Lectures 21-24 Flashcards
On average, how old are continents?
2.4 Billion years old
Basic features of Asia
- Shallow earthquakes (<30km)
- virtually no deep ones so no longer subducting (India under Asia)
- slowed and eventually stopped - Earthquakes mainly at reverse (aka at low angles = thrust) and strike-slip faults
- The Mountains are a U shape as the rock ‘flows’ out
Why is India in the position it is?
Fragment of Gondwanaland
- broke away from Africa and Antarctica at 150-125 Ma
- Collision with Asia through Mesozoic
- roughly 150mm yr-1
- convergence slowed to 50mm at 40Ma
Distribution of faults in Asia
- Thrust faults bound the high mountains (Tibet and Himalaya)
- Strike-slip important throughout
- Conjugate pattern
What are the two main thrust faults and which is active?
Main central thrust
- inactive
Main Boundary Fault
- active
How is most of the energy in stars released?
by fusion of
H1 and H2 into helium and a high energy photon
- EXOTHERMIC
- then helium added to get more elements
- why more abundant at even atomic numbers
What element can fusion go upto
Iron 56
- most stable nuclide
- higher then the reactions is endothermic so no fusion - instead = neutron capture
Beta decay
- neutron into proton and electron - electron ejected
- same atomic mass but increases atomic number
Neutron capture 2 processes
- s-process (slow)
- some fusion reactions produce flux of neutrons
- neutron flux from this relatively low
- lots of time to undergo beta decay after capture
- highest = Bi 209 - r-process (rapid)
- some situations have very high flux of neutrons
- so add neutrons a lot on after the other
- some nuclides only made by this process
How did the solar system form?
nebula containing hydrogen and helium
- exploded
- Gravitational contraction of rotating gas cloud leads to dense central region and more diffuse flattened region
- Dust particles into a disc
- numerous planetesimals - collisions
- Eventually larger capture smaller
When did the Earth’s atmosphere form?
129 I r-process into 129Xe by β decay
- 17Myr half life
- atmosphere contains less 129 Xe, relative to other Xe isotopes, than the mantle
- Some iodine stayed behind in the mantle and the atmosphere had to form while 129 I was still active
- very early development
Isotope geochemistry that can estimate universe age
Re and Os
- in iron meteorites
- 187 Os in s-process but some from decay of 187Re only in R
- Re very long half-life = 4 x 10^10 years
Due to this ratio and also the Re decay indicate 8.65 Ga age minimum
When did the Earth’s core form?
182 Hf formed by r procoess and decays into 182W
- half life of 9Myr
- Hafnium fits preferentially into silicates minerals = mantle and crust
- Tungsten for Iron and Nickel and so core
- look for 182W in relation to other tungsten isotopes
- Mantle higher W than carbonaceous chondrites
iron meteorites have smaller than cc as well
OBSERVATIONS INDICATE
- core of planetesimals and mantle formed within parent bodies while 182Hf still active
- so within 3 Myrs of solar system start
Continent formation
147Sm into 143Nd
- 1.06 x 10^11 half life by α decay
- continental crust separated from mantle by some melting process
- during melting Nd prefer liquid and Sm for solid
- so 143 to 144Nd
- Model age - assumes single melt event
- 1.8-2 Ga
How much sediment is subducted back into mantle?
86Sr much greater in continents and continent-derived sediment than in mantle
- so this signature like a dye for continent sediment
- show very little sediment returned to mantle - usually incorporated back into mountain belts
- CONTINENTS VIRTUALLY INDESTRUCTIBLE
Africa- where is continent separating
rift valleys of East Africa
- 3-4mm per year
- many sub-parallel fault scarps - bound basins
What is a graben?
A basin formed by separating continent
- rarely symmetrical (usually faults in one direction) and the asymmetrical ones are half-graben
Faults around Greece
Arc of earthquakes following southern and western border of Greece and Aegean
- subduction zone where med floor under crete
Evolution of a normal fault
GRAPH
What limits the height of mountains?
- isostatic equilibrium is not stress balance
- after height reached then not supported by isostasy
- spread out from mountain
- flat plateaux made
Why are continents different?
- continental crust is more silica-rich than mantle
- so less dense than mantle - this inhibits its subduction
- Also weaker than oceanic crust and mantle - more susceptible to deformation by creep
- Continents can be very old - unlike oceanic
- So continents have many old faults - these can be reactivated
Energy sources on the Earth
- Plate motions = earthquakes, mountain building, deformation, volcanism, metamorphism
- account for about 2 x 10^11 W - Heat loss about 4 x 10^13 W
- most lost through spreading ridges
- Others are radioactive decay
- residual heat from formation of earth - Sun
- 1.73 x 10^17 - Humans
- 1.2 x 10^13
Conduction in the Earth
- rocks poor conductors
- thermal time contant = l^2/π^2k
k= thermal diffusivity = thermal conductivity/density x specific heat capacity
Rayleigh number
Ra = ratio to show if system transfers heat by convection or conduction
=(ρgαΔT(l^3))/κμ)
Diagrams for temperature versus depth distribution
- in earth
GRAPHS
Pattern of mantle convection
- hard to observe - cant see through plates
- correlate gravity and topography
- where surface raised due to uplift from rising - mantle plume
- reduce gravity as hot is less dense
- diagram
Scales of convection
- large-scale convection
- plates move from ridges to trenches - small-scale convection pattern
- indicated by age-depth relationship in the oceans
- flattening of root t
BOTH = partially decoupled by low velocity zone - most likely also the low viscosity zone
- so probably no close correlation between the two
Do we know how deep the convection currents go into the asthenosphere?
no- some believe entire mantle
- others think upper mantle - shallower than 650km
- essentially independent from that in the lower mantle
What are hot spots?
Sites of volcanism
- above rising convecting plumes in asthenosphere
- known to move relative to each other but slowly compared to the relative motion between plates
- provide an ‘absolute’ reference frame to look at motion of plates relative to the asthenosphere
Little correlation between plate area and absolute velocity
- what does this suggest?
Shear forces on the base of the plates are not important
What is slab pull
- indicated to be significant by the length of the subduction zones on the plate boundary which correlate with plate velocity
- arises from the density differences between the cold dense plate and relatively hot, light surrounding lithosphere
- phase change of basalt rocks of the ocean floor to dense eclogite - significant additional force
- oceanic part of plates are the cold dense, strong conductive boundary layers of convecting system.
- They fall back into the asthenosphere under their own weight
- back in asthenosphere slowly warm up and incorporated into general asthenosphere circulation
Surface of the moon
- rugged and cratered highland regions = mainly igneous rocks 4-4.6Ga - plagioclase feldspar
- darker parts are smoother mare/seas made of younger basalts - 3.8Ga
CRATERS
- meteorite impact structures
- suggest heavy bombardment from formation to 3.8Ga
When did most of the tectonic and volcanic activity stop on the moon
- 5-3Ga
- very different earth which is still active
Contrast moon to earth
1/100th mass of Earth
cooled more quickly due to SA:V
How was the Moon formed
Dont know
- could be 2 different
- Condensation of silicate atmosphere from impact to Earth
- CHECK
Composition/surface of Mars
radius roughly half of earth
- g only a third of Earths
- this could be because
1. onset melting by adiabatic decompression of mantles
2. Lithosphere maybe twice as thick as on earth
-60 degrees
water in ice on polar caps (or in near surface rocks)
- surface - looks like a network of drainage channels - maybe from rivers
Rock magnetism on mars
Southern - stripes but in a different pattern from the one seen from Earth plate tectonics
Mars Atmosphere
thin
- gravity also weak
- so bits knocked off by meteorites
- These then to Earth as Martian Meteorites
- from bubbles
- isotopic composition must be from fractionation of the isotopes
- this needs:
1. Mean free path of nitrogen molecules in upper atmosphere
2. Velocity - depends on temperature
Marian Meteorites
1300Ma
- basaltic breccias with glass that traps bubbles of atmosphere
Composition of Venus
- same size as Earth
- surface temp of 450 degrees as closer to sun by 40 million kilometres
- Atmosphere of CO2 and sulphuric acid
- surface pressure 90x that of Earth
- NO WATER ON SURFACE
Surface of Venus
Impact craters
- only the bigger meteorites survive through dense atmosphere
- use to estimate age of surface
- 500 million years
- lots of evidence for faulting and volcanic
- maybe some plate boundaries
- dont know
- also then how does it renew surface?
Is there mantle convection on venus
Evidence for it
- residual topography shows that some areas must be held up by mantle convection
Io
- density and size between the Moon and Mars
- Orbital resonance with Europa keeps interior hot enough to cause melting
- Voyager 2 observed eruption plumes
- probably both silicate and sulfur
Lack of craters suggest surface is active
Europa
cracked surface- mostly ice, with few craters suggests tectonic activity
- density half of moon
- some liquid maybe from tidal heating
Enceladus
Saturn
- small and icy
- old cratered terrain overlain by smoother areas (no impact craters so young)
- very reflective
- icy eruptions which indicates
- oceans of liquid water under ice crust
Triton
Largest satellite of Neptune
- bright solar cap of frozen nitrogen from plumes
- mostly water-ice with frozen methane and maybe ammonia
- Smooth areas are probably icy lavas
