Michaelmas Flashcards
Secular Variation
Continuous drift in intensity and direction of Earth’s magnetic field
MOHO boundary
Boundary between Earth’s mantle and crust
Low velocity zone
Occurs near boundary of asthenosphere and lithosphere, where solid mantle begins to melt
Evidence for density increasing with depth
Rocks from volcanoes are denser than the ones at the crust
Evidence for temp increasing with depth
Gold mines
Evidence for the structure of the Earth
Earthquakes and seismometers, receiving stations, MOHO boundary
Evidence for composition of the Earth
Spectroscopy of the sun, analysis of meteorites
Reasons for changing velocity of seismic waves
change in state, change in density, change in mineralogy
Uniformitarianism
theory that changes to the Earth’s crust are due to continuous and uniform geological processes
Unconformity
gap in continuous rock layer, show missing section of time
Limitations of
biostratigraphy
Many rocks lack fossils, fossils may not extend beyond a single continent
What does analysis of fossils and rocks show?
Fossils= relative ages of rocks
Rocks= environment at the time
How does creep occur?
Recrystallisation
Discloations
Sliding crystals
What is cataclastic flow?
Movement of rocks, which roll over one another. The small granule size is caused by fractures
What is the difference between crust/mantle and lithosphere/asthenosphere?
Crust/mantle - boundaries of composition
L/A- boundaries between different mechanical properties
What is isostasy?
Gravitational equilibrium between Earth’s crust and mantle, so that the crust ‘floats’
What is Pratt’s theory of isostasy?
That density and height are inversely proportional, so mountains are less dense
What is Airy’s theory of isostasy?
That density is uniform and its the thickness that varies, so mountains have roots
Which compensation theory applies to continental and to oceanic crust?
Continental= Airy
Oceanic= Pratt
How can paleomagnetism be observed?
1)Thermo-remnant- magnetism from rocks that have rapidly cooled
2) Chemo-remnant- Magnetism from phase changes of FeO that occur at very low temp, below Curie point
3) Depositional remnant- Magnetism from magentised particles that are trapped in sediment e.g. shales
What is APWP and what can they show?
Apparent Polar Wander Paths, can show the movement of continents in the past, relative to each other, to help understand how continents were arranged and what has happened since then
Define plate tectonics.
Relative motion of rigid plates, accommodated by narrow deformation bands on their edges
What kind of melting occurs at ridges?
Adiabatic compression melting, where change in pressure causes melting. When the adiabat intersects the solidus
What kind of melting occurs at subduction zones?
Flux melting, addition of water lowers solidus temperature. Which causes earlier melting, however the movement of magma upwards is slow
What do marine magnetic anomalies show?
Shows symmetry in ridge axis. Similar patterns are found on either side which suggest sea floor spreading.
Show possible fracture zones, if the patterns are offset.
What is the Benioff zone?
Zones where earthquakes occur due to the subduction of the oceanic crust underneath the continental crust
Why is there a max thickness to oceanic crusts?
The crust is denser (cooling by conduction not convection) than the mantle below so is gravitationally unstable. Limit to how thick it can get before it becomes convectionally unstable. Small scale convection removes some of the lithosphere.
What is differentiation?
Separation of Iron from silicates, occurring in many planets. Earth undergoes further differentiation, into the crust/mantle and atmosphere
What is nucleosynthesis?
Formation of H, He and Li at the start of the universe
Why does the graph of abundance against atomic mass
i) plateau
ii) why is it spiky
iii) why is there a peak at Fe
i)Plateaus because larger elements are harder to make, require explosions of large stars which are rare
ii)spiky because the even numbered isotopes are favored in fusion reactions
iii)peak at Fe as its the heaviest element to still be exothermic and so help sustain the star
What are the 4 stages of the life cycle of an average star?
1) Pre- main sequence (burn deuterium)
2) Main sequence (burn H)
3) Post- sequence/ Red giant (burn He)
4) Post giant/ White dwarf (no envelope)
What are the two main detection methods and what do they show?
1) Transit- radius of planet
2) Doppler/ Radial vel- mass of planet
In which direction is the planet moving if you see blue shift?
Towards you
Biases in detection methods
1) Semi- major axis- (distance)
2) Size of planet
What is the semi-major axis?
Distance between the planet and the star
With statistical analysis what does it suggests about the distribution of exoplanets?
High abundance of Earths and superearths. Low abundance of giant planets, even though more of them have been detected
What can be used to model the composition of planets?
Equation of state which uses density and mass
Why is detecting an atmosphere of a planet so difficult?
1) accounts for a very small part of the overall radius
2) clouds will reflect the light so atm not detected
What effects the global energy budget
1) albedo
2) Brightness of the star
How can the liquid water habitable zone be calculated?
Relating the T equation with distance, and calculating zone between 0 and 100 degrees celcius
What occurs on the inner edge of the LWHZ?
Water is present as a strong greenhouse gas, causes temp to increase and water to be lost by evaporation
What occurs on the outer edge of the LWHZ?
CO2 is present, and not an effective greenhouse gas, reaches its max at about 8bar. Above 8bar increases albedo. Causing water to freeze and positive feedback
Ways to detect for life
1) Color of the planet
2) Biosignatures (O2 or perhaps CH3)
Describe the changes in insolation with latitude.
Changes in amount of radiation that actually reaches the surface with latitude. Higher latitude has to travel through more atmosphere if its hitting perpendicular to the equator
What is zonal radiation balance?
An overall balance in radiation, even with varying intensities of radiation from different zones in the world. e.g. low at poles but high at low latitudes, creating an overall balance
What does Planck’s law calculate?
Solar irradiance, suns power
What does Wien’s frequency displacement law show?
Maximum wavelength of emission by taking derivative of Planck’s law
What is a greenhouse gas?
A gas that absorbs IR. When IR absorbed it changes its dipole moment
What are the vibrational modes associated with a greenhouse gas?
Bending
Asymmetric stretching
What is radiative forcing?
Budget between incoming and outgoing radiation
Calculated by incoming- outgoing
What are the 3 causes for climate change?
1) change in solar constant
2) change in albedo
3) change in emissivity
Describe how changes in solar constant can occur
1) Sun spots- occurring on an 11 yr cycle
2) Faint sun paradox- sun was 30% dimmer at formation
Describe possible changes in albedo
1) clouds, water vapour= positive and negative feedback
2) Ice- very reflective
3) volcanic eruptions- sulfur dioxides = incr albedo
How was the Keeling curve created?
Taking atmospheric recordings at Mauna Loa since 1958 and accounting for anthropogenic rise and seasonal variation
What’s the other way of measuring atmospheric CO2 and other gases?
Using ice cores that date back 800,000 years. Measuring the conc of gases in the trapped air bubbles within the ice
Name 4 examples of fast feedback mechanisms
1) clouds/ water vapour
2)Ice
3) Atmospheric conc
4) Upper ocean
Name 4 examples of fast feedback mechanisms
1) clouds/ water vapour
2)Ice
3) Atmospheric conc
4) Upper ocean circulation
Name 3 long terms feedback mechanisms
1) Deep ocean circulation
2) vegetation
3) Ice sheets
What are future predictions of temp change based on?
5 different models of socio-economic pathways.
All show increase in temp and anthropogenic release of greenhouse gases, at least a 1 degree rise in global temps
How do you calculate residence time?
volume of carbon/ total fluxes
Size of carbon stores from largest to smallest?
Lithosphere- Deep Ocean- Land Biosphere- Surface Ocean- Atmosphere
Residence time shortest to longest
Atmosphere- Surface Ocean- Biosphere- Deep Ocean- Lithosphere
Why is the delta C13 decreasing?
Burning of fossil fuels of fuels C12
What is the reason for the seasonal fluctuations in conc of CO2 and delta C13?
Seasonal fluctuations due to photosynthesis.
delta C13 higher in summer as C12 is used
CO2 conc lower in summer as more is removed
What are the 3 ways carbon can exist in seawater?
Carbonate ion
bicarbonate ion
carbonic acid
Why can ocean uptake and atmospheric CO2 lead to a positive feedback mechanism?
Warmer water holds less CO2, so more in atmosphere, causing more warming
What is chemical weathering?
chemical breakdown of rock by a weak acid
How does chemical weathering lead to a negative feedback mechanism?
Higher CO2 conc, increases rate, removing CO2, so decreasing rate, allowing CO2 to build up again
What is ocean acidification?
When oceans become more acidic due to dissolving of CO2, expected seawater pH to drop by 0.24 is conc is doubled
What is benthic foraminifera and planktonic foraminifera?
Benthic found on sea floor
Planktonic found in surface water
Why is benthic foraminifera better to look at than planktonic?
At sea floor undergoes less changes in oxygen isotope ratio
How does delta O18 in sea change when there is more ice?
Ratio increases, O16 more likely to evaporate , form clouds and snow. And be trapped in ice, so ratio increases in ocean
What is the oxygen ratio dependent on?
Temperature
Ice volume
What led to the build of ice in Antarctica?
ACC- Antarctic Circumpolar Current, created by tectonic changes causing the circulation of colder water, allowing ice to form
What is the mantle mostly composed of?
Silicates (Si, O)
Define a mineral
An element or chemical compound that is normally crystalline that has formed under geological processes
What is an endemember?
A mineral with just one type of cation in the structure
What is a solid solution?
A mineral with a mix of cations in its structure due to substitution
What is the max size difference for substitution to occur?
15%
How does temperature effect substitution of cations?
Increase temp
- more disorder
- more flexibility
What are bridging and non bridging oxygens?
Bridging form bonds with 2 Si
Reduce overall -ve charge
What is the bond angle and structure of SiO4?
109.5
tetrahedra
Describe the structure and formula of single chain tetrahedra
2x non-bridging
2x bridging
[Si2O6]2-
Describe the structure and formula of double chain tetrahedra
2 3x bridging 1x non
2 2x bridging 2x non
[Si4O11]6-
Describe the structure and formula of continuous
3x bridging
1x non
[Si2O5]2-
Describe the structure and formula of 3D framework
4x bridging
[Si2O]
What is the ideal radius ratio?
Radius ratio which is just large enough to hold anions apart without rattling
Describe the structure of Orthosilicates
Formed by single tetrahedra
Olivine, Garnet
Needs +4 charge to balance it out
Describe the structure of single chain silicates
90 degree cleavage
Form I beam
pyroxene
monoclinic, orthorhombic
Describe the structure of double chain silicates
56 and 120 degree cleavage
Wider I beam formed
Amphiboles
Describe the structure of sheet silicates
Sheets stack up in sandwiches
Weakly bonded by cations
Micas (muscovite & biotite)
Describe the structure of tectosilicates
3D frameworks
No cations requires
If substitution occurs may require cations
Quartz
What is a primitive lattice?
Where the lattice points are on the corners only
What is a non-primitive lattice?
Where the lattice points aren’t only on the corners
What is the body centred lattice?
Point in the middle of the 3D shape
What is a face centred lattice?
When points are in the middle of all 6 faces
What is a C face centred lattice?
When there are lattice points in the middle of the C face
What is a rhombohedral lattice?
Where all the lengths are the same however the angles are not equal to 90 degrees
What is n-fold symmetry?
symmetry where an object rotates by 360/n
What is a diad symmetry?
a 2 fold symmetry where motif rotated by 180 degrees
How do you identify a plane?
Plane is normal to the axis e.g. (100) crosses x axis
What is habit?
Overall shape of the crystal, can help infer the environment
What is twinning?
Intergrown crystals related by symmetry or rotation
How does twinning occur?
Mistakes during crystal growth
Mechanical deformation
Cooling onto a more stable structure
What are the properties of light?
Transverse wave, perpendicular electric and magnetic field
What is unpolarised light?
Light that vibrates in all directions
How can refractive index be calculated?
Vel in vacuum (3x10^8)/ vel in material
What is the refractive index property of an isotropic material?
Same refractive index in all directions
What is the refractive index property of an anisotropic material?
Light vibrating in different directions will have difference refractive indices
What ways do the polarisers polarise light in a petrographic microscope?
Sub-stage= E-W
Eye piece (analyser)= N-S
What can be seen under plane polarised light?
Shape- euhedral
Cleavage planes
Twinning
Pleochroism
Relief
Colour
Moving stage down raises the focal plane, how would the Becke Line Test be carried out?
line moving in = high RI than medium
line moving out = lower RI than medium
What does a large n mean for the speed of light?
Slow moving
What is double refraction?
When light is refracted, creating a ordinary and extraordinary ray
Why is there extinction every 90 degrees?
Light permitted to travel in 2 directions perpendicular to each other, when direction parallel to polariser, extinction occurs.
What is retardation/ optical path difference?
The distance traveled by the fast component before the slow component exits the material.
How do you calculate birefringence?
n1-n2
What are the types of ray combination and their respective visibilities?
Destructive (multiple of wavelength)= light not pass through
Constructive (multiple of half of wavelength)= max light through
What is phase difference?
Difference in wave component (between the 2) when exiting a material
What direction does the wave travel in if they are in phase with each other?
Same as incident (E-w)
What direction does the wave travel if they are out of phase with each other?
90 degrees to incident
N-S
What is the optical indicatrix?
3D representation of relative refractive indices
What is the optical indicatrix for an isotropic material
Sphere shape
What is the optic axis?
View direction perpendicular to isotropic section
What shape is the isotropic section?
Circular
What types of minerals have a uniaxial indicatrix?
tetragonal
trigonal
hexagonal
What is the rotation axis in a unaxial indicatric and what is the optic axis?
Rotational = z axis
Optic axis= parallel to z axis
What is the positive uniaxial indicatrix?
e> w e is up and w is across, shape of a rugby ball
What is the use of an indicatrix?
Can find orientation of thin section, max anisotropy sliced parallel to optic axis
Zero anisotropy when sliced perpendicular to optic axis
What minerals have a biaxial indicatrix?
Orthorhombic
Monoclinic
Triclinic
How is a biaxial different to a uniaxial indicatrix?
Contain 2 isotropic sections and so 2 optic axes
What is the orientation of the indicatrix for Orthorhombic?
Optical aligned with crystallographic, diad symmetry along each axis
What is the orientation of the indicatrix for monoclinic?
one optical axes parallel to y axis only
Free to rotate around y
What is the orientation of the indicatrix for triclinc?
No symmetry at all, any orientation
What is thermodynamics?
Study of how energy is transferred between different states of matter
What are the 3 types of systems?
Isolated- No matter or energy exchange
Closed- only energy exchange no matter
Open- Exchange of matter and energy
What is stable and metastable?
Stable- When a reaction is at equilibrium at lowest energy state
Metastable- stable but at a higher energy state
What is the first law of thermodynamics?
Energy cannot be created or destroyed
What is work and how is mechanical work calculated?
Work is the energy transferred via force to displace something
Mechanical work is -w= pressure x volume
or w= force x distance
What is the second law of thermodynamics?
Heat always flows from hotter to colder, and not all energy is available for work
What is the example of CO2 sequestering rock?
Rock in Oman, Olivine turns into Quartz and Magnesite
The reaction increases volume by 80%
Causes cracking and more rocks to be exposed to CO2 and react
Negative enthalpy, exothermic
Increases rate of reaction of surrounding rocks
How do you calculate Gibbs Free Energy change?
Enthalphy- (temp x entropy)
How is a phase diagram created?
Plotting temp against pressure and using a Clausius- Clapeyron slope with the gradient being dP/dT= dS/dV
How is thermodynamics experimentally used to determine Earth’s interior?
Mixture of rocks (relative composition to mantle)
Exposed to high specific temp and pressure in
laser heated Diamond anvil cell or
large volume press
Then analysed using X-ray diffraction
How does pressure effect mineralogy?
Increasing pressure changes the types of minerals found, Increase pressure turns pyroxenes to garnet
What is the evidence of water on Mars?
hydrated minerals
carved drainage basins
sedimentation by water
What is activity?
Thermodynamic version of concentration which takes into account the availability of the molecule, e.g if it’s in the right state to react
What does K (law of mass action) mean?
calculation of ratio of products over reactants to the stoichiometric power
Why is thermodynamics useful?
Used to predict outcomes
Can be applied to variable compositions such as natural water
Help construct an idea of ancient environments
Ways of heat production from the Earth
Radioactive decay (U, Th)
Heat from formation
Conduction of crust, Convection of mantle due to high Rayleigh number
When was the Last Glacial Maximum (LGM)?
20,000 years
On average how thick is the continental crust and how thick is the oceanic crust?
Continental - 35km
Oceanic - 8km
What is eccentricity?
Ellipticity of the Earth’s orbit
What is precession?
Axial and ellipitical precession
Axial precession is due to Earth’s wobble
What is obliquity?
Tilt of Earth’s rotational axis
How is the lithosphere and asthenosphere different?
Lithosphere- cool and strong, move by power law creep
Asthenosphere- hot and weak, move by diffusion creep
What are the differences between gas giants, ice giants and terran planets?
Gas giants x300 times the Earth, H/He atmosphere
Ice giants x20 times the Earth, ice dominated
Terran Silicates, sharp shift from rock to atmosphere
What is a protoplanetary disk?
Newly forming star
Will form its own solar system
