Earth and Other Planets (Lectures 21-24) Flashcards
What are the idealised types of faults?
Normal faults: extensional in origin, slip is mainly down-dip
Reverse faults: shortening, slip mainly in dip direction, reverse faults dipping less than 30 degrees are “thrust”
Strike-slip: horizontal slip, parallel to strike
Fault types are idealised, what do many faults involve?
Oblique slip
Combination of strike-slip and normal/reverse
What kind of earthquakes are seen in Asia, and why?
Shallow earthquakes
Reverse and strike-slip faults
India collided with Asia
What does the rift valley in East Africa contain?
Sub-parallel fault scarps that bound basins
Volcanoes rich in alkali elements
Hot spot
Normal-faulting earthquakes
What is the arc of earthquakes in Greece and the Aegean sea associated with?
A subduction zone where the Mediterranean slides beneath Crete
What kind of earthquakes are seen in Greece and the Aegean sea?
Normal faulting
What limits the height of mountains?
Mountains are held up by the surrounding lithosphere because isostatic equilibrium is not stress balance
The thicker crust has greater pressure than the thin crust surrounding it
How are continents different from the mantle and oceanic crust?
More silica-rich than the mantle
Less dense than the mantle and so inhibits subduction
Weaker than oceanic crust and mantle because it has a lower melting T, more susceptible to deformation by creep
Continents are far older than oceans
Contain old faults and lines of weakness from prior deformation, these can be reactivated
What is the energy source for plate motion, and what must it account for?
Account for energy lost in earthquakes 2 x 10^11 W
Heat loss from the Earth is 4 x 10^13 W
Heat loss sources: heat production by radioactive decay or residual heat left over from Earth’s formation
What is a good measure of the rate of heat conduction?
Thermal time constant
Tc = (l^2)/((π^2) x κ)
κ is roughly 10^-6 m^2 s^-1
What information can be obtained from the thermal time constant?
For 125 km lithosphere, Tc = 62 Ma
For a conductive Earth, Tc is roughly 10^11 years
So significant cooling could only have occurred down to 1000km depth
How can you test whether a system will conduct or convect?
Use the Rayleigh Number
Ra = (ρ x g x α x T x l^3)/(κ x μ)
If Ra is greater than 1000 convection dominates,
What is the Rayleigh number for the mantle?
About 10^6
Why does convection occur in the mantle beneath the lithosphere?
Homologous temperature is high
Allows convection to occur by solid state creep
How can we tell where there are mantle plumes?
Correlation of gravity and topography
Topography increases
Gravity slightly increases overall because the hot mantle is low density but the topography increases it more
What evidence is there for large scale and small scale convection pattern?
Large scale: Plates move from elevated and hot ridges to low and cold trenches
Small scale: the age-depth relationship in oceans as heat is supplied to the base of the plates
What is a hot spot?
A site of volcanism above rising convecting plumes in the asthenosphere
For the forces in subduction, what suggests that shear forces on the base are not important, and what suggests that slab pull is?
Shear forces: little correlation between plate area and ‘absolute’ velocity
Slab pull: length of subduction zones on the plate boundary correlates with plate velocity
Where does slab pull come from?
Density differences between the cold, dense plate and the relatively hot, light surrounding lithosphere
Also the phase change of basalt rocks to dense eclogite assemblages
What may counteract the viscous drag of the mantle on the plate motion?
Low viscosity under the plates
What happens to subducted slabs?
Fall back into the asthenosphere
Slowly warm up
Incorporated into general asthenosphere circulation
What is isotope geochemistry?
Study of the naturally-occurring isotopes of elements
Most energy in a star is produced by what?
The fusion of hydrogen into helium
One way: 1,1-H + 2,1-H -> 3,2-He + γ
γ is a high-energy photon released by these exothermic reactions
What form is the matter in, in stars?
Nuclei and free electrons
Plasma
Why aren’t elements heavier than 56-Fe made in fusion processes?
How are they made?
Reactions are all endothermic
By successive nucleon capture
Neutron capture occurs by which two processes?
S-process: Flux of neutrons is low, the rate of capture is slower than β decay
18-O + 2 x 4-He -> 22-Ne + 4-He
22-Ne + 4-He -> 25-Mg + 1-n
continues up to 209-Bi
R-process: Flux of neutrons is very high, the rate of capture is higher than β decay
What is the sequence of steps in the formation of the solar system?
Gravitational contraction of a rotating gas cloud leads to a dense centre and a diffuse flattened region
Dust particles from nebula settle into a disc
Accretion of dust into planetesimals (km in diameter)
Collisions between planetesimals occur, large ones form
Outline carbonaceous chondrites
Similar elemental abundances to the Sun
Except for very volatile elements and some used in fusion reactions
Not uniform, contain mineral inclusions called chondrules
Outline basaltic achondrites
Similar to terrestrial basalts
Age close to carbonaceous chondrites
What do basaltic achondrites show?
That early planetary differentiation and magmatic activity occurred on planetesimals within a few Myr of solar system formation
What isotopic ratio is used to identify the age in carbonaceous chondrites and why?
Contain plagioclase feldspar with 26-Mg
26-Mg comes from β decay of 26-Al
So the ratio of 26-Al to 26-Mg
Which isotopic ratio gives a minimum age of the universe of 8.65 Gyr?
Where does it come from?
187-Os to 187-Re
Proportions in iron meteorites
β decay of 187-Re into 187-Os
Which isotopic ratio is used for the age of the Earth’s atmosphere?
129-Xe to 129-I
β decay of 129-I
Which isotopic ratio is used for the age of the Earth’s core?
182-Hf β decays into 182-W
Looking at the abundances of 182-W, what do we find, and what does this indicate?
Relative abundance for 182-W: Earth’s mantle > carbonaceous chondrites > iron meteorites
Cores of planetesimals and Earth’s mantle formed when 182-Hf was still active
Estimate cores of planetesimals formed within 3 Myr of the start of the solar system, and Earth’s core within 30 Myr
What do the terms: lithophile, siderophile, chalcophile and atmophile mean?
Lithophile elements remain close to the surface because they react readily with oxygen
Siderophile elements tend to be in the core
Chalcophile elements remain close to the surface because they react readily with sulphur
Atmophile elements tend to be on or above the surface
Which isotopic ratios are used for the age of the formation of the continents?
Why?
What age does this give?
147-Sm α decays into 143-Nd
Use 143-Nd to 144-Nd ratio in the continents and 147-Sm to 144-Nd ratio in carbonaceous chondrites
Continental crust separated from the mantle by melting
During melting, Nd prefers liquid phase, Sm prefers solid residue
1.8-2.0 Ga
What are the ages of the various areas of the Moon’s surface?
Cratered highland regions: igneous rocks, mostly plagioclase feldspar 4.0-4.6 Ga
Darker areas ‘seas’: basalt lava averaging 3.8 Ga, youngest 3.2 Ga
When did the Moon’s tectonic activity likely stop?
Why?
2.5-3.0 Ga
1/100th of the mass of the Earth so fewer radioactive elements
Cooled faster due to higher surface area/volume ratio
The value of g on Mars is a third of the value on Earth and its radius is half of Earth’s, what are the consequences?
The onset of melting by adiabatic decompression starts much deeper than on Earth and continues for longer so much more melt forms
If heat production scales with volume and heat loss scales with surface area, we expect the lithosphere to be twice as thick as Earth’s
What is the surface temperature on Mars?
Average of -60 degrees C
Where does water exist on Mars?
Form of ice in the polar caps
Or in near-surface rocks
What are the properties of Martian meteorites?
Age of roughly 1300 Ma
Basaltic breccias with glass that traps atmospheric bubbles with a nitrogen isotope ratio different to Earth’s
The isotopic composition of Mars’s atmosphere must arise from fractionation, what does this require?
Mean free path of N molecules in the upper atmosphere must be comparable to the planet’s size
The velocity of the molecules must reach the escape velocity
What tectonic activity is visible on the surface of Venus?
Faulting
Volcanic activity
Possible plate boundaries like on Earth, including subduction zones
Why is it not clear if Venus is still tectonically active?
The average age of crust is nearly the same everywhere
Using residual topography on Venus reveals which features that support convective flow in the mantle?
Thick crust - like Tibet
Convective support - like Hawaii
Why might there be no plate tectonics on Venus?
No water to return to the mantle, nor to lubricate the plate motion
