Rocks
What Rocks Mean
Sedimentary Rocks
• Record of the earth's ancient surface environments
and of life.
• Record of crustal stability or disturbance.
Igneous Rocks
• Indicate crustal disturbance
• Unusual heat in the crust or mantle.
Metamorphic Rocks
• Indicate crustal disturbance
• Indicate uplift and erosion
• Indicate orogenic (mountain-building) events
What Rocks Tell Us
Rock Type
Igneous
Sedimentary
How Classified
Composition
Texture
Chemical
Composition
Grain Size
Composition
Metamorphic
Mineral Makeup
Texture
What it Tells Us
Tectonic Setting
Cooling History
Surface
Environment
Energy of
Environment
Original Rock Type
Temperature,
Pressure
Degree of Change
Igneous Rocks Cool from the
Molten State
• Volcanic -- Erupted on Surface
• Plutonic -- Solidify Within Earth
Large Grain Size ---> Slow Cooling
• Volcanic Rocks -- Fine Grained
• Plutonic Rocks -- Coarse Grained
Porphyritic Texture:
Large Crystals in Fine-grained Setting
Igneous Rock Classification
How Much Silica? (account for Si)
• Excess - Rock Has Quartz
• Just Enough to Form Other Silicates
• Deficient - Silica-Poor Minerals (Like Olivine)
What Feldspars? (Account for Al, Ca, K, Na )
• Potash Feldspar KAlSi3O8
• Plagioclase Series
NaAlSi3O8......CaAl2Si2O8
What Other Minerals Are Present? (Account for
Fe, Mg)
Bowen's Reaction Series
• The geologist N.L. Bowen found that
minerals tend to form in specific sequences
in igneous rocks
• These sequences could be assembled into a
composite sequence.
Bowen's Series and Igneous Rocks
Bowen's Series and Igneous
Rocks
Volcanic Rocks
(Rare) Basalt Andesite
Rhyolite
Plutonic Rocks
Dunite Gabbro Diorite
Granite
1200 C
Melting Point
700 C
Mg, Fe
Rich In...
Si, Na, K
Rapid
Weathering
Slow
Usually Dark
Color
Often Light
Bowen's Series and Volcanoes
Volcanic Rocks
(Rare) Basalt Andesite
Rhyolite
Plutonic Rocks
Dunite Gabbro Diorite
Granite
Fluid
Lava Is...
Viscous
Mild
Eruptions
Violent
Type of Volcano
Shield Volcano Stratovolcano Plug Dome
Some Igneous Rocks Are
Named on Textural Criteria
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•
•
•
•
Pumice - Porous
Obsidian - Glass
Tuff - Cemented Ash
Breccia - Cemented Fragments
Porphyry - Fine Matrix, Large Crystals
What Igneous Rocks Mean
Basalt
• What you get if you melt average planetary material
Andesite
• Mantle material mixes with continental crust
• Volcanoes on converging plate boundaries
Rhyolite
• Forms from melting continental crust
• Long period of crustal heating
Granite
• Nothing in the earth can melt directly to yield granite
• Long continued crustal activity and re-working of rocks
Sedimentary Rocks are the
Principal Repository for
Information About the Earth’s
Past Environment
Environmental Clues in
Sedimentary Rocks
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•
•
•
•
Grain Size - Power of Transport Medium
Grading - Often Due to Floods
Rounding
} Transport, Reworking
Sorting
Cross-bedding - Wind, Wave or Current
Action
Environmental Clues in
Sedimentary Rocks
• Fossils
– Salt Water - Corals, Echinoderms
– Fresh Water - Insects, Amphibians
– Terrestrial - Leaves, Land Animals
• Color And Chemistry
– Red Beds - Often Terrestrial
– Black Shale - Oxygen Poor, Often Deep Water
– Evaporites – Arid Climates
Bedding or Stratification
• Almost Always Present in Sedimentary
Rocks
• Originally Horizontal
• Tilting by Earth Forces Later
• Variations in Conditions of Deposition
• Size of Beds (Thickness)
– Usually 1-100 Cm
– Can Range From Microscopic to 50m
Sedimentary Rocks
Clastic Rocks
• Made of Fragmentary
Material
• Deposited by
– Water (Most
Common)
– Wind
– Glacial Action
– Gravity
Biochemical
Sedimentary Rocks
• Evaporation
• Precipitation
• Biogenic Sediments
Clastic Rocks
Classified by:
• Grain Size
• Grain Composition
• Texture
Sediment Sizes and Clastic
Rock Types
Rock Type
Sediment
Grain Size
Shale
Clay
less than 0.001 mm
Siltstone
Silt
.001-0.1 mm
Sandstone
Sand
.01-1 mm
Conglomerate Gravel
1mm +
Sedimentary rocks made of silt- and clay-sized
particles are collectively called mudrocks,
and are the most abundant sedimentary rocks.
Diagenesis
Compaction
Cementing
• Quartz
• Calcite
• Iron Oxide
• Clay
• Glauconite
• Feldspar
Alteration
• Limestone - Dolomite
• Plagioclase – Albite
Recrystallization
• Limestone
Chemical Sediments
Evaporites -Water
Soluble
• Halite
• Gypsum
• Calcite
Precipitates
Example: Ca(sol'n) +
SO4 (Sol'n) = CaSO4 • Gypsum
• Limestone
• Iron Formations
Alteration After
Deposition
• Dolomite
Biogenic Sediments
• Limestone - Shells,
Reefs, Etc.
Organic Remains
• Coal
• Petroleum
Fossil Fuels
Coal
• Delta, continental
environments
• Carbonized Woody
Material
• Often fossilized trees,
leaves present
Petroleum
A hydrocarbon molecule
What organisms make these?
Answer: None
Petroleum
• Lots of organisms make these, however
• Fatty Acids
• Probable source: Marine plankton
Petroleum is a Fluid
• It Migrates
• It Floats Upward on Water
• It either reaches the surface or accumulates
under some trap
• There is no such thing as an “oil pool”
Petroleum Traps
What Sedimentary Rocks Mean
Clastic Rocks
• Power of transport mechanism
Mineral Makeup of Clastic Rocks
• Stable vs. Unstable Settings
Carbonate Rocks
• Virtually all are biological in origin.
Evaporites
• High evaporation settings, deserts or hot tidal flats
Locale for Petroleum
Tilting or Fracturing of Crust
Metamorphism
•Changes in Rock Composition or Texture
•Due to Heat, Pressure and Action of Fluids
We Do Not Live at “Normal”
Conditions
• By the standards of Earth’s interior, we live
in a frozen vacuum
• Things that look “abnormal” to us are normal
behavior for materials
– Solids can flow
– Solids can react chemically with each
other
– A given material can have several
different atomic structures
Chemical Changes in Rocks
Weathering
• At Surface
Diagenesis
• Sedimentary Rocks
Metamorphism
• Starts about 200 C
• Outside range of
normal near-surface
conditions
Where Do the Heat and Pressure
Come from?
Heat: Radioactive decay of Uranium,
Thorium, Potassium-40
Pressure: Weight of Overlying Rocks
• Air Pressure = 14 P.s.i. (1 Atmosphere or
100 Kpa) = 10 m water = 3.5 m rock
• Pressure in Deepest Part of Ocean = 1000
Atmospheres = 100 Mpa = 3.5 km rock
Types of Metamorphism
Contact
• Around Intrusions
• Shallow: 0-6 Km
• Low Pressure
• Local
Regional
• Wide Areas
• 5-20 Km, Sometimes
30+
• High Pressure
• Usually Accompained
by Deformation
What Happens During
Metamorphism
Minerals React to Form New Minerals
Minerals Change Form
New Materials Are Added
(Metasomatism)
• Minerals in Solution == Ore Bodies
Recrystallization
Why Don't Rocks
"De-metamorphose"?
Reactions Can't Reverse Because Ingredients
Lost
• Almost all reactions involve loss of water or
CO2
Reactions "Freeze" at Low Temperature
Sometimes it Does Happen if Fluids Present
• Retrograde Metamorphism
• On the surface we call it weathering
Metamorphic Grade Degree to Which the Rock Has Changed
Composition
• Can Often See Original Bedding
• Can Sometimes Even See Deformed Fossils
• At High Grades, Rocks Can Often Lose All
Trace of Their Original Appearance
Polymorphism
Al2SiO5
• Andalusite
• Kyanite
• Sillimanite
Ice - 6 high pressure
forms
Diamond - Graphite
Calcite - Aragonite
Quartz • - Tridymite Cristobalite (increasing
temperature)
• - Coesite - Stishovite
(increasing pressure)
Metamorphic Facies
Depth\Temp
300C
400C
5 km
Zeolite
Contact Metamorphism - Andalusite forms
10 km - 3 kb
Greenschist
Blueschist Chlorite,
Biotite form
•Slate
•Greenstone
•Quartzite
•Marble
Amphibolite
Garnet,
Staurolite,
Kyanite form
•Schist
•Amphibolite
•Quartzite
•Marble
•Gneiss
Not Found
Eclogite (Mantle)
15 km
20 km - 6 kb
25 km
30 km - 9 kb
35 km
40 km - 12 kb
500 C
600 C
700 C
800 C
Granulite
Sillimanite forms
Muscovite breaks
down to Kfeldspar
Partial Melting
•Gneiss
Mantle Rocks
What Metamorphic Rocks Mean
Originally far below the surface and got to the
surface by uplift and erosion.
Greenschist Metamorphism
• Margins and upper levels of mountain belts
Amphibolite Metamorphism
• Cores of mountain belts
Granulite Metamorphism
• Extreme metamorphism. Rocks of the lower crust.
Signify extreme uplift (30 km and more).
Blueschist Metamorphism
• Somehow got very deep and then back to the surface
quickly. Plate collision boundaries.