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
