2.2 Geothermal energy Flashcards
Where (in our planet) are the highest temperatures found?
the core
Which part of the planet stores the greatest amount of heat energy and why?
The mantle stores the greatest amount of heat energy because silicates are poor conductors of heat.
What is meant by the term geothermal gradient?
The increase in temperature with depth.
What is the geothermal gradient near the Earth’s surface?
25°C km-1
What are the three sources of heat associated with the formation of the Earth?
- the collisions of planetoids transferring kinetic energy into thermal energy
- the gravity settling (creating core, mantle and crust) transferring potential energy into thermal energy
- the change in state at the inner core - outer core boundary releasing latent heat
At what rate does the inner core grow?
approximately 0.5 mm per year
How does the Earth continue to generate heat (i.e. heat not associated with its formation)?
radioactive decay of K, Ur and Th
Why was the Earth’s temperature gradient much higher in the past (with higher rates of mantle convection)?
the radioactive decay of isotopes with short half-lives (now decayed)
What is the evidence of much higher rates of mantle convection and higher geothermal gradients in the past?
the generation of ultramafic lavas (such eruptions are unknown today)
How is heat transferred within the Earth?
- advection
- convection
- conduction
What is thermal flux?
the rate of heat energy transfer through a given surface per unit time
What is the mean heat flow at the Earth’s surface in continental crust?
65 mWm-2
What is the mean heat flow at the Earth’s surace over oceanic crust?
100 mWm-2
What are the likely interpretations of higher than usual heat flow at the Earth’s surface?
- Magma or hot water is rising through the crust transferring thermal energy by a process called advection
- The underlying mantle is moving upward, at mid-ocean ridges and hot spots, by a process called convection
What happens to the geothermal gradient with increasing depth?
It decreases (the heat increases but more slowly)
Which process is responsible for most of the heat transport through the crust?
convection
Which process is responsible for most of the heat transport through the mantle?
advection
Why is heat production by radioactive decay lower in the mantle than in the crust?
the crust contains the highest concentrations of uranium, thorium and potassium
What is meant by the term advection?
the process by which thermal energy is transferred through a medium by a fluid
What is meant by the term convection?
the process by which thermal energy is transferred by a substance (fluids or rheids) due to buoyancy differences within the substance
What is meant by the term conduction?
the process by which thermal energy is transferred (from atom to atom down a thermal gradient) through a substance with no overall movement of that substance
Which is more efficient: convection, advection or conduction?
convection
Which is the least efficient: convection, advection or conduction?
conduction
Who first proposed that convection in the mantle drives the Earth’s geological processes?
Arthur Holmes (in 1928)
What is now thought to drive convection in the mantle?
the transfer of thermal energy from the lithospheric plates to the atmosphere and the sinking of the cold lithospheric plates by negative buoyancy into the hot mantle by subduction
What is now thought to drive convection in the mantle?
the transfer of thermal energy from the lithospheric plates to the atmosphere and the sinking of the cold lithospheric plates by negative buoyancy into the hot mantle by subduction
