April 2, 2004 | David F. Coppedge

How Little We Know What Lies Below

Those cutaway views of the earth, with its core, mantle and crust, make nice diagrams in textbooks.  But without a Hollywood-style probe and time machine to the center of the earth, how do we know what’s down there, and how it got that way?  We know surprisingly little, admits David Stevenson (geologist, Caltech) writing in the April 1 issue of Nature.1  He poses a series of unanswered questions:

The basic divisions of Earth’s internal structure (crust, mantle and core) have been known for a long time.  But the evolutionary path that gave us this structure, and that provides the dynamics of plate tectonics, volcanism and magnetic-field generation, remains poorly understood.  Why do we have plate tectonics?  What is the nature and extent of melting deep within Earth?  How does the core manage to keep generating such a richly complex magnetic field?

These questions were addressed at a recent workshop on earth’s deep interior.  From Stevenson’s viewpoint as a participant, “it is evident that we need a better knowledge of the processes that govern deep-Earth history, and the material parameters that control those processes, before any kind of ‘standard model’ can be constructed.”
    One parameter that cannot be violated in modelmaking is the First Law of Thermodynamics, the principle that energy cannot be created or destroyed (a scientific law with no known exceptions).  Yet the earth has been losing heat through its crust; that heat must come from somewhere.  Geologists invoke heating caused by radioactive decay to arrive at estimates of earth’s steady-state heat output over geologic time.  [Radioactivity supposedly overcame Lord Kelvin’s argument from thermodynamics that the earth could not be as old as evolutionists claim; see 02/02/2004 commentary.]  But Stevenson admits there are not enough radioactive sources known, and little is also understood about the viscosity of the mantle, despite the simple models:

Models of this kind are easy to construct and boringly monotonic.  Furthermore, they cannot explain the widely accepted factor-of-two ratio for current Earth heat output to current radiogenic heat production.  Our planet was more eventful than these simple models allow.  Whereas Earth scientists have no desire to repeal the first law of thermodynamics, they are willing to challenge almost everything else.  Recently, major disagreements have emerged in attempts to understand the energy budget of Earth’s core, and there are still many uncertainties over how to incorporate the effects of plates, water, melting and layering into our picture of mantle circulation.

One topic on which there is “publication activity … but no consensus” is earth’s magnetic field.  Presumably, electrical currents in the fluid mantle keep it running, and the inner core is one of the “main contributors to the energy budget available to the dynamo,” but there are problems sustaining this dynamo for 4.5 billion years (see 12/15/2003 entry):

Standard evolutionary models have difficulty explaining how the inner core has existed for more than the past billion years or so, yet Earth’s magnetic field has existed throughout most of geological time.  There is no direct evidence on the age of the inner core, and the dynamo may operate without an inner core.  Still, it would be surprising if it were a recent feature of Earth’s structure.  This is one of several reasons why some scientists wonder whether there is an additional energy source in the core.

Stevenson suggests some additional radioactive elements that might supply the missing energy to power the dynamo, but each candidate is not without problems, such as how you get the elements to separate from their ores during core formation.  Maybe it was a non-radioactive energy source, like gravity.
    Plate tectonics is another puzzle often oversimplified but still poorly understood.  Even if they get convection models to work in the present, can those processes be extrapolated back billions of years?

It is an unfortunate feature of simple models of convection that they can mimic many of the characteristics of plate tectonics, but cannot explain some essential features of plates.  The danger of these simple pictures is that they may not provide an adequate predictive framework for how plate tectonics evolves through geological time.  Some models suggest possible solutions, but the lack of agreement between these various approaches means that we are not close to a final resolution.

The lack of agreement has led to “provocative ideas” on these subjects.  Stevenson is hopeful that models that incorporate water and carbon dioxide in the mantle and core might help, but at this time they have a “poorly understood effect on melting in the mantle.”  We need more information and new ideas, he concludes, as he meekly suggests one preliminary line of inquiry:

It seems likely that we will not understand the origin of Earth’s magnetic field until we know how the mantle controls heat flow in the core.  But we cannot understand the mantle side until we have a better understanding of plate tectonics.  This may in turn depend on understanding Earth’s water cycle.  Could it be that magnetism, like life, depends on water?

1David Stevenson, “Earth science: Inside history in depth,” Nature 428, 476 – 477 (01 April 2004); doi:10.1038/428476a.

This article may come as a shock to those who took high school physical science and were accustomed to boring, confident-sounding textbook drawings and films about the earth and how things work.  (Geologists working the surface of the earth have their own problems, too: see 10/09/2003 entry).  Notice how little is known.  The origin of the earth’s magnetic field, vital to life as we know it and dropping in strength rapidly, has them still at square one.  Plate tectonics, after 50 years the dominant paradigm, is still poorly understood (especially in terms of operation over long ages).  The size, chemical makeup and viscosity of the core and mantle are matters of conjecture by armchair scientists trying to get their models to work.  And if you were told the earth’s heat comes from radioactive decay, thus rendering Lord Kelvin’s upper limit on the age of the earth obsolete, were you aware that estimates are off by a factor of two?
    Stevenson jokingly teeters on the edge of “repealing the first law of thermodynamics,” which he knows, of course, would be absurd.  But there is another boundary no secular geologist would dare cross, even if they are willing to challenge everything else, and that is the assumed old age of the earth.  Even if there is no way to explain the rapidly-depleting magnetic field, even if plates cannot be kept moving that long, and even if there is more heat coming out of the crust than known sources permit, that is one parameter not open to question, because it would not allow enough time for Darwinian evolution from molecules to man.  And if evolution did not occur, the alternative is unthinkable.  (Also, no geologist would risk the scorn, ridicule and ostracism associated with being labeled a young-earther.)

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Categories: Physical Science, Physics

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