2.2 Geothermal energy

Question 1

Where (in our planet) are the highest temperatures found?

Answer 1

the core

Question 2

Which part of the planet stores the greatest amount of heat energy and why?

Answer 2

The mantle stores the greatest amount of heat energy because silicates are poor conductors of heat.

Question 3

What is meant by the term geothermal gradient?

Answer 3

The increase in temperature with depth.

Question 4

What is the geothermal gradient near the Earth’s surface?

Answer 4

25°C km^-1

Question 5

What are the three sources of heat associated with the formation of the Earth?

Answer 5

1. the collisions of planetoids transferring kinetic energy into thermal energy
2. the gravity settling (creating core, mantle and crust) transferring potential energy into thermal energy
3. the change in state at the inner core - outer core boundary releasing latent heat

Question 6

At what rate does the inner core grow?

Answer 6

approximately 0.5 mm per year

Question 7

How does the Earth continue to generate heat (i.e. heat not associated with its formation)?

Answer 7

radioactive decay of K, Ur and Th

Question 8

Why was the Earth’s temperature gradient much higher in the past (with higher rates of mantle convection)?

Answer 8

the radioactive decay of isotopes with short half-lives (now decayed)

Question 9

What is the evidence of much higher rates of mantle convection and higher geothermal gradients in the past?

Answer 9

the generation of ultramafic lavas (such eruptions are unknown today)

Question 10

How is heat transferred within the Earth?

Answer 10

1. advection
2. convection
3. conduction

Question 11

What is thermal flux?

Answer 11

the rate of heat energy transfer through a given surface per unit time

Question 12

What is the mean heat flow at the Earth’s surface in continental crust?

Answer 12

65 mWm^-2

Question 13

What is the mean heat flow at the Earth’s surace over oceanic crust?

Answer 13

100 mWm^-2

Question 14

What are the likely interpretations of higher than usual heat flow at the Earth’s surface?

Answer 14

1. Magma or hot water is rising through the crust transferring thermal energy by a process called advection
2. The underlying mantle is moving upward, at mid-ocean ridges and hot spots, by a process called convection

Question 15

What happens to the geothermal gradient with increasing depth?

Answer 15

It decreases (the heat increases but more slowly)

Question 16

Which process is responsible for most of the heat transport through the crust?

Answer 16

convection

Question 17

Which process is responsible for most of the heat transport through the mantle?

Answer 17

advection

Question 18

Why is heat production by radioactive decay lower in the mantle than in the crust?

Answer 18

the crust contains the highest concentrations of uranium, thorium and potassium

Question 19

What is meant by the term advection?

Answer 19

the process by which thermal energy is transferred through a medium by a fluid

Question 20

What is meant by the term convection?

Answer 20

the process by which thermal energy is transferred by a substance (fluids or rheids) due to buoyancy differences within the substance

Question 21

What is meant by the term conduction?

Answer 21

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

Question 22

Which is more efficient: convection, advection or conduction?

Answer 22

convection

Question 23

Which is the least efficient: convection, advection or conduction?

Answer 23

conduction

Question 24

Who first proposed that convection in the mantle drives the Earth’s geological processes?

Answer 24

Arthur Holmes (in 1928)

Question 25

What is now thought to drive convection in the mantle?

Answer 25

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

Question 26

What is now thought to drive convection in the mantle?

Answer 26

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