ROCKSGEOL 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 rocks3. 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

• Feldspar

• Quartz 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 = less than 1/256 mm• silt = 1/256 to 1/16 mm• sand = 1/16 to 2 mm• granule = 2 to 4 mm• pebble = 4 to 64 mm• cobble size = 64 to 256 mm• boulder = greater than 256 mm

Quartz-rich sandy beach

Calcium carbonate sand grains

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 Loose grains Single grain

1. conglomerate gravel pebble2. sandstone sand sand3. shale mud, clay clay4. limestone shells, "lime" mud shell, etc. 5. crystalline loose crystal

crystal "evaporite" "mush"

RELATIVE ABUNDANCE OF ROCK TYPES(Volume % estimates)

• Continental Crust:– Granitic average composition– Igneous & Metamorphic rocks = 93%– Sedimentary = 7%

• Oceanic Crust:– Basaltic average composition– Igneous (averages about 5-6 km thick) = 89%– Sedimentary veneer (up to 1 km thick) = 11%

• fine-grained CaCO3 skeletons of• " " SiO2·nH2O micro-organisms• " " clays (silicates)

ROCK CYCLE

MAGMA

SEDIMENTS

IGNEOUS ROCKS METAMORPHIC

ROCKS

SEDIMENTARY ROCKS

Cooling &

Crystallization

Weathering

Erosion

Transportation

Deposition

Cementation (=Lithification)

High Temperature

and/or

High Pressure

and/or

Hot Fluids

(=Metamorphism)

Melting at Higher

Temperatures

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– O = Oxygen

– K = Potassium– Na = Sodium– Al = Aluminum

Form silicatetetrahedrons

A Little More Chemistry Review

• Si = Silicon• O = Oxygen

• Al = Aluminum • Fe = Iron• Mg = Magnesium• Ca = Calcium

Form silicatetetrahedrons