Lecture 7: Cooling the Earth Flashcards
What physical state was Earth in when it formed?
Molten
What generated heat when the Earth formed?
accretion
differentiation
radioactive decay of short-lived isotopes
What has happened to the short-lived isotopes present when Earth was created?
They’ve all disappeared now
How did radioactive decay create heat?
emits particles whose kinetic energy of is then transformed into heat energy
What is proof that the Earth’s interior is still hot?
volcanoes, hot springs, high temperatures in mine shafts, etc.
How much does temperature increase by per kilometre descended near the Earth’s surface
30 degrees
How is the temperature of the planet changing and why?
Slowly cooling because of the flow of heat from the interior to the surface
Is rate of heat flow constant across the Earth’s surface?
No
What is the total heat loss from the Earth’s interior?
10^21 J y^-1
How much smaller is the heat energy lost from the Earth’s interior than heat energy received from the sun?
5000 times - although the sun only heats the surface of the crust
Where does the energy that drives the Earth’s processes come from?
The sun
Where does the energy that drives the Earth’s internal processes and plate tectonics come from?
energy from within the Earth
What are the four main types of isotopes on Earth?
Uranium 238
Uranium 235
Thorium 232
Potassium 40
What is the half life of uranium 238
4.5 Ga - roughly the same age as the Earth
What is the half life of uranium 232
0.71 Ga
What is the half life of Thorium 232
13.9 Ga
What is the half life of Potassium 40
1.3 Ga
What type of mineral is Uranium?
A trace mineral
What is the concentration of uranium on the typical continental crust?
4 ppm
What is the concentration of uranium in the mantle peridotite
0.01 ppm
How can heat flow/be transferred?
Conduction
Convection
Radiation
For what depth range is convection the main heat flow mechanism?
100 to 6370 km
What is convection?
Hot material rising buoyantly
What is the main heat flow mechanism at less than 100km depth?
Mainly convection - the lithosphere it to rigid to convect
How is heat conducted by conduction?
Heat energy makes atom vibrate
Heat conducted by atoms hitting each others
What does the intensity of vibrations determine in conduction?
Temperature
What happens when atoms vibrate in conduction?
Transfers vibrational energy from hotter to colder region
How good are rocks as conductors?
Very poor
Continental crust is - Thicker or thinner More complex in structure and composition or simpler (list types) Older or younger (give age) than oceanic crust
Thicker
More complex in structure and composition (igneous, metamorphic and sedimentary rocks)
Older (Up to approx 4 Ga old)
Why does continental crust act as an insulating layer?
Because it is thick, old and cold
What does continental crust do to heat flow?
Reduces heat flow from the Earth’s deep interior
What proportion of continental heat is generated in the Continental crust
Approx 2/3
Why does the continental crust generate so much energy?
Contains high concentrations of long-lived radioactive isotopes
`What does heat flow on the continents correlate with?
Regional geology
What is heat flow like on areas of recent rifting/volcanism?
q>80 mW per metre squared
What is heat flow like on areas of average continental crust?
q approx 65 mW per metre squared
What is heat flow like on areas of old stable cratonic regions?
q approx 40 mW per metre squared
What does heat flow correlate with in the ocean basins?
age of oceanic crust
What is heat flow like on oceanic areas near MORs
q approx 100-300 mW per metre squared
What is heat flow like on oceanic areas of intermediate age?
q approx 60 mW per metre squared
What is heat flow like on oceanic areas of oldest seafloor?
q approx 30 mW per metre squared
Where is heat flow highest in oceans?
Over ridges (hot lava erupted)
Where is heat flow lowest in oceans?
At trenches (subduction carries cold rock into the mantle)
Does heat flow still vary over different ridges?
Heat flow is higher at fast-spreading ridges such as the East Pacific Rise, than slow ones such as the Mid-Atlantic Ridge
What % of oceanic crustal heat flow originates from within the continental crust?
7%
Which typically has higher heat flow, oceans or continents?
Oceans
When was hydrothermal circulation discovered?
1977
What happens with hydrothermal circulation?
Hot water and minerals expelled at hydrothermal vents
What % of the planet’s total surface heat flow does hydrothermal circulation account for?
25%
Why must the mantle convect?
because conduction is too inefficient to have cooled the mantle sufficiently to be solid
What speed does the mantle flow?
very slowly (like a very very viscous fluid) by solid state creep.
What is the temperature at the base of the continental crust?
1000 degrees
What is the temperature at the core-mantle boundary?
3500 degrees
What is the temperature at the centre of the Earth?
5000 degrees
How do we detect temperature variations in the mantle?
Seismic topography
Why do subducted slabs show up as cold on tomography images?
Subducting oceanic lithosphere stays colder than the surrounding mantle for >200 Ma because its thermal conductivity is so low
What is the average velocities range for mantle movement?
1.5 to 9.5 cm/y.
How does slab pull work?
As subducting plates sink into the mantle due to negative buoyancy, they exert a gravitational pull on the plate at the surface.
Subducting lithosphere represents the downwelling process of mantle convection
The subducting plates provide the main driving force for plate tectonics.
How does continental drag work?
Thick roots (keels) of continental lithosphere may act as brake to horizontal plate motions.
What is the main driving force of the plates?
Subduction
What is plate motion moderated by?
Continental keels and collisions
For plate tectonics to work what must happen?
Descended lithospheric material must be recycled through the mantle and return to the crust as new lithosphere
How does the Earth cool?
Transport of heat by conduction
Transport of heat by convection (more efficient)
