The geophere Flashcards
What was the Theory of Earth?
Paper published in 1795 by James Hutton
Said laws governing nature are independent of time
- processes operating on and within the earth follow physical and chemical laws
- given enough time could account for all features of rocks
James hutton recognised renewal of the regolith and showed igneous rock can be intrusive
This observation indicated that sequence alone did not account for the age - showed different rocks formed in different processes
What theory did Alfred Wegner propose?
Continuental drift
-proposed that continuents were able to move slowly through oceanic crust
Evidence sited: congruence of coast lines and major structural features, similarity of palaeontological remains in different contients, evaporite deposits in northern latitudes
What did Harold Hess propose?
Seafloor spreading
Marine surveys revealed Guyots: flat topped seamountswhich Hess proposed wre eroded volcanic islands
He suggested the oceanic crust is young - sediment is slow
Ocean ridge systems caused by upward flow of hot mantle derived manterial
Deep sea trenches regions of crustal destruction.
Outline of plate techtonics
10 major rigid plates
Divergent margine - plates move apart, cause shallow quakes
Convergent margins - pleates come together, andesitic lava
Transform margins - plates slide against each other, movement only
Rigid lithosphere moves on top of plastic asthenosphere
What can earthquake locations tell us?
Shallow earthquakes generally associated with seamounts of ridge systems which are constructive plate margins
Deep earthquakes often parallel to trenches termed Benioff zones which destructive plate margins - Oceanic crust is forced under continuental crust as it is less dense
What do oceanic ridges represent?
Ocean ridge systems are constructuve plate margin
Evidence: volcanic acitivity (basic magma), black smokers, shallow earthquakes
New oceanic crust is created from rising mantle-derived material
Rising plumes of hot mantle form upward limbs of convection cells
What part do convection cells play?
Oceanic crust sinks into the asthenosphere
Can be subducted under other oceanic crust
Plate movement drivemn by convection cells
Radioactive decay provides energy
What is the evidence for plate movement?
Island chais provide evidence e.g: Hawaii-Emperor chain
Series of volcanic islands decreasing in age
Plate moves over a hot spot which forces hot magma up to form a new island
Dog leg implies a chang eof direction
Describe the Seismeic discontinuities (layers)
5-40km: Mohoroviviv discontinuity - Felsic minerals (crust) give way to mafic minerals (upper mantle) - associated large change in density
400km discontinuity - ultramafic (periodite) give way to high pressure minerals
650 km - phase change to very high density minerals
Outer core - Viscosity similar to water, convects readily so cannot support temperature or chemical gradients
What is the Asthenosphere?
100-200km low velocity zone
region of the upper mantle with partial melting of periodite mantle rock
Depth of melting is limitted by increase in melting point with depth
Zone can undergo plastic deformation on geological timescales
What happens in a continuental-continuental collision?
Creates mountain ranges
Deep earthquakes and intrusions of granite
Extensive regional metamorphism
Very high pressure leads to uplift, overturning, rotation or folding, can cause metamorphism
What crystal forms can be seen in minerals?
Crystal habits: prismatic, tabular, equant
Crystal form reflects the space lattice, hence chemical structure and can be described by one of the Bravais lattices:
Primitive, Face centerred cubic and body centered cubic
What crystal systems are possible?
The seven Bravais lattices each representing unique symmetry assemblages
Each of the non primitive lattices has the same symmetry as one of the primative lattices
Six crystal families
Hexagonal and trigonal differ only by angle of the crystalographic axes hence are one crystal family
How are crystals defined?
Axes - equal, long or unequal
angles
Miller indicies
How do crystals behave optically?
light waves passsing between materials of differing densities are refracted - more dense material bends light towards surface normal
The angle of refraction depends on the obliquity and relative velocity of the light in each medium
Snell’s law: sin i /sin r = n
n = index of refraction = vair/vmineral = 1/vmineral
n depends on l of light:lower speed of shorter l causes greater n
Crystals thus have different refractivw indicies for different wavelengths - known as dispersion
What are isotropic crystals?
crystals that have one refractive index
These crystals have high symmetry which means that light moves through the crystalwith equal velocity in all directions
What are anisotropic crystals?
Crystals that have more than one refractive index
Crystals with lower symmetry - the velocity of light varies with crystalographic direction: light moves through the crystal with different velocities depending on direction
This is a consequence of different relationship of the axes
Light passing into an anisotropic crystal is split into two polarised rays vibrating in mutually perpendicular planes
How are thin sections of rock prepared?
Slice of rock is sawn off and stuck to a glass slide with epoxy resin (has similar refractive index, both isotropic)
Slide is clamped and the rock sawn down to a thickness of a few mm (light velocity depends on thickness)
Sliver is ground down further and finally polished to 30mm
Contrast in mineral hardness and nature of rock can influence the quality of the thin section - holes
What happens in a polarisding microscope?
Light vibrates in all directions perpedicular to its direction of propogation?
Speed of light changes in different media but frquency is unchanged
Given v = c/λ wavelength must change
What are the optical properties of opaque or colourless minerals?
Opaque minerals transmit no light in polarised microscope
Colourless minerals: relief - feature of refractive index
Observed as the prominence of mineral edges and features
What are the optical characteristics of coloured minerals?
Natural colours oly observed in ppl but depend on light intensity
Pleochromism - a feature relating to the strength of absorption of different wavelengthd in different crystal orientation
Rotate stage to observe intensity changing systematically
What is Birefringence?
In antisotropic minerals the orthogonally polarised rays recombine on leaving the mineral grain
Rays out of phase (slow and fast ray) and different wavelengths produce interference patterns that alter with rotation of the stage
This is evident in cross polarised light
What are extinction and twinning?
Extinction: rotate the stage and mineral grains wll go from light to dark at particular angles relative toi a feature of the crystal axes - reference to elongation, cleavage, twinning - meausre angle of extinction
Twinning: crystals that have intergrown in different orientations - only happens when lattice is compatible in more than one orientation
Evident in cross polarised light
What is an igneous rock?
Rocks formed by cooling and solidification of molten rock
Comprsed of different minerals that interlock
Grain size amd distribution depend on cooling rate
Mafic - melanocratic - basic - dark
Felsic - leucocratic - acidic - light
What structures can igneous rocks form?
Molten rocks originate deep within the earth and structures form based on cooling which reflects the temperature and composition of melt
Intrusive forms: subteranian sheets (sills and dykes) and discrete bodies (plutons: e.g. baoliths/lacoliths)
Plutons = large bodies of rock formed underground
How are acidic igneous rocks formed?
Formed by emp;acement:
Magama rises through continuental crust
incorporates some host rock by melting
This s exposed by erosion of overlaying strata
What are extrusive igneough structures?
Lava flows:
can be very extensive
Column jlining can develop as lava flow cools
What is the composition of igneous rocks?
The most abundant elements on earth dominate magma: O, Si, Al, Fe, Mg, Ca, Na, K, S
Relatuve abundances, temperatyures, pressures and water content control behaviour amd rock formed on cooling
How does composition change relation to exture, colour and silica content?
Rocks that cool quickly such as basalt have smaller/fine grains
Slow cooling rocks such as granite are coarse grained
Lighter higher silica content is generally acidic whereas darker, dense rocks with low silica content are more basic
How does composition affect erruptions?
Basic magma - basault lava flows
Intermediate magma - andesitic lava flows
The more silica the more viscous the lava and the slower the movement
What is a rock and what are the types of rock?
A naturally formed, non-living, firm and coherent aggregate mass of solid matter that constitues mart of a planet
Igneous - rock formed from solidification of molten magma
Sedimentary - rock formed from chemical precipitation or by sedimentation and cementation of mineral grains
Metamorphic - rock in which minerals or textures have have been changed by reactions in the solid state as a result of exposure to elevated T, P or hydrothermal fluids
What is regolith?
irregular blanket of loose uncemented particles that cover the surface of the earth
Clastic / detrital sediments - fragments of rock formed by erosion by wind, water, ice, abrasion
Ice is particularly effective: produces angluar fragments with a wide distribution in particle size
Chemical sediments - formed by precipitation + deposition
Biological sedimets - deposition of remains of dead organisms
How are sedimants classified?
By size
How are sediments transported?
Watre, ice, wind
PArticle shape and distribution change in transport
Greater energy and distance increases roundness of gains and sorting of particle size distribution (becomes narrow with transport)
How can the method of deposition be discovered?
Fine grained and low density material is transported the furthest
Bedding can be used to indicate driving force for transport
Rhythmic bedding: alternation of parallel layers - indicates control by natural cycle
Graded bedding: coarse to fine upward sequence - indicates deposition from flow (cross bedding)
Chaotic: no sorting - indicates rockfall mudflow, glacial deposit
What is lithification?
process by which loose sediment is converted into rock
Diagenesis: chemical, physical and biological processes that affect a sediment after its initial deposition and during lithification:
compaction excludes water
cememntation occurs through precipitation from pore waters
recrystalisation may produce new authigenic minerals
oxidation may remineralise convert organic matter to CO2 (reducing conditions favour preservation of organic matter)
What are clastic sedimentary rocks?
Sediments formed by breakdown of rocks accumulate at the Earth’s surface typicallu grains are discrete minerals
Rocks formed by lithification:
conglomerate: formed from gravel
Sandstone: formed from sand
Shale fromed from silt and mud
What are evaporites?
Gypsum precipitates from salt solution - precipitation occurs in reverse order of solubility: carbonate, gypsum, halite, sylvite
The sequence of evaporites is closely controlled by climate
What are biogenic sediments?
White cliffs of Dover are massive accumilations of cocolith shells (CaCO3) - cannot distinguish between fossils visually but can microscopically
Shelly limestone typically comprises accumilations of shell fragments
Organic sediments = peat / coal
What are strata?
Distinct layers that accumilates at the earth’s surface
Layers defined by thickness or character
Bedding: a layered arrangement of strata in a body of sediment or sedimentary rock
Bedding plane: top of bottom surface of a bed
What is the relationship between sedimentary strata?
Conformity: layers deposited without interruption
Unconformity: deposition of layers interruption by break in sedimentation
What are metamorphic rocks?
Metamorphism can occur for igneous and sedimentary rocks
Mediated by high T and P or hydrothermal fluids
Rock altered without melting, produces new material and textures
Changes occur within the solid state (could be localised areas of melt - partial)
What is regional and burial metamorphism?
buried rocks experience gradual heating and high pressure - overburden - steeper geothermal gradient in ocean associated with proximity to upper mantle
Regional scale lateral compression generates high pressure at temperatures ranging from low (surface) to high (deep)
- Associated with plate techtonics
Can cause deformation, folding + uplift
Textures reflect effects of pressures
How does texture change in burial and regional metamorphism?
Burial - horizontal slaty cleavage
Regional - verticle slaty cleavage
How does foliation occur?
Pressure leads to preferred orientattion of mineral grains - long axis perpendicular to stress
Bedding and foliation form zones of weakness
Does does the metamorphic grade change the rock?
New mineral grains segregate as they develop - micas form from alteration of clay minerals - created of schistocity
Increasing the metamorphic grade ehances mineral segregation - distinct mineral bands develop
- creation of greissic texture
What are isogrades?
Zones of equal metamorphic grade defined by mineral assemblage
These can be mapped out
Metamorphic minerals form in sequenve as conditions becomes more extreme
Increasing grade towards the focus of metamorphism (regional and contact)
What is contact metamorphism?
When an igneous body intrudes into a body of rock it is very hot and can cause metamorphism of rock in contact with it
The metamorphic grade is invcreased cloer to the igneous rock (heat source)
An example of this is a dolerite intrusion nto sandstone where the heat forms hornfels around the dolerite and sandstone remains further away
What is metamorphic aureole?
Contact metamorphism keads to compositional changes that scale with proximity to the igneous body
zonation of metamorphic minerals
increase in metamorphic grade towards contact with an kgneous body
What are metamorphic facies?
Facies describe particular assemblages of minerals that relate to particular temperatur/ pressure regime
Allows recognition of the conditions that caused metamorphism
Temperature and pressure conditions are associated with plate techtonic environments and processes
What are the grampian metamorphic rocks
The gramphian orogery was a ountain building event that caused extensive metamorphism of rocks in the highlands
What is the geostrophic cycle?
Igneous, sedimentary and metamorphic rocks are connected via the geostrophic (rock) cycle
The process can be interrupted at any stage by techtonic activity
What did Willian Smith and Cuvier do?
Smith: unique fossil assemblages in particular sedimentary rock units
Fossils indicate time equivalent units
Can correlate rocks in different locations
Constructed first geological map
Curvie: Stratigraphic sequences of terrestrial vertebrates and marine invertebrates in Paris Basin
Seqences extinction and advancing complexity
How can we map the geochronology of rocks?
Isotopic dating of rocks:
eploits radioactive decay of selected nuclei
Gives absolute not relative ages
Consider Rb/Sr decay series
What is the equation for time in geochronology?
Assumes:
λ is constant
Changes in amounts of D and P are due only to radioactive decay - largest source of error
How does Rb decay?
One radio active isotope decays to non-radioactive Sr isotope by β decay
8737Rb —> 8738Sr + 0-1e
Potential for use in dating rocks proposed by Hahn and Walling (1938)
First age determined by Hahn in 1943
Where are rubidium and strontium found in the crust?
The crust is <1% of the mantle and is enriched 200-400 time is trace elements like Rb and Sr
Rb substitues for K in micas and feldspars
Sr substitutes for Ca in carbonates amd plagioclase
During cooling of a melt Sr concentrates in earlky formed crystals and Rb remains in melt
How can Rb and Sr be used in the fundamental equation?
Some Sr is trapped in the minerals during crystalisation (common strontium)
How do we correct for common strontium?
We use 86Sr to correct for it
What is the final fundamental equation for Rb and Sr dating?
If 87Sr is high the technique is relativelky insensitive to
(87Sr/86Sr)
Value for recent basic volcanic rock is used (0.704)
Otherwise determine from the isochron
How can we show the relationship between (87Sr/86Sr)m and 87Rb/86Sr
Rearrange the fundamental equation
For constant 87Sr/86Sr and t, (87Sr/86Sr)m and 87Rb/86Sr are realted linearly
The equation holds if proportions of Rb and Sr change only as a result of radioactive decay
What is an isochron?
Members of a so-magmatic suite on the isochron
Determines age of crystallisation and initial 87Sr/86Sr ratio
Use minerals where
(87Sr/86Sr)p>> (87Sr/86Sr)i to get a good spread of values
High Rb/Sr = biotite, muscovite, K-feldspar, whole rocks
acidic igneous rocks better than basic
How can we measure the ratio of strontium isotopes?
Mass spec - high resolution magnetic sector instrument
Surface ionisation for solid samples (Rb and Sr salts)
Use of Faraday cups
When is the model age used?
The model age equated to the true age if the minerals have remained in a cloed system
Model age of different minerals of the same rock mau be concordant (agree with each other) or disconcordant (disagree with each other)
If disconcordant whole rock measurements may be applied to rocks of a comagmatic suite if:
Sr remained isotopically homogenous during cooling or the cooling period was shirt
Rocks of the same age are plotted on the isochron
