Heat and Convection in the Earth
We have seen how seismology, etc. enables us to determine density, seismic velocity etc. as a function of depth in the Earth.
Observation also tells us that the Earth is active - volcanoes, earthquakes, mountain belts and magnetic fields.
These must be due to an internal energy or heat source.
What is the thermal structure of the Earth?
Temperature is the most poorly constrained planetary parameter.
It cannot be directly obtained from seismology - need to carry out other studies, e.g. heat-flow, etc.
We know - Earth has a hot interior:
-hot springs, etc.;
-hot mines at depth.
Heat flows from centre of Earth to surface via:-
conduction-thermal vibrations: every atom is physically bonded to its neighbours in some way. If heat energy is supplied to one part of a solid, the atoms vibrate faster. As they vibrate more, the bonds between atoms are shaken more. This passes vibrations on to the next atom, and so on:
Silicates are poor conductors and therefore conductive heat transfer is only important in cold lithosphere.
We will see that in the Earth, convective transfer occurs.
Radiative heat transport (e.g. heating like an electric bar fire) may play a minor role deep in Earth where T > 2000 K. But minerals are probable too opaque for this to me very significant.
Two important questions:-
(1)How does heat-flow from Earth compare with other energy sources?
(2)Where does Earth's heat energy come from?
Energy Sources on Earth
Energy sources responsible for Earth Processes are:-
- gravitational energy from Sun and Moon
-Earth's internal heat
-Earth's rotational and gravity field
The external sources tend to dominate surface processes such as ocean circulation and tides, atmospheric processes, biological activity.
Internal sources responsible for volcanism, earthquakes, metamorphism, mountain building, etc.
Despite massive effects of internal sources (e.g. creating the Alps), they are much less than external energy:
Sun ->1.7 x 1017 W to Earth, of which 60% reaches the surface.
Earth heat-> 4 x 1013 W to surface.
i.e. ~4500 times more energy from Sun than from Earth's interior!
The energy reaching the surface of the Earth from within can be measured to get heat flux, q.
q = - k dT/dz
Units of heat flux = Wm-2 which is equivalent to Js-1m-2, and k is the thermal conductivity (Wm-1K-1).
The average heat flow from the Earth gives a q of approximately 0.08Wm-2 (equivalent to 80mWm-2).
But the flow is very uneven. Some areas, such as volcanoes and mid-ocean ridges have very high q ~400 mWm-2.
Could we use the average heat flow as an industrial energy source?
If we collect all the energy that flows through a football pitch (100 x 70 m2), then:
Total power=7 x 103x0.08=560 W
=5.5 light bulbs!!
Not a generally viable power source. But locally
Grand Prismatic Geothermal Pool in Yellowstone National Park
Geothermal power only possible in areas of anomalous heat flow (eg Iceland or Japan).
Nevertheless the internal energy in the Earth can generate large scale changes on geological time scale.
What are the sources of the Earth's internal heat processes?
Sources of the Internal Heat of the Earth
(i) primordial heat - generated during Earth formation.
(ii) radioactive heat - generated by long-term radioactive decay
Primordial Heat Sources
These are somewhat speculative as they depend on the hypotheses of Earth formation.
(a) Accretion energy - conversion of K.E. of smaller planetary objects into heat as they collided on accretion.Collision -> seismic shock -> internal heating.
(b) Adiabatic compression - as compresses something cause it to heat up (c.f. bicycle pump) -> adiabatic heating.
As more particles accreted in planet those at centre squashed by growing gravitational load -> adiabatic heating.
(c) Core formation Energy - settling of Fe to centre of Earth converts P.E. of iron to heat energy.
(d) Decay of short-lived radio-isotopes - in early-solar system have isotopes such as Al26, Cl36, Fe60, with half-lives of approximately 0.3 Ma.
Heat released early in Earth's history.How important these were depends on how rapidly the Earth accreted.
If <20 Ma, Al26, etc. would have been present and given heat, but if > 100 Ma little effect on heat from Al26 etc.
Relative importance of these four depends on formation models.Prior to Rutherford, Curie, etc. the non-radiogenic primordial heat sources were the only known source of heat energy in the Earth.
This led to calculations by Lord Kelvin of the age of the Earth, based on cooling rates, of <100 Ma - not 4.5 Ga!
All radioactive decay -> heat, but only break-down of isotopes with large half-life will have made a continuing contribution to heat source over geological time.
Four long-lived isotopes occur in sufficient abundances as to be important heat sources:-
IsotopeHalf-life (x 109 y)Heat generation (mWkg-1)
K401.32.8 x 10-2
Th23213.92.6 x 10-2
U2350.756.0 x 10-2
U2384.59.6 x 10-1
The total contribution to global heat production depends on the abundance of the isotope.
That abundance has varied throughout geological time because of half-life.
AbundanceNow109 years ago4.5x 109 yrs ago
Heating effect of U235 much more important at beginning of Earth's history than today as a result of being x80 more abundant.
To assess importance of radio-active heating need to know true abundances and distribution of isotopes.
Cannot sample core and lower mantle, therefore some uncertainty.
Variation of K, Th and U in rocks means that some rocks generate more heat from radioactive decay than others, e.g.
Granodiorite3.5 ppm K40
(Continental Crust)18.0 ppm Th232-> 96.4 x 10-8mWkg-1
3.97 ppm U238
0.03 ppm U235
Peridotite1.2 x 10-3 ppm K40
(Mantle)0.06 ppm Th232-> 0.26 x 10-8 mWkg-1
0.01 ppm U238
7 x 10-5 ppm U235
Gabbro->18.63 x 10-8 mWkg-1
Continental crust has concentration of radioisotopes (they happen to be incompatible elements), therefore heat generation in continental crust is more concentrated.
Mantle has less heat producing isotopes per kg, but has a much larger volume than crust.
What about global heat production from these elements?
Geochemical models of Earth suggest that the Earth has same chemistry as a chondritic meteorite.Can this be used to estimate radio-isotope heating effect?
In chondrite have K400.1 ppm
This gives total heat generation of 0.48 x 10-8 mWkg-1 of chondrite, of which K40 and Th232 contribute major part.
If the Earth (mass =5.97 x 1024 kg) was chondritic this would give a heat flow of 28 TW.
In Earth we have:-
Upper Cont. Crust96.4 x 10-8 mWkg-18 x 1021 kg
Lower Cont. Crust40.0"8"
-> 0.38 x 10-8 mWkg-1 of silicate in Earth -> 23 TW
This suggests possible lost K in core??
Recall global heatflow is ~40 TW, so we can conclude that heat-flow in Earth is dominated by radio-active decay heat energy.
Estimates 60 70% of heat flow is due to radioactive heat, and so 30-40% is contributed from loss of primordial heat.
If losing primordial heat the Earth must be cooling slowly.Estimates range from:
5 to 10 K per 100 Ma -> 230 to 460 K over life span of Earth.
Explains the formation of the inner core - crystallising as the Earth cools.
But how does heat escape and what how does it affect the nature of the Earth???
Introduction Heat is transferred through convection, conduction and radiation. Convection is the transfer of heat through the movement of matter. Conduction is the transfer of heat or energy between two objects that are in contact with each other. Radiation is the transfer of energy through electromagnetic waves. In this lab we conducted four separate experiments. The experiments test to see how convection currents cause effects on an ecosystem.
Experiment 1 In this experiment, we tested to see if liquids in an ecosystem could cause convection currents. We poured hot water into a flask. We colored it with half a teaspoon of potassium permanganate (just to distinguish it from others). The flask was plugged with a double-holed rubber stopper. The purple colored water in the flask sunk to the bottom of the graduated cylinder.
Analysis 1 The purple dyed water came out of one of the glass droppers into the large portion of hot water, which wasn?t as hot as the water in the flask. The purple water reached the surface while the clear water was being pushed down. This illustrated convection of liquids. The currents eventually reached thermal equilibrium and convection stopped. All the water was purple and reached one temperature.
Experiment 2 There was a cardboard juice box container that was cut in the middle to create a window. Two holes were made on the top and sticking out of the holes were cardboard toilet paper roles. Inside the box was a small crucible containing hydraulic acid, ammonium hydroxide and directly under the toilet paper roll was a lit candle. The window was covered with plastic wrap so that nothing could...
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