GEOLOGY GL1

Revision cards for WJEC GL1 Geology

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Mineral Diagnostic Features

FORM/HABIT (SHAPE)

  • crystalline
  • massive
  • fibrous
  • mammilated

TWINNING (WHEN TWO/MORE MINERALS GROW INTO EACH OTHER)

  • simple twinning (share a crystal face)
  • repeated/multiple twinning (repeated simple twinning)
  • interpenetrating (share a crystal face and internal structure)

ORTHOCLASE FELDSPAR: SIMPLE TWINNING

PLAGIOCLASE FELDSPAR: REPEATEAD TWINNING (stripy)

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Mineral Diagnostic Features

FRACTURE (HOW A MINERAL BREAKS)

  • uneven fracture (rough fracture surface)
  • conchoidal fracture (curved fracture with conchoidal rings)

CLEAVAGE (HOW A MINERAL SPLITS ALONG THE LINES OF WEAKNESS)

  • one perfect cleavage (muscovite mica)
  • three good cleavages at 60/120 degrees (calcite)
  • two good cleavages at 90 degrees (feldspars)
  • four good cleavages (flourite)

HORNBLENDE: 3 good cleavages at 60/120

AUGITE: 2 good cleavages at 90 

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Mineral Diagnostic Features

COLOUR

STREAK (COLOR OF POWDER WHEN MINERAL DRAGGED ACROSS STREAK PLATE)

LUSTRE (HOW A MINERAL REFLECTS LIGHT)

  • metallic (galena, pyrite, chalcopyrite, haematite)
  • dull (haematite)
  • silky (crystalline gypsum)
  • pearly (fibrous gypsum, biotite mica)
  • adamantine (diamond)
  • vitreous/glassy (quartz, orthoclase feldspar)
  • resinous (amber, barites)
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Mineral Diagnostic Features

Transparency (how light passes through a mineral)

  • transparent
  • transluscent
  • opaque

Harndess (Moh's scale)

(http://www.oocities.org/collegepark/lab/8822/fall99_mohs.jpg)

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Weathering

WHAT IS WEATHERING?

 Weathering is the breakdown of rock insitue (no movement). There are 3 different types of weathering:

BIOLOGICAL WEATHERING

the breakdown of rock in situe due to biological processes:

  • bacteria
  • tree roots
  • animals burrowing

PRODUCTS OF BIOLOGICAL WEATHERING:

  • rock fragments (scree)
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Weathering

PHYSICAL WEATHERING

the breakdown of rock insitue due to physical processes

  • Freeze thaw/ frost shattering (water freezes inside a joint and expands by 9% inside the joint so it can seep deeper into the rock when it melts, repeated freeze-thaw action makes the joint expand so much that rocks break off and fall off into piles of scree)
  • Exfoliation (minerals expand when heated and contract when cooled, repeated expansion and contraction weaken the rock so the outside of the rock "peels off" in layers)

PRODUCTS OF PHYSICAL WEATHERING

  • rock fragments (scree)
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Weathering

CHEMICAL WEATHERING

the breakdown of rock in situe due to chemical processes

  • Carbonation (rain water picks up carbon dioxide and becomes slightly acidic, this dissolves calcite/limestone)
  • Oxidation (reaction of iron rich rocks with oxygen)
  • Hydrolysis (reaction of rock with water, important for feldspars)

PRODUCTS OF CHEMICAL WEATHERING

  • ions in solution
  • new mineral
  • resistant materials
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Transportation

ICE (HIGH ENERGY) 

BOULDER CLAY

  • coarse grained
  • angular, moderate-low sphericity
  • poorly sorted
  • blue-grey/ brown (depends on lithics its made of)
  • made of lithics with sand/clay matrix

WIND (HIGH ENERGY)

ORTHOQUARZITE

  • medium grained
  • rounded-well rounded, high sphericity
  • well sorted
  • pale-yellow/ beige
  • made of mostly quartz, contain lithic fragments and feldspar, quartz/haematite cement


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Transportation

DEEP SEA (LOW ENERGY)

SHALE

  • very fine grained
  • platey, low sphericity
  • well sorted
  • grey
  • made of clay minerals

BEACH (HIGH ENERGY)

SANDSTONE

  • medium grained
  • sub-rounded- sub-angular, low sphericity
  • well sorted
  • dark beige
  • made of quartz, lithics, shell + calcite/quartz cement
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Transportation

RIVER SOURCE (HIGH ENERGY)

BRECCIA

  • coarse grained
  • sub-rounded- sub-angualr, low-moderate sphericity
  • poorly sorted
  • colour varies
  • fine sand matrix

RIVER MOUTH (LOW ENERGY)

SANDSTONE

  • medium grained
  • rounded, high sphericity
  • well sorted
  • light beige
  • made of mostly quartz (with lithics and feldspar)
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Transportation/Erosion

GRAVITY (SCREE)

BRECCIA

  • very coarse grained
  • angular, low sphericity
  • poorly sorted
  • colour depends on parent rock
  • made of lithics + matrix (gravels/sands)

EROSION

breakdown of rocks with movement

  • abraison (friction between rocks rubbing together erode the rock surface)
  • attrition (rocks banging against each other split into pieces/ pieces break off)


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Lithification

LITHIFICATION

when loose sediment is turned into solid sedimentary rock by:

  • compaction ( upper layers press down on lower layers; pressure squeezes out water and grains pack closer together)
  • cementation ( water carries ions in solution and moves through the sediment slowly; ions are precipitated and lock the sediment together)

Rock cycle:

WEATHERING
TRANSPORTATION+ EROSION
DEPOSITION
BURIAL
LITHIFICATION 

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Sedimentary rocks

SANDSTONES:

1. (ITS JUST SANDSTONE IF NONE OF BELOW)

2.ARKOSE (more than 25% orthoclase)

  • medium grained (500-1000 um)
  • sub-rounded, moderate-high sphericity
  • moderate-well sorted
  • beige-grey
  • made of orthoclase (>25%), quartz, lithics

3.GREYWACKE (more than 15% clay)

  • medium (1000 um)
  • sub-angular, low sphericity
  • poorly sorted
  • greeny-grey
  • made of clay (>15%), quartz, lithics, orthoclase
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Sedimentary rocks

4. ORTHOQUARZITE (more than 90% quartz)

  • medium grained (500-700 um)
  • rounded, high sphericity
  • well sorted
  • beige-yellow/ red brown (if desert sandstone)
  • made of quartz (>90%), lithics, orthoclase + quartz cement

CONGOLMERATE

  • coarse grained (2-5mm)
  • well rounded
  • poorly-moderately sorted
  • if red/brown: wadi deposit (desert river), if no red/brown: river deposit
  • contains lithic fragments and quartz + matrix
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Sedimentary rocks

BRECCIA

  • coarse grained
  • angular, low sphericity
  • poorly sorted
  • if red has haematite cement (arid environment)
  • made of lithic fragments + sand matrix

SHALE

  • fine grained
  • platey+laminated
  • well sorted
  • dark  blue-grey
  • made of mostly clay minerals
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Sedimentary structures

WHAT ARE SEDIMENTARY STRUCTURES?

Structures within the bed that help tell us about the formation of sedimentary rocks and the environment they were found in.

BEDS (layer of rocks thicker than 1cm)

  • each bed is one pulse of sedimentation ; each bedding plane is a pause of sedimentation
  • large pulses of sedimentation can show high energy (if grains are coarse)

* beds do not tell us anything without looking at the grains*

CROSS BEDDING 

  • grains are picked up by the wind on the windward slope and are deposited on the lea slope when they fall out of suspension
  • the slope angle increases and reaches the criticial angle of sand (30 degrees) ; grains collapse
  • each time grains slump they form a cross bed; over time dune migrates towards current flow
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Sedimentary structures

cross bedding shows:

  • environment (desert dunes/rivers/coastal dunes)
  • energy level (moderate-high)
  • current direction (uni-directional)
  • way-up (has been overturned if it is convex)

GRADED BEDDING

coarse grains are at the bottom and grains fine up as they move upwards due to a rapid decrease in energy

 graded bedding shows:

  • environment (rivers flowing into lake/sea/lower energy river)
  • energy level (rapid decrease)
  • way-up (if it fines down it has been overturned)


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Sedimentary structures

ASYMMETRICAL RIPPLE MARKS (micro/mini dunes-form the same way as cross-bedding)

asymmetrical ripple marks show:

  • environment (rivers, desert/beach dunes)
  • energy level (moderate)
  • current direction (uni-directional)
  • way-up (but not without sand and clay)

SYMMETRICAL RIPPLE MARKS

  • when the current goes one way the rock is eroded and deposited on the other side; the current direction switches and the rock does the same

symmetrical ripple marks show:

  • environment (beaches: waves)
  • energy level (moderate)
  • current direction (bi-directional)
  • way-up ( if cross-laminations and sand/clay are wrong way round)
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Sedimentary structures

LAMINATIONS (thin beds less than 1cm)

laminations don't show us anything without looking at the grains

DESSICATION CRACKS

  • water-logged fine sediment is exposed to the air and the mud cracks because it loses volume when water evaporates
  • contraction happens on wetter parts of the mud and causes tension and forms cracks (V-shaped in cross-section because more water is lost from the surface)

dessication cracks show:

  • environment (flood planes, temporary lakes)
  • energy level (low)
  • way-up (if upside down V in cross-section)
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Sole structures

WHAT ARE SOLE STRUCTURES?

Sole structures are sedimentary structures at the bottom of the bed.

LOAD AND FLAME STRUCTURES

  • form after deposition as sand loads down; clay flames up (sand displaces the shale/clay)

load and flame structures only show us way-up

FLUTE CLASTS

  • sediment is deposited; turbulent flow hollows out (erodes) sediment below in scoop shapes ; more sediment is deposited in the hollows making flute clasts
  • current flows from the deep (closed) end to the shallow (open) end

flute clasts show us current direction

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Limestone

BIOCLASTIC LIMESTONE (shelly)

  • course grained
  • angular
  • poorly sorted
  • beige/blue-grey
  • made of broken fossils (100% calcite) + calcite cement/matrix

OOLITHIC LIMESTONE (egg rock)

  • medium-coarse grained
  • very well rounded, high sphericity
  • well sorted
  • made of ooliths (100% calcite) + calcite cement/calcite mud matrix

How ooliths are formed: (NOT LIKE A SNOWBALL)

  •  calcite precipitates onto the grain
  • grain rolls and more calcite precipitates onto it
  • concentric rings of calcite precipitated onto the grain = OOLITH
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Limestone

CHALK

  • very fine grained (shape too fine to see)
  • very well sorted
  • white
  • made of coccoliths (dead plankton)  (100% calcite)

LIMESTONE ENVIRONMENT - Shallow carbonate seas

*clean and shallow water, tropical and warm

  • energy is lost on the reef and creates a calm, low energy lagoon
  • calcite dissolves in cold deep water so only forms in shallow and warm places
  • the reef is broken up by the high energy and broken fossils (bioclastic limestone) are deposited on the talus slope
  • plankton dies and is depostied as ooze which turns into chalk
  • oolithic limestone is found on land because the waves create a bi directional current which form ooliths
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Coal

Coal is fossilised plant material from the carboniferous period

Environment (swamps)

  • tropical and warm
  • humid
  • lush plant growth
  • anaerobic, low energy

HOW IS COAL FORMED?

  • Plant material dies and is buried by more plant material
  • Low energy in the swamp (in stagnant water) slows down decomposition of the dead plant material
  • Compaction due to overlying sediments (squeeze out water + volatiles = increase in carbon)
  • Increase in carbon = production of coal
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Evaporites

  • A desert environment has wet and dry seasons. 
  • During the wet season, water flows down mountains into desert wadis. There are ions in solution which come from chemical weathering.
  • The river deposits material as the energy drops (coarse, poorly sorted, sub-angular,graded bedding)
  • An alluvial fan is formed and a temporary lake (playa lake)is created
  • As the river evaporates and shrinks, surface area decreases and salts start to show the least soluble salt will precipitate first

Calcite (least soluble- precipitates first)

Gypsum

Halite

Potassium salt/ Potash (most soluble- precipitates last)

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Geological time

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Ammanoids

Ammanoids have a coiled and chambered shell with a suture line where chambers intersect  with the outside of the creature.

EVOLUTION OF THE SUTURE LINE:

GONIOTITE

  • carboniferous+ permian (oldest ammanoid)
  • rounded saddle
  • pointed lobe

CERATITE

  • triassic
  • rounded saddle
  • fluted lobe

AMMONITE

  • jurassic + cretaceous (youngest ammanoid)
  • fluted saddle
  • fluted lobe



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Trilobites

(http://www.kgs.ku.edu/Extension/fossils/gifs/Isotelus_diagram.gif)

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Bivalve

(http://www.fao.org/docrep/007/y5720e/y5720e06.jpg)

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Bivalve

Pallial sinus: shows where siphons could be retracted into shell (burrower)

To close shell: adductor muscles contract

To open shell: adductor muscles relax, ligament springs shell open

MUSCLE SCARS:

  • isomyarian: 2 muscle scars, same size 
  • anisomyarian: 2 muscle scars, different size
  • monomyarian: 1 big muscle scar

GAPE:

  • where the shell never closes
  • indicate very long siphons and burrowing lifestlye

Bivalves are INEQUILATERAL and EQUIVALVE

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Brachiopods

(http://paleo.cortland.edu/tutorial/Brachiopods/Brachiopod%20Images/brachdraw3.GIF)

(interior)               Pedical Valve                                     Brachial Valve

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Brachiopods

(http://www.geo.arizona.edu/geo3xx/geo308/FoldersOnServer/2003/4brachiopds_files/image008.jpg)

(exterior)

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Brachiopods

Brachiopods are EQUILATERAL and INEQUIVALVE

BIVALVES

  • INEQUILATERAL
  • EQUIVALVE

BRACHIOPODS

  • EQUILATERAL
  • INEQUIVALVE
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Graptolite

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Graptolite

STIPE ATTITUDE

  • pendant (ordovician)
  • horizontal
  • reclined
  • scandent
  • uni-serial (silurian)

THECAL SHAPE

  • simple (ordovician)
  • hooked
  • lobate
  • isolate (silurian)

STIPE NUMBER

  • quadroserial (ordovician)
  • biserial
  • uniserial (silurian)
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Fossil Assemblages

What is a fossil assemblage?

A collection of living things preserved together.

LIFE ASSEMBLAGE

A collection of living things preserved in the position in which they would have lived.

  • whole
  • different sizes (different stages of life)
  • aligned in life position (all same way)

DEATH ASSEMBLAGE

A collection of living things preserved in the position which they were deposited after transportation

  • broken (show signs of abraison)
  • similar size (have been sorted)
  • random orientation (aligned with current)
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Derived fossil

HOW IS A DERIVED FOSSIL FORMED?

  • living organism is buried and preserved in a rock
  • the fossil is eroded out of older rock and transported
  • fossil is deposited in a new, younger sediment 

fossil is derived from an older rock

LAW OF UNIFORMITARIANISM:

the present is the key to the past

(we use what is happening today to work out how things worked in the past)

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Zone fossils

Zone fossils are fossils we use to date rocks. They need to be:

  • wide spread(found in lots of places to make comparisons between rocks/areas)
  • plentiful (lots of them so we can find them easily)
  • ready preservation (easily fossilised)
  • fast evolution (to date accurately)
  • high degree of facies independance (lived in lots of different environments)
  • easily identifiable

Good zone fossils:

  • Ammanoids 
  •  Graptolites (but graptolites are very delicate so are poorly preserved)
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Relative dating

What is relative dating?

Giving a rock, fossil etc. a date relative to something else (ex: X is older than Y)

LAW OF SUPERPOSITION

  • the oldest thing is on the bottom (unless it has been overturned)

WAY UP STRUCTURES

  • tell us if the bed has been overturned (cross bedding, dessication cracks, graded bedding, flute clasts, load+flame structures, fossils, included fragments)

EVOLUTION OF FOSSILS

CROSS-CUTTING RELATIONSHIPS

  • the thing that cross cuts is younger

INCLUDED FRAGMENTS

  • the rock (A) inside another rock (B) is older (A is older than B)
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Relative dating

UNCONFORMITIES

  • the rocks above the plane of unconformity are the youngest

How is an unconformity formed?

  • sedimentary rocks deposited horizontally
  • rocks folded and uplifted due to tectonic pressure (not in depositional environment)
  • rocks eroded and more sediment deposited on top of plane of unconformity (goes back to depositional environment)

RELATIVE DATING:

  • law of superposition
  • way-up structures
  • evolution of fossils
  • cross cutting relationships
  • included fragments
  • unconformities
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Absolute dating

WHAT IS ABSOLUTE DATING?

Giving a rock, fossil etc. an actual age in millions of years based on the radioactive decay of unstable isotopes (ex: potassium to argon, carbon 14 to carbon 12)

Unstable parent isotopes decay into stable daughter isotopes.

  • to date rocks etc. you look at an unstable isotope's half life

What is a half life?

The time taken for half of the parent isotope to decay into the daughter isotope. (after on half life: 50-50, after two half lifes: 25-50)

THERE NEEDS TO BE A CLOSED SYSYEM FOR ABSOLUTE DATING OTHERWISE

  • if parent leaks in: higher percentage of parent = date younger
  • if parent leaks out: lower percentage of parent = date older
  • if daughter leaks in: higher percentage of daughter = date older
  • if daughter leaks out: lower percentage of daughter= date younger
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Absolute dating

Rocks that are good for absolute dating:

IGNEOUS ROCKS (good)

  • crystallised so crystals become enclosed in a closed system (weathering will open the closed system = inaccurate dating)

Igneous rocks are good for absolute dating unless they have been weathered.

METAMORPHIC ROCKS (not good)

  • take hundreds of millions of years to form 
  • during recrystallisation isotopes can move in/out

SEDIMENTARY ROCKS (not good)

  • don't have a closed system
  • dating a grain from a sedimentary rock gives the date of the igneous rock it orignated from

GLAUCONITE (green mineral forms green sand- sandstone) forms a cement (closed system) during diagenesis (forms at time of rock formation) SO IS GOOD FOR ABSOLUTE DATING!


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Carbon 14 Dating

WHAT IS CARBON 14 DATING?

Absolute dating of young rocks etc. in the quarternary (short 1/2 life).

  • the ratio of C14 to C12 in a living thing remains the same as it is constantly replenished from the atmosphere by respiration
  • when it dies it is no longer able to take in C14 from the atmosphere
  • ratio of C14 to C12 decreases because C14 is unstable and decays into C12
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Fossil Preservation

TO BE PRESERVED A FOSSIL MUST BE:

  • rapidly buried (otherwise organism will be weathered,eroded,eaten etc.)
  • be in an anaerobic environment (low oxygen so no decay)
  • be in a low energy environment (no erosion)

TO HELP PRESERVATION:

  • hard parts (ex: shell, skelenton because soft parts are usually eaten/decomposed before preservation)

Rocks are most commonly preserved in fine grained sedimentary rocks:

  • mudstone
  • shale
  • siltstone
  • limestone

Because the rocks form in environments where many organisms live, the fossils won't be crushed by large grains, they undergo rapid sedimentation (fast burial) and are found in anaerobic environment.

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Fossil Preservation

HOW ARE FOSSILS PRESERVED?

REPLACEMENT/MINERALISATION/PETRIFICATION

Replacement of the hard parts with minerals dissolved in water present in the sediment (replaced molecule by molecule):

  • pyritisation- replaced molecule by molecule by pyrite (ex: ammonites, graptolites)
  • calcification- replaced molecule by molecule by calcite (ex: bivalves, brachiopods)
  • silicification- replaced molecule by molecule by silica (quartz) (ex: wood)

ALTERATION

  • many shells are a mixture of aragonite and calcite
  • aragonite is unstable so changes to calcite by recrystallisation (destroys the internal structure of shell layers but not the shape of it)

CARBONISATION

  • organims with high carbon content (ex; graptolites, plants) are buried and compressed
  • volatile organic molecules driven off but a film of carbon stays (IS NOT ADDED)
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Fossil Preservation

CAST AND MOULD PRESERVATION

  • the physical characterestics of organisms are impressed onto rocks
  • when organisms are buried/trapped in mud/clay/other materials these harden around the organism (cast)
  • bodies decay and leave a mould of the organism
  • cast=solid
  • mould=empty

External mould and cast: external views

Internal mould and cast: internal views

  • empty and internal features : internal mould
  • empty and external features: external mould
  • solid and internal features: internal mould
  • solid and external features: external mould
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Destruction of fossils

Fossils are very easily destroyed. They can be destroyed by:

  • acidic groundwater which dissolves fossils
  • metamorphism
  • melting of rock
  • erosion (the rock cycle can destroy fossils)
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Igneous bodies

Major igneous intrusions: pluton/batholith

Minor igneous intrusions: sills, dykes

Igneous extrusions (on surface): lava flows

PLUTONS/BATHOLITHS

  • very large structures, many km across (ex: south west granite- 250 km across)
  • the pluton is younger than the country rock it intrudes into 
  • a chilled margin on the outside of the intrusion: made of finer crystals as this part cools quicker because it is in contact with the "cold" country rock
  • a metamorphic aureole forms around the pluton because the heat of the intrustion metamorphoses the country rock around it
  • plutons are discordant to the country rock (cross cut it)
  • intrude at depths 2 km below surface
  • slow rate of cooling = course grained igneous rocks
  • GRANITE AND GABBRO
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Igneous bodies

DYKES

  • intrude at shallow depths (less than 2km below surface)
  • 2 chilled margins
  • 2 baked margins where the heat of the dyke has metamorphosed the country rock around it
  • discordant to country rock
  • medium rate of cooling= medium grained
  • DOLERITE

SILLS

  • form at shallow depths (less than 2 km below surface)
  • 2 chilled margins
  • 2 baked margins
  • concordant to country rock
  • medium rate of cooling = medium grained
  • DOLERITE

transgressive sills move along bedding planes (concordant) and then up the joint (discordant)

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Igneous bodies

LAVA FLOWS

  • extrusion (on the surface)
  • concordant
  • 2 chilled margins
  • 1 baked margin underneath
  • reddened, weathered surface
  • included fragments of lava in rock above
  • fast cooling= fine crystals
  • BASALT

How to tell if it is a lava flow (not a sill):

  •  1 baked margin under the lava flow
  • fine grained
  • reddened, weathered surface
  • xenoliths from below only
  • vesicular (has gas bubbles in it)
  • lath shaped crystals (phenocrysts) aligned to direction of flow
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Igneous rocks

  • Size (coarse = slow cooling, medium=medium cooling, fine= fast cooling)
  • Shape (euhedral = slow cooling), subhedral, anhedral)
  • Colour (light, grey, dark)
  • Composition (what are the crystals made of?)
  • Texture (glassy, porphyritic, equigranular, vesicular)

TEXTURES OF IGNEOUS ROCKS

PORPHYRITIC   2 crystal sizes = 2 rates of cooling (euhedral phenocrysts formed slower and first and are surrounded by anhedral ground mass which formed fast and last)

 EQUIGRANULAR  all crystals roughly the same size = 1 rate of cooling ( check measurement for rest of cooling history)

GLASSY no crystals= super fast cooling underwater 

VESICULAR  filled with vesicles (gas bubbles) = fast cooling on surface/close to surface (gas is dissolved in magma under pressure, lava erupts and pressure is released so gas comes out and gas bubbles are frozen in the lava as it crystallises)

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Igneous rocks

GRANITE (SILICIC- HIGH Si CONTENT)

  • coarse grained (slow cooling)
  • light
  • form in pluton/batholith
  • made of quartz, orthoclase, biotite/muscovite mica (some plagioclase if near intermediate)

GABBRO (MAFIC- LOW Si CONTENT)

  • coarse grained (slow cooling)
  • black
  • forms in pluton/batholith
  • made of plagioclase, augite, hornblende (can have some olivine)

DOLERITE (MAFIC)

  • medium grained (medium cooling)
  • black
  • forms in sill/dyke
  • made of same as granite/gabbro



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Igneous rocks

BASALT (MAFIC)

  • fine grained (fast cooling)
  • black
  • forms in lava
  • made of same as gabbro

PERIDOTITE (ULTRAMAFIC)

  • coarse grained (slow cooling)
  • black
  • forms in pluton/batholith
  • made of mostly olivine + augite

the only difference between gabbro, basalt and dolerite is grain size (SCALE IMPORTANT)

IN THIN SECTIONS:

plagioclase= stripy (repeated twin), quartz= wavy, orthoclase= 1/2 black 1/2 white (simple twin), mica = weathered, augite= 2 90 degree cleavage, hornblende= 60/120 degree cleavage, olivine= brightly coloured

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Igneous structures

COLUMNAR JOINTS (ex: Giant's courseway)

  • form in lava flows 
  • as lava cools it loses volumes and sets upstresses in the rock which fracture

PILLOW LAVA

  • when lava erupts underwater a crust forms 
  • crust splits as lava is pumped into it
  • the pillows collapse into each other and mould around each other because they are soft (can be used for way-up)

LAVA SURFACES

Pahoehoe lava - crust forms as lava cools and wrinkles (ropey appearance)

Aa lava- rubble like surface that is painful to walk on


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Evidence for the internal structure of the earth

EVIDENCE FOR THE INTERNAL STRUCTURE OF THE EARTH:

  • seismic waves
  • ophiolites
  • ocean basins
  • magnetism
  • gravity
  • meteorites

THE EARTH IS MADE OF:

  • Crust: solid, oceanic crust: 10km, 3g/cm3 , continental crust: 30 km ,2.7g/cm3
  • Upper mantle: solid, 700km
  • Lower mantle: solid, 2900km
  • Lithosphere: between the upper mantle and the crust, solid
  • Astenosphere: between the lower and upper mantle, PLASTIC
  • Outer core: liquid, 5100km
  • Inner core: solid, 6371 km
  • Lehmann discontinuity: 5100km, boundary between outer core and inner core
  • Gutenberg discontinuity: 2900km: boundary between outer core and mantle
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Evidence for the internal structure of the earth

SEISMIC WAVES

An earthquake is the release of three types of energy:

  • P waves
  • S waves
  • L waves

P waves (primary/push-pull waves)

  • compressional body waves
  • primary waves = arrive FIRST
  • affected by INCOMPRESSIBILITY (more incompressibility=faster)
  • P waves pass through liquids at a slower rate (lower incompressibility)
  • 4-7 km/s

S waves (secondary/transverse waves)

  • 2-5 km/s
  • affected by RIGIDITY (more rigidity=faster)
  • vibrate at right angles
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Evidence for the internal structure of the earth

L-waves (love waves)

  • arrive last
  • affected by COMPITENCY
  • surface waves

P waves:

  • speed up in the crust, mantle, inner core (high incompressibility because they are solid)
  • slow down in the astenosphere, outer core (low incompressibility because plastic/liquid)

S waves

  • speed up in the crust and mantle (high rigidity because they are solid)
  • slow down in the astenosphere (low rigidity because it is plastic)
  • STOP in the outer core (no rigidity because it is liquid)

L waves

  • only travel through the crust (speed depends on rock type)
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Evidence for the nature of the crust

OCEANIC CRUST

Can be directly observed by dredging and drilling/where mid-ocean ridge above sea level/ ophiolites on land (oceanic crust which is pushed onto continental crust).

  • density= 3 g/cm3
  • made of basalt
  • 10km thick
  • layered basaltic structure (ophiolite sequence)
  • younger than continental (oldest is Triassic)

CONTINENTAL CRUST

Can be directly observed with surface rocks/ drilling or deep mining/ geological mapping

  • density= 2.7 g/cm3
  • made of granite
  • 30km thick
  • deformed (faulted and folded) structure
  • older than oceanic crust (3900 million years)
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Evidence for the nature of the core

MAGNETIC FIELD

The Northern lights and compasses show us we have a magnetic field.

Earth's magnetic field shows that the core has to be made of something magnetic and it must be partly solid and partly liquid to move (outer core is liquid, inner core is solid)

METEORITES

Meteriotes come from the asteroid belt between Jupiter and Mars. Two types of meteorites:

  • iron meteorites: the core of exploded planets (made of iron and nickel)
  • stone meteorites: the crust+mantle of exploded planets

EARTH'S GRAVITY FIELD

The average density of earth is 5.5g/cm3. The crust and mantle do not reach this density (2.7-3.0 g/cm3 and 3.3-5.4g/cm3). The most density is in the core and only iron and nickel have this density.

~THE CORE HAS TO BE MADE OF IRON AND NICKEL~

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Metamorphic rocks

CONTACT METAMORPHISM

HIGH TEMPERATURE-LOW PRESSURE

TEXTURES OF CONTACT METAMORPHIC ROCKS:

  • Granoblastic (roughly same size crystals)
  • Poikilitic/spotted (spotted texture, incomplete recrystallisation)
  • Porphyroblastic (euhedral porphyroblasts surrounded by finer groundmass)

~ all contact metamorphic rocks are crystalline and unfoliated (randomly orientated ~

CONTACT METAMORPHIC ROCKS

Spotted rock 

  • poikilitic texture
  • pelite parent rock (clay)
  • low grade (fine grained) so has some sedimentary structures
  • contains blebs of biotite/cordierite
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Metamorphic rocks

Hornfels

  • granoblastic texture
  • made of minerals such as hornblende
  • splintery 
  • pelite parent rock (clay)
  • high grade (coarse/medium grained)

Metaquarzite

  • granoblastic texture
  • made of quartz
  • sandstone parent rock
  • contact/regional metamorphism (quartz crystals are round so can't align)

Marble

  • granoblastic texture
  • made of calcite
  • limestone parent rock
  • contact/regional metamorphism 
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Metamorphic rocks

WHAT AFFECTS THE WIDTH OF A METAMORPHIC AUREOLE?

  • temperature of intrusion (cool-thin, hot-wide)
  • temperature of country rock
  • rock type of country rock (pelite-wide, sandstone-thin)
  • size of the intrusion
  • composition of the intrusion (granite=thin, gabbro=wide)
  • angle/shape of the intrusion

REGIONAL METAMORPHISM (destructive plate boundaries in fold mountains)

HIGH TEMPERATURE-HIGH PRESSURE

Granoblastic- MARBLE + METAQUARZITE

  • unfoliated because crystals are round

Slatey cleavage- SLATE

  • slatey texture, can contain lumps of pyrite, aligned to pressure maximum
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Metamorphic rocks

Schistosse SCHIST

  • platey ,can have garnet poryphyroblasts if it is high grade

Gneissose GNEISS

  • white and black minerals stick together (don't mix) colours are aligned to pressure maximum 

(highest grade) Gneiss----->Schist----->Slate (lowest grade)

Metamorphic rocks:

  • Slate
  • Schist
  • Gneisse
  • Metaquarzite
  • Marble
  • Hornfels
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Structural Geology

FAULT: a fracture in the rock with movement

DIP SLIP FAULT (VERTICAL MOVEMENT)

  • Normal fault
  • Reverse fault

NORMAL FAULTS

  • constructive plate margins
  • tensional stress
  • crustal thinning + crustal extension
  • footwall (under fault plane) moves up, hanging wall moves down (FUN= footwall up=normal)

REVERSE FAULTS

  • destructive plate boundaries
  • compressional stress
  • footwall moves down, hanging wall moves up (FDR= footwall down=reverse)
  • in a THRUST FAULT the footwall goes down at a low angle
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Structural Geology

STRIKE SLIP FAULTS (HORIZONTAL MOVEMENTS)

  • transform fault
  • transcurrent/tear/shear fault

TRANSFORM FAULT

  • offset mid ocean ridges at constructive plate boundaries
  • shear stress

TRANSCURRENT/TEAR/SHEAR FAULT

  • conservative plate boundaries (ex: San Andreas fault)
  • dextral: movement to the right
  • synistral: movement to the left
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Structural Geology

BEDDING

  • horizontal bedding
  • vertical bedding (vertical things will be straight on the surface)
  • dipping beds (direction of dip, strike, amount of dip)

FOLDS

  • folds are a flexure (bend) in a rock
  • compressional stress
  • destructive plate margins
  • limb= straight part of a fold between 2 hinges
  • hinge= part of fold where angle of dip changes
  • axial plane= plane that joins hinges together
  • axial plane trace= surface expression of axial plane
  • antiform= closes upwards (A)
  • synform= closes downwards (V)
  • anticline= oldest beds in the core
  • syncline= youngest beds in the core


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Structural Geology

Antiformal anticline: closes upwards, oldest beds in the core

Antiformal syncline: closes upwards, youngest beds in the core

Synformal anticline: closes downwards, oldest beds in the core

Synformal syncline: closes downwards, youngest beds in the core

Symmetrical: both limbs similar length

Asymmetrical: 2 different length limbs

To describe a fold:

  • antiform/synform
  • anticline/syncline
  • asymmetrical/symmetrical


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Structural Geology

UNCONFORMITIES

  • hiatus (gap) in geological time

How unconformities are formed:

1) sediment deposited (high sea level)

2) rocks folded (compression+uplift) on destructive plate margins (low sea level: earth rises above sea level)

3) weathering + erosion (low sea level)

4) deposition of more sediment on top (high sea level)

unconformity with angular discordance: beds have different dips above and below the plane of unconformity

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