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Introduction to LandformsGEOG 1113K

Dr. Thieme

Lecture 6: Earth Materials

• The 8 most abundant elements in Earth’s crust are: Oxygen (O): 46.6% Silicon (Si): 27.7% Aluminum (Al): 8.1% Iron (Fe): 5.0% Calcium (Ca): 3.6% Sodium (Na): 2.8% Potassium (K): 2.6% Magnesium (Mg): 2.1% All other 95 elements: 1.7%

• These 8 elements most available to form rocks and minerals

•mineral - a naturally occurring, inorganic substance that has a definite chemical composition and characteristic crystal structure

•rock - an assemblage of one or more minerals• three major categories of rocks:

1) igneous2) sedimentary3) metamorphic

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Mineral Groups1) silicates – Si and O base usually combined

with one or more metal cations2) oxides – O base combined with one or

more metal cations3) sulfides – S base combined with one or

more metal cations4) carbonates – metal cations combined with

the carbonate anion (CO3-2)

"Rock-forming" Minerals

• quartz, SiO2

• Feldspars • potassium (Kspar) • plagioclase (sodium and calcium)

• mica• muscovite (white or tan)• biotite (black)

• Ferromagnesian• olivine (Fe- and Mg SiO4)• pyroxene (Augite)• amphibole (Hornblende)

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Native copper

Pyrite (FeS2)

Galena (PbS2) - both shiny and dull

Metallic Luster

Color• often due to minor impurities rather than the mineral's crystalline structure

Fluorite (CaF2) comes in manycolors with the same octahedral form.

Colored varieties of quartz (SiO2) include amethyst, citrine, rose quartz, smoky

quartz, etc...

Streak• the color obtained by rubbing the mineral across a piece of

unglazed porcelain

Both of these samples of hematite (Fe2O3) have a reddish brown streak.

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Hardness

Cleavage• tendency of a mineral to break along planes of weak bonding

4 at < 90o

3 at 90o

3 at < 90o

Quartz has such strong bonds between Si and O in all directions that it has no cleavage. Instead, it breaks apart by conchoidal fracture.

Obsidian (volcanic glass) also has conchoidal fracture.

Fracture

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Specific Gravity

• ratio of the weight of a mineral to the weight of an equal volume of water

• Quartz (SiO2) = 2.65• Galena (PbS2) = 7.5• Gold (Au) = 20

Special Properties•Taste – some minerals can be identified by taste, example, halite.

•Effervescence – a drop of dilute HCl will cause calcite to effervesce

•Double Refraction - clear samples of some minerals will bend light, giving a double-image if placed over an object or writing.

Double refraction in a sample of calcite.

Figure 13-4

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Figure 13-5(a)

Igneous Rocks

Igneous Rocks•form when melted rock cools and solidifies•magma – molten rock beneath earth’s surface

• lava – molten rock above earth’s surface•intrusive rocks form when magma cools below the surface

•extrusive rocks form when lava cools above the surface

Igneous Rock Textures•Aphanitic – igneous rocks with very fine-grained crystals, individual crystals are too small to be seen with the unaided eye.

•Phaneritic – igneous rocks with individual mineral crystals large enough to be seen with the naked eye.

•Glassy – igneous rocks that cooled so quickly there was no time for any alignment of minerals

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Phaneritic

Aphanitic

Glassy

Phaneritic cooled slowly below ground

Aphanitic cooled quickly above ground

Glassy cooled very rapidly (quenched) above ground

Igneous Rock Textures

Igneous Rock Composition

Felsic – igneous rocks that are composed mostly of quartz and feldspar, with some minor additions of biotite mica and amphibole.

Mafic – igneous rocks composed mostly of ferromagnesian minerals with some minor additions of feldspar

Intermediate – igneous rocks that fall between rocks with felsic or maficcompositions

Felsic

Mafic

Intermediate

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Viscosity• resistance to flow, depends upon composition of magma or lava1. Felsic magmas have high viscosity (are thick) because their high silica content makes them “sticky”

2. Mafic magmas have low viscosity (are thin) because of their low silica content

3. Intermediate magmas have intermediate viscosity, and ultramafic magmas have very low viscosity

Naming Igneous Rocks• based upon texture and mineralogy

Granite – a phaneritic, felsic rock. Most common intrusive

Rhyolite – an aphanitic, felsic rock Andesite – an aphanitic, intermediate rockDiorite – a phaneritic, intermediate rockBasalt – an aphanitic, mafic rock. Most common

extrusiveGabbro – a phaneritic, mafic rockObsidian – a glassy, felsic rockPumice – a glassy, felsic rock with numerous

vesicles

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• gives the order that minerals crystallize in as magma cools

Forming Sedimentary Rocks

•diagenesis – describes any chemical, physical, and/or biological changes that take place after sediments are deposited

• lithification – the process by which unconsolidated sediments are transformed into solid sedimentary rocks•compaction – compressing of sediment by the weight of overlying sediment

•cementation – precipitation of minerals in pore spaces leading to a linkage of the sedimentary particles

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CEMENTATIONCOMPACTION

Figure 13-11

Sedimentary Rock Classes

•detrital (clastic) – sedimentary rocks composed of material (sand, silt, clay) that was deposited

•chemical – sedimentary rocks formed when material is precipitated out of solution

•organic – sedimentary rocks formed from organic material

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Top left: detrital sedimentary rock,

Top right: chemical sedimentary rock,

Bottom left: organic sedimentary rock.

Detrital Sedimentary Rocks• Most are composed of clays and quartz• Particle size is the primary basis for distinguishing detrital rocks:

• shale – consists of silt and clay-sized particles where the particles are aligned parallel to one another

• sandstone – consists primarily of sand-sized grains• conglomerate – consists primarily of rounded gravel

Parallel alignment of clay particles in a sample of shale.

Detrital Sedimentary Rocks• sorting – the degree of similarity in particle size in a sedimentary rock

• well sorted – most of the particles are of the same size

• poorly sorted – the particles are of highly variable size

Well Sorted

Poorly Sorted

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Detrital Particles rounded - particles have smooth, curved edges angular - particles have sharp, sudden direction changes

along edges

Rounded particles

Angular particles

Chemical Sedimentary Rocks• Classification based on composition

• limestone – composed mainly of calcite (fizzes in acid)

• chert – hard rock made of silica (SiO2)

Limestone

Chert arrowhead

Biochemical Sedimentary Rocks

• limestone – composed of shell material• coquina – limestone made of shells and shell fragments• chalk – made of the tests (shells) of microscopic marine organisms

• coral – coral reefs are made up of limestone secreted by the coral polyps and algae

• coal – formed when plant materials are preserved in a swamp, buried under sediments, and then compressed by weight at depth

• lignite – a soft brown coal• bituminous – a harder black coal formed by further compression of lignite

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Sedimentary Structures•beds – horizontal layering of sediment shown in the rock

•bedding planes – the boundary between beds, flat surfaces along which rock tends to break

Beds and bedding planes shown in a Utah rock outcrop.

Sedimentary Structures•cross-bedding – beds deposited on an incline relative to horizontal

•graded beds – particles within a single bedding layer gradually change from coarse at the bottom to fine at the top

Cross-beds

Graded beds

Sedimentary Structures• ripple marks – wave-like features that develop on a sand surface•asymmetrical – formed by a current • symmetrical – formed from a back and forth motion

•mud cracks – indicate a sediment that was alternatively wet and dry

Ripple marks

Mud cracks

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Fossils•Fossil - evidence of past life

• remains – preserved bones, skin, etc• trace – footprints, coprolites, etc

•By combining rock type (detrital, chemical, organic) and name, sediment size, shape, and sorting (if detrital), structures, and fossils, we can determine sedimentary environments, important in landform genesis.

Remains

Trace fossils

Sedimentary Environment

• A geographic setting where sediment is accumulating

• Determines the nature of the sediments that accumulate (grain size, grain shape, etc.)

Continental Transitional MarineFloodplain Delta Continental Shelf

Alluvial Fan Barrier Island Continental Slope

Playa Lake Lagoon Continental Rise

Desert Tidal Flat Deep-sea Fan

Glacier Margin Abyssal Plain

n = 14

Sedimentary Environments

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Continental Environments• fluvial or alluvial – stream dominated environment• glacial – environment dominated by moving ice• lacustrine – lake environment• eolian – wind dominated environment

Fluvial Glacial Lacustrine Eolian

Transitional Environments•beaches – sandy deposits along the shoreline• tidal flats – mud-covered areas that are alternately flooded with shallow seawater and the exposed to air as the tide rises and falls

•barrier islands – elongated islands of sand that run parallel to the coast

Beach Tidal Flat (salt marsh) Barrier Island

Transitional Environments• lagoons – area of sheltered water between a barrier island or reef and the mainland

•deltas – complex accumulation of sediment built out into the ocean or a lake at the mouth of a river

Lagoon Delta

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Marine Environments• shallow marine – water up to about 700 feet

deep• deep marine – water over 700 feet deep

Figure 13-5(c)

Metamorphic Rocks

Metamorphism• The transition of one rock into

another by temperatures and/or pressures unlike those in which it formed

• Metamorphic rocks are produced fromIgneous rocksSedimentary rocksOther metamorphic rocks

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Metamorphism

•Type•Contact (thermal)•Hydrothermal•Regional•Impact

•Grade (from Low to High)

Contact Metamorphism

Hydrothermal Metamorphism

• Chemical alteration caused when hot, ion-rich fluids, called hydrothermal solutions, circulate through fissures and cracks that develop in rock

• Most widespread along the axis of the mid-ocean ridge system

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Regional Metamorphism

• Produces the greatest quantity of metamorphic rock

• Associated with mountain building

Ocean-Continental Plate Convergence

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Subduction Zones• Mountainous terrain adjacent to

subduction zones has distinct linear belts of metamorphic rocksHigh-pressure, low-temperature

zones nearest the trenchHigh-temperature, low-pressure

zones further inland in the region of igneous activity

Continent-Continent Plate Collisions

• Compressional stresses deform the edges of the plate

• Formation of the Earth’s major mountain belts including the Alps, Himalayas, and Appalachians

Figure 13-13(a)

Foliation

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LOW GRADE

HIGH GRADE

Naming Metamorphic Rocks

•Metamorphic rocks are named based on their foliation, or if lacking foliation, based on their composition (chemistry)

•Chemistry is inherited from the -•Parent rock – the rock from which a metamorphic rock forms

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foliation – any planar arrangement of mineral grains or structural features within a rock

•slaty cleavage – closely spaced planar surfaces along which rocks split

•schistosity – layered appearance or orientation of mica and chlorite grains

•gneissic banding – segregation of minerals in the rock produces a banded look

Slaty Cleavage Schistosity Gneissic Banding

Phyllite

Slate

Foliated fine-grained rocks

Garnet-mica schist

Amphibolite gneiss

Foliated coarse-grained rocks

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Foliated Rocks• slate – a fine-grained rock composed of mica flakes

too small to be seen with the unaided eye. Parent rock: shale

• phyllite – a rock composed of slightly larger mica flakes, large enough to cause a sheen when rotated in light. Parent rock: slate

• schist – medium-grained rock, platy minerals large enough to be seen with the unaided eye dominate. Parent rock: phyllite

• gneiss – a coarse-grained rock with banding, mineralogy is usually similar to granite. Parent rock: schist or granite

•rocks that do not display any orientation of minerals

Examples of two nonfoliated metamorphic rocks

Non-foliated Rocks

Marble, a nonfoliated metamorphic rock formed from limestone.

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Quartzite, a nonfoliated metamorphic rock formed from quartz sandstone.

Nonfoliated Rocks•marble – a coarse-grained rock composed of calcium carbonate. Parent rock: limestone or dolostone

•quartzite – a medium-grained very hard metamorphic rock composed of quartz. Parent rock: quartz-rich sandstone

•anthracite – a shiny black organic rock. Parent rock: bituminous coal

Impact Metamorphism

•Occurs when high speed projectiles called meteorites strike Earth’s surface

•Products are calledimpactites ortektites

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Impact Metamorphism

Tektites from Nullarbor Plain, Australia