Download 103-04-RocksIntrod-2006(Lesson08)

ROCKS
GEOL 1033
General Oceanography
Review Lesson 8 in the Study Guide
THREE TYPES OF ROCKS
• Geologists
classify
Earth
materials
according to naturally occurring minerals &
rocks, rather than by elements or chemical
compounds.
• According to the genetic classification of
rocks, there are 3 types of rocks:
1. Igneous rocks
2. Metamorphic rocks
3. Sedimentary rocks
1)
Igneous rocks:
• Once molten:
– Cooled, crystallized, hardened
– Lava is extrusive
– Magma is intrusive
• Examples:
– Granite (K & Na Al silicates) of continental crust
– Basalt (Fe, Mg, & Ca Al silicates) of oceanic crust
– Iron (Fe) & magnesium (Mg) silicates of Earth’s mantle
1)
Igneous rocks:
• Minerals of granitic continental crust:
–
–
–
–
Quartz (SiO2) abundant
K-feldspar (= K-rich Al silicates ) more abundant than quartz
Only a few % dark Fe & Mg silicate minerals
Form light-colored rocks that are not as dense as oceanic crust
• Grey granites
• Pink granites
• Etc.
GRANITE
• From a magma
rich in silica
•
• Quartz
Feldspar
Muscovite
Biotite
Hornblende (Orthoclase)
1)
Igneous rocks:
• Minerals of basaltic/gabbroic oceanic crust
–
–
–
–
Pyroxene (=dark Fe, Mg, & Ca-rich Al silicates)
Na & Ca feldspars abundant (instead of K-feldspars of granite)
No quartz
comparison with continental crust:
• Denser
• Darker
• Composition & minerals different
GABBRO/BASALT
• From a
Silicate
magma
rich in
Fe & Mg
Feldspar
Pyroxene (Fe+Mg-rich) Olivine
(Plagioclase)
CLASSIFICATION OF IGNEOUS ROCKS
(Small cx)
(Large cx)
2)
Metamorphic rocks:
• Deformed rocks by
– 1) High temperature &/or
– 2) High pressure &/or
– 3) Hot fluids
• Found associated with deformational
mountains, folds, faults, i. e., where
cont. & oceanic plates collide.
• New minerals, but not much chemical
change in rock composition
• Examples:
– Slate forms from shale
– Marble forms from limestone
– Quartzite forms from quartz sandstone
• Not a common oceanic rock type (least
important of the 3))
3)
Sedimentary rocks:
• Sedimentary Rocks come from sediments
• Sediments:
– Abundant on the seafloor (veneer the basalt layer of crust below)
– Mostly loose grains (eroded from previously existing rocks)
– May be skeletal debris (shells, etc.), plant materials (coal)
– May be chemical precipitates from seawater (salts)
• 4 basic components:
– a. Grains
(larger particles)
– b. Matrix
(fines)
– c. Pores
(air, H2O, oil, or gas may fill them)
– d. Cement
(chemical precipitates in pores after burial,
usually post-depositional)
What are the two most important
properties of sediments & sedimentary
rocks?
• 1. Grain size
• 2. Composition
1. Grain size
• Varies from clay sizes to boulders:
•
•
•
•
•
•
•
clay sizes
silt
sand
granule
pebble
cobble size
boulder
= less than 1/256 mm
= 1/256 to 1/16 mm
= 1/16 to 2 mm
= 2 to 4 mm
= 4 to 64 mm
= 64 to 256 mm
= greater than 256 mm
Calcium carbonate sand grains
Quartz-rich sandy beach
2. COMPOSITION – TWO MAJOR ABUNDANCE
CATEGORIES OF MARINE SEDIMENTS
• A. Silicates
– Dominated by the elements silicon (Si) & oxygen (O)
– Fine-grained (clays, mud, etc.)
• Clay minerals (silicates) dominate
• Tiny fragments of other silicate minerals
– Sands
• Quartz dominates (Silica = SiO2)
• Some feldspar grains (Na, K, Ca-rich Al silicates)
• B. Carbonates
– Dominated by calcium carbonate (CaCO3) [Ca, C & O]
– Characterize coasts with
• Warm climates (low latitudes) &
• Isolated from abundant silicate sediments
• Invertebrates and algal shells/skeletons dominate
coarse shallow-water carbonate sediments
– Characterize deep-sea sediments that
• Have little clayey sediment &
• Not too deep
• Fine-grained calcareous "oozes" cover 48% of the
deep-sea floor
Examples of Sedimentary Rocks:
Cemented rock
1. conglomerate
2. sandstone
3. shale
4. limestone
5. crystalline
"evaporite"
Loose grains
gravel
sand
mud, clay
shells, "lime" mud
loose crystal
"mush"
Single grain
pebble
sand
clay
shell, etc.
crystal
RELATIVE ABUNDANCE OF ROCK TYPES
(Volume % estimates)
• Continental Crust:
– Granitic average composition
– Igneous & Metamorphic rocks =
– Sedimentary =
93%
7%
• Oceanic Crust:
– Basaltic average composition
– Igneous (averages about 5-6 km thick) =
– Sedimentary veneer (up to 1 km thick) =
• fine-grained CaCO3
• "
"
SiO2·nH2O
• "
"
clays (silicates)
skeletons of
micro-organisms
89%
11%
ROCK CYCLE
Cooling &
MAGMA
Crystallization
Melting at Higher
Temperatures
IGNEOUS
ROCKS
METAMORPHIC
ROCKS
Weathering
High Temperature
Erosion
and/or
Transportation
High Pressure
Deposition
and/or
Hot Fluids
(=Metamorphism)
SEDIMENTS
Cementation (=Lithification)
SEDIMENTARY
ROCKS
END OF FILE
A little Chemistry Review
• Silicates = complex compounds dominated by
Silicon & Oxygen
• Elements symbolized by letters, e. g., C = Carbon.
• Also know:
– Si = Silicon
Form silicate
– O = Oxygen
tetrahedrons
– K = Potassium
– Na = Sodium
– Al = Aluminum
A Little More Chemistry Review
• Si = Silicon
• O = Oxygen
•
•
•
•
Al = Aluminum
Fe = Iron
Mg = Magnesium
Ca = Calcium
Form silicate
tetrahedrons