ROCKS

GEOL 1033 General Oceanography Review Lesson 8 in the Study Guide

THREE TYPES OF ROCKS

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Geologists classify Earth materials according to naturally occurring minerals & rocks, rather than by elements or chemical compounds.

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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:

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Once molten:

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Cooled, crystallized, hardened Lava is extrusive Magma is intrusive

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Examples:

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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:

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Minerals of granitic continental crust:

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Quartz (SiO 2 ) 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

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Grey granites Pink granites Etc.

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From a magma rich in silica GRANITE

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Feldspar

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Quartz Muscovite Biotite Hornblende ( Orthoclase )

1) Igneous rocks:

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Minerals of basaltic/gabbroic oceanic crust

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Pyroxene (=dark Fe, Mg, & Ca-rich Al silicates) Na & Ca feldspars abundant ( instead of K-feldspars of granite ) No quartz comparison with continental crust:

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Denser Darker Composition & minerals different

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From a Silicate magma rich in Fe & Mg GABBRO/BASALT Pyroxene (Fe+Mg-rich) Olivine Feldspar ( Plagioclase)

CLASSIFICATION OF IGNEOUS ROCKS (Small cx) (Large cx)

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2) Metamorphic rocks: Deformed rocks by

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1) High temperature &/or 2) High pressure &/or 3) Hot fluids

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Found associated with deformational mountains, folds, faults, i. e., where cont. & oceanic plates collide.

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New minerals, but not much chemical change in rock composition

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Examples:

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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:

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Sedimentary Rocks come from sediments Sediments:

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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:

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a. Grains (larger particles)

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b. Matrix (fines)

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c. Pores d. Cement (air, H 2 O, oil, or gas may fill them) (chemical precipitates in pores after burial, usually post-depositional)

What are the two most important properties of sediments & sedimentary rocks?

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1. Grain size

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2. Composition

1. Grain size

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Varies from clay sizes to boulders:

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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 Calcium carbonate sand grains Quartz-rich sandy beach

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2. COMPOSITION – TWO MAJOR ABUNDANCE CATEGORIES OF MARINE SEDIMENTS A. Silicates

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Dominated by the elements silicon (Si) & oxygen (O) Fine-grained (clays, mud, etc.)

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Clay minerals (silicates) dominate

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Tiny fragments of other silicate minerals

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Sands

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Quartz dominates (Silica = SiO 2 ) Some feldspar grains (Na, K, Ca-rich Al silicates) B. Carbonates

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Dominated by calcium carbonate (CaCO 3 ) [ Ca, C & O] Characterize coasts with

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Warm climates (low latitudes) & Isolated from abundant silicate sediments Invertebrates and algal shells/skeletons dominate coarse shallow-water carbonate sediments

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Characterize deep-sea sediments that

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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 1. conglomerate gravel 2. sandstone sand 3. shale 4. limestone mud, clay shells, "lime" mud 5. crystalline "evaporite" loose crystal "mush" Single grain pebble sand clay shell, etc.

crystal

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RELATIVE ABUNDANCE OF ROCK TYPES (Volume % estimates ) Continental Crust:

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Granitic average composition Igneous & Metamorphic rocks = Sedimentary = 93% 7%

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Oceanic Crust:

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Basaltic average composition Igneous (averages about 5-6 km thick) = Sedimentary veneer (up to 1 km thick) =

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fine-grained CaCO 3 skeletons of " " SiO 2

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nH 2 O micro-organisms " " clays (silicates) 89% 11%

Cooling & Crystallization

ROCK CYCLE

MAGMA Melting at Higher Temperatures IGNEOUS ROCKS Weathering Erosion Transportation Deposition METAMORPHIC ROCKS High Temperature and/or High Pressure and/or Hot Fluids (=Metamorphism) SEDIMENTS SEDIMENTARY ROCKS Cementation (=Lithification)

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A little Chemistry Review

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Silicates = complex compounds dominated by Silicon & Oxygen Elements symbolized by letters, e. g., C = Carbon.

Also know:

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Si = Silicon

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O = Oxygen Form silicate tetrahedrons

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K = Potassium

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Na = Sodium

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Al = Aluminum

A Little More Chemistry Review

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Si = Silicon O = Oxygen

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Al = Aluminum Fe = Iron Mg = Magnesium Ca = Calcium Form silicate tetrahedrons