Geology of the Lithosphere
1. Earth’s Heat Flow
• What is the Earth’s heat flow?
• What is the origin of this heat flow?
• How does heat flow vary with depth?
• How is rock strength related to temperature?
• How and why does heat flow vary across the Earth’s surface?

What is the Earth’s heat flow?
What is Heat Flow?
Heat flow is ….
the rate at which heat flows from the Earth’s interior through one square metre of the Earth’s surface.
How is Heat Flow measured?
Heat flow is measured ….
in watts per square metre.
W m -2
What is the average Heat Flow over the Earth’s surface?
Average heat flow over the Earth ….
0.08 W m -2
How does Heat Flow from the Earth’s interior to the Earth’s surface?
Conduction… Process by which heat in a material is transferred by the interaction of atoms & molecules within a material
Convection… Process by which heat is transferred by a moving mass (e.g. a liquid) carrying the heat with it.

What is the Earth’s heat flow?
How many 100-watt light bulbs could be continuously illuminated, in theory, by the heat emitted from an area the size of 1 football pitch?
Football pitch = approx. 100m x 70m
Average heat flow = 0.08 Wm -2
Area of football pitch = 100 x 70 = 7000
0.08 x 7000 = 560
= 5.6 100-watt light bulbs

What is the Earth’s heat flow?
Average continental heat flow (0.06 W m -2 )
Average global heat flow (0.08 W m -2 )
Average oceanic heat flow (0.095 W m -2 )

What is the origin of the this heat flow?
1. Residual or primordial heat form the Earth’s formation
2. Heat released from iron crystallisation to form solid inner core
3. Heat emitted by radioactive decay of isotopes of uranium, thorium & potassium in the crust.

How is heat flow transferred in the Earth?
Temperature C
Core

How does heat flow vary with depth?
Lithosphere
Geothermal gradient = 30 ° C/km
(Conduction)
Asthenosphere geotherm
Geothermal gradient
= 10 ° C/km
(Convection)

How is rock strength related to temperature?
Continental crust
Oceanic crust
Upper mantle
Asthenosphere

How and why does heat flow vary across the Earth’s surface?

What is the relationship of surface heat flow with plate tectonics?

Define meanings & give figures. Say what you see!
Explain & give reasons for
With the aid of a labelled diagram, describe and account for the variation in heat flow across an ocean basin and an adjoining arc system.
(25)
How heat is transferred from inside the Earth to the outside
• Read & dissect the question (doing words & key words)
• Plan an outline & write out the essay question
• Use paragraphs as basic structuring unit in the essay
1. Introduction
2. Main Body
Say what you are going to do by defining your version of the title. Show you have understood every significant word of it. Define key words.
Theme of paragraph & should link with concluding sentence of paragraph above.
Topic sentence
•Paragraph 1
•Paragraph 2
• Presentation
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Make it easy for them to read.
• supporting evidence
• supporting evidence
• supporting evidence
•Paragraph 3
•Paragraph 4
•Paragraph 5
•Paragraph 6
Each paragraph should be on a different aspect of the topic.
Clear & well presented.
Spelling & punctuation.
Concluding sentence
3. Conclusion
Briefly summarise the essay by referring to the title

With the aid of a labelled diagram, describe and account for the variation in heat flow across an ocean basin and an adjoining arc system.
(25)
Introduction:
Heat flow is the rate at which heat is moving upwards through a particular part of the Earth’s crust. This rate is measured in Watts produced per square metre (Watts/m²). This essay will describe how this heat flow varies across an ocean basin and adjoining volcanic arcs from its highest at MOR to its lowest at ocean trenches, and explain how this is directly related to plate tectonics and mantle processes within the lithosphere.
Paragraph 1:

The average global heat flow is about 0.08 Watts/m
²
, though it varies considerably from place to place. The average heat flow from ocean basins (0.095 Watts/m
²
) is higher than this global average, although this too varies across an ocean basin. The highest heat flow is from oceanic lithosphere up to 35 Ma old, such as near spreading oceanic ridges, for example the East
Pacific Rise in the Pacific Ocean. Here, the heat flow is >0.3 Watts/m
²
, whereas for oceanic lithosphere older than 65 Ma, the mean heat flow is much lower at <0.05 Watts/m
²
. The lowest heat flow values (<0.04 Watts/m
²
) of all come from ocean trenches, such as the Peru-
Chile Trench off the west coast of South America. All oceans show a similar distribution of heat flow.
In contrast, the average heat flow from the continents is about 0.06 Watts/m², but just like in oceanic crust, there is a decrease in heat flow with increasing age of continental rocks.
However, the rate of decrease is very different between the two types of crust. In oceanic areas heat flow decreases quickly, whereas in continental areas heat flow decreases much more slowly. The highest heat flow on the continents is at volcanic arcs such as beyond the
Peru-Chile trench in the Andean mountains of South America, here the heat flow can reach
0.2 Watts/m
².

a). Describe the variation in surface heat flow across a spreading ocean basin and an active continental margin.
b). Discuss the importance of surface heat flow in supporting the theory of plate tectonics.
(25)
Ocean Average
Continental Average
MOR (<35Ma) East Pacific
Rise
0.095 W/m2
0.06 W/m2
Oceanic Lithosphere
(>65Ma)
Ocean Trenches
Volcanic Arcs
Hot Spots
Pacific
Ocean
Peru-Chile
Trench
Andes
Hawaii
>0.3 W/m2
<0.05 W/m2
<0.04 W/m2
>0.2 W/m2
>0.2 W/m2
Lithosphere pulled apart allowing asthenosphere to rise nearer surface (more convection & less conduction).
Erupting magma takes heat with it to surface
(advection).
Seawater penetrates cracks in crust, gets heated up and rises carrying heat with it in hydrothermal vents (convection)
Further away from heat source (MOR).
Lithosphere cools & thickens away from MOR
(more conduction & less convection).
Old, cold oceanic lithosphere cools mantle.
Dehydration of subducting plate cools descending slab.
Rising magma (advection).
Rising magma (advection).

a). Describe the variation in surface heat flow across a spreading ocean basin and an active continental margin.
b). Explain how surface heat flow relates to plate tectonic and mantle processes within the lithosphere.
(25)

With the aid of a labelled diagram, describe and account for the variation in heat flow across an ocean basin and an adjoining arc system.
(25) a). Describe the variation in surface heat flow across a spreading ocean basin and an active continental margin.
b). Explain how surface heat flow relates to plate tectonic and mantle processes within the lithosphere.
(25) a). Describe the variation in surface heat flow across a spreading ocean basin and an active continental margin.
b). Discuss the importance of surface heat flow in supporting the theory of plate tectonics.
(25)