In fact almost everything we have worked out about the insides of the Earth comes from a study of one thing -- earthquake waves. When an earthquake occurs, it produces shock waves which run through and around the Earth.
These waves do not just radiate out and dissipate, they actually bounce off various levels within the Earth, called discontinuities, where the physical properties of the rocks change. Observations of the waves reveal just where under the surface the discontinuities are located, and give an indication of the properties of the rocks on either side.
Fig. 9.1. Structure of the Earth, from surface to centre
Figure 9.1 shows a conventional summary of what has been deduced about the properties of the Earth, from its surface to its centre (about 6370 km down). A fuller explanation can be found in any modern text (eg [64]), but the main assumed features are these:
- Feature A • Four discontinuities, at depths of around 20, 200, 2900, and 5150 km, where there are abrupt changes in density;
- Feature B • Density increasing from about 2.7 (g/cc) at the surface to about 13.6 at the centre;
- Feature C • Assumed chemical composition changing from oxides high in silicon and aluminium (Sial) or high in silicon, magnesium, iron and calcium (Sima) at the surface, down through layers with increasingly less silicon and more iron, until a core region is reached containing mostly iron oxides;
- Feature D • Temperature increasing from around 20 (ºC) at the surface to 3000 at the centre.
The Lithosphere
Virtually everything in the area of domainography which has been mentioned so far concerns the region called the lithosphere, which lies beween the uppermost solid surface and a depth of around 60-100 km.This region is made up of the Crust and the uppermost division of the Mantle. The boundary between these two is the first discontinuity, called the Moho (after Mohorovicic, who discovered it). There the density changes abruptly, from 2.9 to 3.3.
The Moho varies in position from about 10 to 35 km below sealevel. Under the seabeds, where the oceanic basic rock or Sima is exposed, it is at its shallowest, while it is furthest down under the continents, which consist mostly of continental acidic rocks, or Sial. Where Sial exists, in the continental areas, it is nevertheless believed to have Sima underneath it, so the Sima extends over the whole of the Earth. The Moho also extends over the whole Earth, apparently almost entirely within the Sima.
There is no doubt that the Moho, and the other three discontinuities mentioned in Feature A, all exist. What is not certain is what their nature is. They have been associated with changes in chemical composition to some extent, but this is purely speculative. Even the Moho is only just within the range of feasibilility for reaching by modern drilling techniques (the current record depth is around 11 km), but to date no drill core recovered has provided hard evidence that rock below the Moho is of different composition to that above.
From all the evidence produced to date, it is clear that extensive domain movements have occurred in the past, and it seems that the influence of these movements has extended at the very least to the base of the lithosphere, up to 100 km down. On the old plate tectonic theory, the 'plates' were supposed to be drifting on the mushy rock layer, the asthenosphere, immediately below the lithosphere.
It is interesting that as long ago as 1782, Benjamin Franklin speculated that "the surface of the globe might be a shell, capable of swimming on an internal fluid". We will see later that this attractive idea, which has now actually reached general acceptance, is false.
It seems logical that if such 'floating' movements occurred, they could not leave intact a boundary based on differences in chemical composition. Even on Continental Drift principles, it can be taken for granted that one such 'chemical boundary', that between the Sial and the Sima, would have already been broken up by domain movements.
It therefore seems likely that the Moho is a physical boundary. Its most probable nature is that of a phase change, brought about by increasing pressure. A given mineral or compound can exist in different states or 'phases', caused by the application of external pressures -- an example is diamond and graphite, both of which are phases of carbon. Diamonds can be made from graphite by application of very high pressures -- amusingly enough, they have actually been made from peanuts, after extracting the carbon from these.
Proposition 9A
The Moho discontinuity represents a phase change boundary
where the rocks are changing their phase in response to increasing
pressure
From this Proposition it follows directly that if the Moho is dependent on pressure, it will alter its position as the pressures change, as will happen when domains move and break up.
Proposition 9B
The position of the Moho will change as the pressure of
overlying rock changes in consequence of domain movement
These propositions are in accord with Features A and B above, but Feature C looks a bit more shaky. Let us now look further at the matter of chemical compositions.
Internal Chemistry of the Earth
Once we descend below the 10 km or so of the Crust which can be analysed directly, our knowledge of the Earth's chemistry is purely speculative. So we are quite within our rights to question some of the assumptions made, assumptions often repeated as fact, from textbook to textbook through the decades, without any real evidence.One of these 'facts' is that the core of the Earth is very rich in iron and poor in silicon. According to the text used to construct Fig. 9.1 [64], some 90% of the core consists of iron oxides, and another 8% of nickel oxide, leaving 2% unspecified. Where did this idea come from?
Well, firstly there is the fact that the core material is very dense, and both iron and its oxides are also dense (though not as dense as the core material). So this does not get us far. Perhaps the strongest argument comes from material assumed to be derived from outside the Earth, from meteorites.
Meteorites are solid objects falling to the Earth from space, of a size great enough to survive vaporization through the friction of falling through the atmosphere (another instance of the great heat available from friction). They are of two types, either 'stony' (made of rocks similar to those found on Earth), or 'iron' (not similar to any Earth-surface rocks).
It has been suggested that meteorites are the remains of a planet similar to Earth which broke up at some time in the past. The proportion of stony to iron meteorites falling on Earth is similar to the proportion of iron to stone within the Earth if its core was made of iron. And this appears to be the main argument for an iron core.
This connection is not only tenuous, it is also flawed. The iron in meteorites is native metal, while the iron assumed to be in the core is represented as iron oxides. No mechanism has ever been suggested whereby, if a planet broke up, the bits of iron oxide from its core could all be converted to iron metal.
There is one further possible link, with magnetism. The Earth has an considerable magnetic field, and iron and its compounds are normally linked with magnetism. However, any possible connection with the core is ruled out by the fact that magnetic materials lose their magnetism when heated up -- this is the basis of the technique of paleomagnetism, mentioned in NU003, whereby newly-created rock from volcanos took on the direction of the local magnetic field as it cooled down.
So it appears that the idea that the Earth has an iron-rich core is without firm basis.
Proposition 9C
The Earth does not have an iron-rich core
What then is the basis for the density discontinuities in the Earth's inner structure? I think it is reasonable to assume that all four of the known discontinuities mentioned in Feature A are, like the Moho, the result of pressure-induced phase changes.
Proposition 9D
The four discontinuities marking the boundaries between the
Earth's Crust, Upper Mantle, Lower Mantle, Outer Core, and
Inner Core are all due to pressure-induced phase changes
This Proposition is at least as reasonable as any other. It is supported by the manner in which masses of molten rock material segregate. Some segregation might be expected, perhaps with lighter components rising closer to the surface (as with the Sial and Sima), but not a sharp division based on chemical composition.
A corollary of Proposition 9D is that all pressure-dependent discontinuities may be expected to alter their position as the Earth expands, as suggested for the Moho.
Proposition 9E
All the density discontinuities within the Earth may be expected to change position as internal pressures change with Earth expansion
Heat of the Earth
Of the four Features listed above, the one most open to attack is the one which assumes that the Earth's temperature increases continuously towards its centre. After all, why should it?When confronted with this question, probably the responses of most geophysicists would fall into two areas. One is to say, whatever the reason for the phenomenon, it describes known behaviour -- it does get hotter as you go downward in the Earth. The other response might be to say that pressures are very great within the Earth (this is undisputed), and high pressures and high temperatures are usually associated.
Neither of these responses hold much water. The most obvious manifestation of heat from within the Earth is through geothermal phenomena such as volcanos. We have already seen that these essential local phenomena are, in fact, an expected outcome of domain rubbing (Proposition 8D).
Well then, what about the observed fact that temperatures increase as you go downwards in mines? This is perhaps the strongest argument which can be produced, and we need to look at it more closely.
Temperatures down mines
As you go progressively deeper in the Earth, temperature variations due to seasonal and climatic effects smooth out, and in an undisturbed site the temperature is virtually constant when a depth of 20-30 m is reached. Of course this constant temperature is different for different sites and depths.As you go deeper still, the temperature invariably rises regularly. The rate of rise, at least for the more shallow mines, is around 1ºC for each 40 m of descent (equivalent to 25ºC/ km), but it does vary by a factor of two, and even varies at different depths in the same mine. This effect has been known since mines were first dug.
More recently, information has been gained from deep oil wells, showing a similar picture [8]. Producing oil wells have been drilled, in different parts of the world, down to depths of around 8 km. The temperature of the oil produced varies according to the area of production and the geology of the host rocks, but invariably increases with the depth of origin. Oil from a depth of 1 km might be at a temperature of 55ºC, that from the deepest wells could reach 240ºC. Again these figures represent rises of around 25ºC/ km.
The pertinent question now is whether these rises increase regularly as you go even deeper, and if so, what the source of the deep heat is. For the first part, a regular rise of the type found in the accessible top 10 km implies a temperature at the Earth's core of over 150,000ºC. This is the sort of temperature believed to exist only in the interiors of very hot stars, and no one has ever suggested it as a real possibility for the Earth -- 3000ºC is a conventional figure.
This last figure represents an average rise of less than half a degree per kilometre, so even on the conventional view it is accepted that any rate of rise must fall off as you go deeper. For the second part, the accepted origin of the heat rising from the Earth's interior is that it comes from the molten core, presumed to be left over from when the planet was first formed in a molten state. We now look at the flow of this heat.
Heat Flow in the Earth
The matter of the flow of heat through the body of thy Earth has received considerable attention in the past. Rocks are poor conductors of heat, and so if the centre of the Earth was once very hot, it can be expected that it would take some time for the inner heat to escape. But just how long?Towards the end of the last century, the famous physicist Lord Kelvin (William Thomson) looked at this problem in an attempt to work out the age of the Earth. Kelvin was an expert in matters relating to heat flow, in fact the absolute scale of temperature, that starting from absolute zero (around -273ºC), is measured in degrees K (K for Kelvin).
Kelvin concluded that if the Earth started off very hot, it could not take more than 400 my for it to cool down to its present temperature, and it could be as little as 20 my. His study was one of the earlier scientific attempts to work out the age of the Earth, and his result of 20-400 my was accepted as valid. It had already been realised that previously assumed values, of a few thousands of years only, were far too low, but the full extent of the Earth's age had not yet been guessed at.
After the discovery of radioactivity, it was realized that the radioactive decay of elements in rocks provided a way of working out their age. Many elements have forms, called radioactive isotopes, which are unstable, that is they break down to form other elements and give out radioactivity. For example, a uranium isotope may change slowly into lead.
Each radioactive isotope decays at a fixed rate, called its half-life (the time taken for half of all the original isotope atoms to change). This half-life may vary, for different isotopes, from many thousands or millions of years down to a fraction of a second. Isotopes with long half-lives can be used to work out ages of rocks. This is done by analyzing the amount of a particular radioisotope in a mineral and the amount of the element it is changing into -- the ratio and the half-life give the time since the original mineral was formed.
What this method showed, when applied to rocks, was that most of them were very much older than Kelvin's result suggested. Confirmation of this great age came from other results also, so the Earth was clearly too old for its internal heat to be 'left over' from when it was first formed.
The next suggestion for the origin of this heat again was connected to radioactivity. When radioisotopes decay, they give off heat -- it is this heat which is used in nuclear power stations. All rocks contain a larger or smaller amount of radioactive material, and a currently suggested cause for the Earth's heat is that it stems from the decay of radioactive material within the crust, perhaps at depth.
There are problems with this theory. One is that the rocks which are richer in radioactive material are known to be concentrated in continental-type rocks, rather than oceanic ones. Upward heat flow does vary in different parts of the Earth, but not in a manner which has any relationship with content of radioactive materials. Another problem is that the content of radioactive material in the rocks is quite insufficient to produce the amount of heat actually recorded, especially since the evidence is that concentrations of radioactives, always relatively uncommon, are mostly confined to the topmost continental Sial layer of the Earth.
The Encyclopaedia Britannica article on the Earth's heat [12] concludes that "no satisfactory theory ... yet formulated explains the Earth's thermal constitution". In summary, it appears that there is no convincing evidence that the core of the Earth is very hot.
Proposition 9F
The core of the Earth is not especially hot
Where then does the observable heat flow originate? We already have an answer -- from domain movements. In this case, however, we probably need to include the deeper domains, down to a depth of around 100 km. We will see later, in NU016, that there are other possible contributions to this heat, but this is suggested as the main source.
Proposition 9G
The principal source of the heat observed to flow from the
depths of the Earth is friction from movement of domains,
including deeper domains
Is the Earth Made of Brown Sugar?
The Earth is not made of any sort of sugar. But in some respects we get a better intuitive picture of what is happening with the Earth if we regard it as made of a substance similar to sugar crystals, rather than of a baked-clay material as implied by the term 'plate'.The hardness and physical strength of various rock materials are readily observed and measured, and so people have an intuitive feel for how a boulder or pebble will behave. There is a difficulty when this feeling is extended to something on quite a different scale, such as a mountain range. The behaviour of a microdomain is not the same as that of a boulder of the same material, scaled up in proportion.
An instance of this has already been looked at, in Rule 3 of NU007, where it was noted that rotation of domains was not normally observed, say when one collided obliquely with another. What actually happens in such an impact is that the material at the point of impact just crumples up, like a mass of sugar crystals, rather than a whole 'plate' swinging about the point of first contact.
The sugar picture also gives a better feel for what happens with deeper domains and under-domains. If a surface domain is split apart by Earth expansion, its edges will be 'soft and crumbly' on the larger scale, and if sub-surface domains split, like sugar cubes embedded in a mass of crystals, the higher material will tumble down into the gaps.
Figure 9.2 is a representation of how surface and subsurface domains might be represented in a cross-section of the top 100 km of the Earth's surface. If they are looked at as lumps and cubes of sugar in a sugar crystal matrix, this may give a better feel for the situation than one showing them as single rigid flat plates floating on a molten surface (the conventional picture). Another image might be that of a dry-stone wall, pieces of rock fitted together with only smaller rock fragments and no mortar in the gaps.
Another advantage of these pictures is that they make it easier to visualize the independent movement of domains and sub-domains at different levels. The old tectonic plate image only considers movement of one-layer massive plates of more or less uniform thickness, around 100 km.
Having looked inside the Earth, we now return to its surface, but this time we look not at the solid land, but at its rolling oceans.
(Full list of references at NURefs)
[8]. Peter Bergman. Personal communication. 1989.
[12]. Encyclopedia Britannica. 1974 edition. Vol.6 p26.
[64]. The Physical Earth. Mitchell Beazley.
NU010: The Rolling Oceans
NU008: Making Mountains Out Of Movements
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Version 2.0, 2004, PDFs etc on World Wide Web (http://www.aoi.com.au/matrix/Nuteeriat.htm)
Version 3.0, 2014 Sep 23, Reworked from Chapter 9 of "Nuteeriat" as one article in a suite on the World Wide Web.