphic rock
• Metamorphic rock under extreme heat/pressure -gt
  melt into magma (start over)

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The Rock Cycle (cont)

• Rock does not always make it through entire cycle
• Igneous rock not exposed to surface will never
  become sediment may go directly to metamorphic
• Igneous and sedimentary may melt directly into
  magma
• All 3 types may form sediments when exposed

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The Rock Cycle
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Igneous Rock

• Two types classified by where magma cools
• intrusive cool deep below crust
• May intrude into other rock layers
• Cools slowly
• extrusive lava cooling on surface
• Cools rapidly

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Igneous Rock (cont)

• Texture determined by rate of cooling
• Intrusive cools slowly
• Course-grained texture
• Ex granite
• Extrusive cools rapidly
• Little time for crystal formation
• Fine-grained texture
• Ex basalt

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Igneous Rock (cont)

• Some extrusive rock cools slowly at first then
  more quickly as it nears the surface
• Produces both large and small crystals
• Porphyry
• Extremely rapid cooling produces rock without
  crystals
• Gases escaping during rapid cooling may form
  small bubbles in rock

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Igneous Rock (cont)

• Composition of igneous rock is determined by
  chemical composition of magma.
• 3 Families based on composition
• Granite
• Basalt
• Diorite

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Igneous Rock (cont)

• Granite Family
• Felsic magma high silica
• Light color
• Main minerals orthoclase feldspar quartz
• Ex granite (intrusive)
• rhyolite and obsidian (extrusive)

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Igneous Rock (cont)

• Basalt Family
• Mafic lava low silica, high iron
• Dark color
• Main minerals plagioclase feldspar and augite
• Ex basalt gabbro

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Igneous Rock (cont)

• Diorite Family
• Medium colored
• Main minerals plagioclase feldspar, hornblende,
  augite, and biotite.
• Little or no quartz
• Ex diorite andesite

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Igneous Rock StructuresIntrusions underground

• Batholith
• Largest (cover over 100km²)
• Form cores of many mountains
• Stock
• Similar to batholith but cover less area
• Laccolith
• Magma pushes overlying rock layer into an arc
• May form dome mountains
• Sills
• Sheet of magma flows between layers of rock
  (horizontal)
• Dikes
• Magma flows upward through fractures and cut
  across rock layers vertically

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Igneous Intrusions
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Igneous Rock StructuresExtrusions at surface

• Volcanic neck softer parts of volcanic cones
  wear away exposing solidified rock in vent.
• Many are simply flat lava flows
• Lava plateaus lava flows out long cracks in the
  surface
• Cover vast areas
• Fill in valleys and cover hills to form flat
  plateaus

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Igneous Extrusions
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Sedimentary Rock

• Compaction and cementation form sedimentary rock.
• Compaction weight of overlying sediments causes
  pressure that squeezes air and water out of
  sediments
• Cementation water carries dissolved minerals
  through sediments which provide a cement to hold
  fragments together.
• Sedimentary rocks are classified by the kind and
  size of their sediments.

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Sedimentary Rock (cont)

• Clastic sedimentary rocks
• Rock fragments are carried by wind, water and ice
  become compacted and cemented together
  classified by size
• Conglomerate rounded, gravel-sized pebbles
  cemented by minerals
• Breccia gravel-sized fragments (angular with
  sharp edges)
• Sandstones are made up of sand-sized grains
• Major component quartz
• Many are porous enough to allow liquids (water,
  crude oil) to move through.
• Shale consists of clay-sized particles
• Usually pressed into flat, flakey layers

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Sedimentary Rock (cont)

• Chemical Sedimentary Rock
• Forms from minerals once dissolved in water
• Minerals may precipitate out due to temperature
  changes
• Evaporites form when water evaporates and leaves
  minerals behind

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Sedimentary Rock (cont)

• Organic Sedimentary Rock
• Forms from remains of living things
• Coal forms from dead plant remains buried and
  compacted
• Limestone forms from the shells of marine
  organisms that use calcite
• Chalk is limestone made of shells of one-celled
  marine organisms

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Sedimentary Rock Features

• Stratification layering occurs when type of
  sediment changes
• May be due to change in river current, sea level,
  etc.
• Thickness of layers (beds) depends on how long
  the sediment is deposited
• Most water-deposited layers are horizontal
• Wind-deposited layers may exhibit cross-bedding
• Graded bedding different kinds sizes of
  material are deposited in the same layer as they
  settle out.

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Sedimentary Rock Features (cont)

• Ripple marks
• Form from action or wind or water on sand.
• Mud cracks
• Muddy deposits dry and shrink causing shrinking
• River flood plains, dry lake beds
• When area floods new deposits fill in and
  preserve the cracks.

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Sedimentary Rock Features (cont)

• Fossils
• The remains or traces of ancient plants and
  animals
• Are preserved in sedimentary rock when remains
  are buried
• Harder parts may remain but most often its only
  the impression

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Sedimentary Rock Features (cont)

• Concretions
• Lumps or nodules of rock with a composition
  different from the main rock body
• Precipitated minerals build up around existing
  rock
• Geodes groundwater deposits dissolved quartz or
  calcite inside a cavity and crystals form

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Metamorphic Rock

• Changing from one type of rock to another by
  heat, pressure and chemical processes
  metamorphism
• Minerals may change into other chemicals
• Minerals may change size, shape, or separate into
  bands (layered look)
• Hot fluids in magma may change mineral
  composition by dissolving some minerals and
  adding others

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Metamorphic Rock (cont)

• Two types of metamorphism
• Contact metamorphism
• Hot magma is near or touching rock
• Changes structure and/or mineral composition
• May also include hot chemical fluids working
  through fractures
• Regional metamorphism
• Occurs over large areas during tectonic activity
• Plates moving against each other causes
  tremendous heat and pressure at the edges
  causes chemical changes in rock minerals
• Most metamorphic rock is formed this way.

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Metamorphic Rock (cont)

• Classification is according to structure
• Foliated
• Visible parallel bands
• Extreme pressure flattens mineral crystals into
  bands
• Minerals of different densities may separate into
  bands -gt alternating light and dark

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Metamorphic Rock (cont)

• Common foliated rocks
• Slate
• Pressure on shale produces thin layers that split
  into sheets
• Schist
• Pressure on slate courser grained
• Gneiss
• Intense heat and pressure on schist -gt minerals
  separate into bands of different densities

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Metamorphic Rock (cont)

• Unfoliated
• Rocks do NOT have bands of crystals
• Quartzite sandstone is highly compacted
• Marble compression of limestone