Download 6.2 Igneous Rocks

Chapter 6
A. What is a Rock?
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Minerals bound
together
Can be two, can be
many in different
quantities
Found in Earth’s Crust
and Mantle
B. Rock Classification
 Rocks are classified according to the process by which
they are FORMED
 Igneous –cooling of magma
 Sedimentary- compacting of sediment layers
 Metamorphic- rocks subjected to heat and pressure
C. The Rock Cycle
 The process (cycle) by
which rocks continually
change from one type to
another type.
 What causes change?
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Weathering of rocks
Melting of Rocks
Subjecting rocks to
intense heat and
pressure underground
 Any rock can change into any other type of rock
 It can even turn into another form of its original type
 Ex- Granite (igneous) can melt underground and turn
into Obsidian (also igneous)
 Some rocks can even be formed by things outside of
the rock cycle
 Ex- Shells from living things can weather into sediment
or become part of a rock as a whole (Fossiliferous
Limestone)
A. Igneous Rock Formations

Classified by mineral composition and texture
Most form directly from magma
Location of magma determines cooling rate
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Underground – Intrusive
Above Ground- Extrusive
1. The Starting Material
 Flesic Magma
 Thick, slow moving
 Large amounts of Silica
 Makes light-colored
rocks
 Mafic Magma
 Thinner, more fluid
 Large amounts of Iron
and magnesium
 Makes dark rocks
2. Underground Magma
 Cools slowly
 Makes large crystal
grains
 Can take thousands of
years
 Intrusive
 Appear when erosion
occurs
3. Surface Magma
 Magma becomes Lava at
surface
 Cools, forms volcanic
(extrusive) rock
 Cools more quickly
 Creates smaller or no
crystals at all
B. Igneous Rock Descriptions
 Grouped by mineral composition
 One family can include both:
 Intrusive and Extrusive
 Coarse and Fine grained
1. Granite Family
 Form from Felsic Magmas
 Usually coarse grained
 Quartz, Feldspar, Mica, Hornblende
1A. Granite
 Granite – Feldspar usually determines color
 White to Grey to Pink
 Intrusive
 Quarried or found at surface after erosion
1B. Obsidian
 Volcanic, glassy texture
 Chemical Composition resembles granite
 Many Iron Oxides
Obsidian
Windchimes
1C. Pumice
 From silica-rich lava
 Gas bubbles trapped in it
 Can float on water
2. Gabbro Family
 Mafic Rocks
 Pyroxene, Olivine, plagioclase feldspar, amphibole,
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biotite (all dark in color)
Gabbro- coarse grained
Basalt – fine grained
Diabase – in between
Scoria – like pumice, but darker and denser
Basalt Glass
Gabbro
(Coarse)
Diabase
(Medium)
Scoria
Basalt
(Fine)
Basalt Glass
3. Diorite Family
 Characteristics of both mafic and felsic
 Medium grays and greens
Diorite
Andesite
C. Igneous Intrusions
 Pluton- any igneous formation below Earth’s Surface
Sill
Dike
Laccolith
Volcanic Neck – Devil’s Tower, Wyoming
Batholith – Half Dome, Yosemite National Park, Calif.
1. Formation of Sedimentary Rocks
Basically made through the
compacting and cementing of
sediments
Can happen in three different ways.
A. Clastic Rocks
 Formed from fragments of other rocks
 Those fragments come from weathering
 Can be many different sizes
 Pebbles, gravel, sand, silt, clay
1. Formation of Clastic Rocks
 Movement and relocation of rock fragments
 Mostly carried by water
 Fragments are smoothed by friction
 Heavier material drops out before lighter
material
 Determines what rock forms where
2. Cementation
 All water contains dissolved minerals
 When minerals fill the spaces between fragments, the
fragments are bound together
 Type of cement determines rock color
 Silica, calcite- light rocks
 Iron based – dark, red, rusty
3. Types of Clastic Rocks
 Conglomerate
 Coarsest
 Sandstones
 Quartz, sand grains
 Usually porous and permeable
 Shale
 Silts and Clays
 Impermeable, smooth
B. Chemical Sedimentary Rock
 Water in lakes, seas, swamps and
underground reservoirs contain
dissolved minerals
 Minerals precipitate out of solution by
evaporation or chemical action
Halite (Rock Salt)
Rock Gypsum
C. Organic Sedimentary Rock
 Sediments from plant and animal remains
 Limestone
 Contains Calcite
 Water dissolves calcite, organisms use it to
make shells
 Organisms die, shells settle to bottom
 Become cemented into limestones
D. Sedimentary Rock Features
 Stratification
 Change in sediment layers
 Bedding plane- separates layers

Usually horizontal, sometimes angled
 Fossils
 Commonly found in sedimentary rock
 Impressions or actual remains
 Ripple marks
 Wind, streams, waves make ripples
 Preserved in rock
 Mud Cracks
 Wet clay dries and contracts
 Fills with other sediment, fossilizes
 Nodules
 Found in limestone and chalk
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Light: Chert
Dark: Flint
 Concretions
 Found in Shale
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Made of Calcium Carbonate
 Geodes
 Found in Limestone
 Made of silica rock
Formed from “Parent Rocks” – Metamorphic rocks usually
resemble their parent rocks.
A. Metamorphic Processes
 A change by heat and pressure
 Can originate from
 Earth’s internal heat
 Overlying weight
 Rock deformation from mountain building
 Can differ from parent rock in chemical composition,
structure and texture
1. Regional Metamorphism
 Occurs in large areas
 Mountain Building
 Rocks are subjected to lots of pressure
and heat from below
 Degree of metamorphism depends on
amount of heat, pressure and
fluids/gasses
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Extreme – high temp/pressure
Low- lower temps/pressure
2. Local Metamorphism
 Contact Metamorphism
 Hot magma moves into rock
 Small changes, small size (no larger than 100 m)
 Deformational Metamorphism
 Low Temp, high pressure
 At faults where rocks move against each other
 Heat from friction and pressure changes the rock
Contact
Metamorphism
Regional
Metamorphism
B. Metamorphic Rock Descriptions
 Quartzite
 Comes from sandstone
Sandstone
Quartzite
 Marble
 From Limestone
 Mostly Calcite
Limestone
Marble
Slate (Low metamorphism)
Phyllite (Higher
Metamorphism)
Shale
Schist (Highest Metamorphism)
 Gneiss
 From shale, granite, conglomerate
 Highly metamorphosed
 Named for chemical composition