Science at the Frontier (1992) / Chapter Skim
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1 Geology: The Great Heat Engine: Modeling Earth's Dynamics
1 Geology: The Great Heat Engine: Modeling Earth's Dynamics
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From page 1... ...
They make news. Geologic time, by contrast, is measured in millions and billions of years and yet marks the unfolding of an even more dramatic scenario the movement of entire land masses over thousands of kilometers, though at a rate of only a few centimeters a year, and the creation of mountains, ridges, valleys, rivers, and oceans as a consequence.
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From page 2... ...
, to an early-20th-century theory called continental drift, to the major set of ideas that unify today's earth sciences. Plate tectonics is no causal force, but merely the surface manifestation of the dynamics of this "great heat engine, the Earth," said presenter David Stevenson.
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From page 3... ...
Another contributor, Jeremy Bloxham from Harvard University, has conducted an exhaustive study of the Earth's magnetic field by reexamining worldwide historical data spanning centuries and has made some strong inferences about the Earth's core. This deep region has also been explored by simulating the conditions of high pressure and temperature that prevail there, an approach described for the symposium's audience by Russell Hemley of the Carnegie Institution of Washington, where pioneering work has been accomplished in the Geophysical Laboratory through the use of diamond-anvil cells.
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From page 4... ...
. The heat engine aspect of the Earth corresponds to this diagram: the closer to the center of the Earth's core, the hotter the temperature and the greater the pressure, reaching a peak of perhaps in excess of 6600°C and over 3.65 million times the atmospheric pressure found at the surface.
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From page 5... ...
Together with measurements of the Earth's magnetic field, they provide baseline information for modeling that allows scientists to search for anomalies. At the center of the Earth, 6370 kilometers from its surface, is its inner core.
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From page 6... ...
The uppermost region, the lithosphere, is rigid, is on average about 150 kilometers thick (thereby embracing the crust and part of the upper mantle) , and actually slides around on the surface of the top zone of the upper mantle, known as the asthenosphere, which convects and is considered to be less viscous than the zones immediately above and below it, and may be partially melted in some places.
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From page 7... ...
Although many observed, based on evidence provided by ever better maps, that the outlines of the continents seemed to dovetail as if they were separate pieces broken off from one original continuous land mass Wegner called Pangaea Wegener's ideas were not embraced for decades, and he died on expedition in Greenland, in search of corroborating evidence that that island was indeed drifting away from Europe. By the 1960s, observations of a great Mid-Atlantic ridge on the ocean floor almost precisely midway between Europe and America revealed it to be a crack in the Earth, from which spewed molten rock.
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From page 8... ...
The Earth's dozen major lithospheric plates illustrate demonstrably at their boundaries their underlying movement with respect to one another (Figure 1.2~. At divergent boundaries such as the MidAtlantic ridge, as the plates move apart in more or less opposite vectors, molten material erupts in the rift and forms a mountainous ridge, actually adding to the Earth's crust.
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From page 9... ...
Closely related to this improvement have been advances in the data collection itself, with more and more accurate- measuring instruments and seismic wave collection centers being established all over the world, and a better and more elaborate campaign of magnetic and gravitational data surveys as well. Moreover, simulations are not limited to the realm of the computer: a new process called diamondanvil cell technology has been developed to recreate the high-temperature and high-pressure conditions thought to exist in the lower mantle and the core itself.
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From page 10... ...
Seismic tomography employs heavy computer power in a process closely analogous to computerized tomography in medicine. Instead of undergoing variable absorption, as do x rays in the body, seismic waves confer information by altering their speed when they encounter differing materials.
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From page 11... ...
The Earth's most common mineral is probably magnesium silicate perovskite, a denser structure type than most other constituents and one that is not thermodynamically stable for silicates at normal atmospheric pressure. The normal olivine structure in which a silicon atom is surrounded by four oxygen atoms has been shown by diamond-anvil cell experiments to undergo a series of transformations, ultimately
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From page 12... ...
This occurs at just about the pressures that have been determined by seismology to prevail at the 670kilometer boundary region between the upper and lower mantle (Knittle and leanloz, 1987~. Said Stevenson, "The reduction of bandgaps and preference for compact phases may greatly change our view of the differences between and interrelationships among silicates, oxides, and metallic oxides." Hemley believes that discovery of these phase transitions and measurements of properties of the perovskite minerals favor the mantle convection models that employ more complicated patterns than the single, deep-convection theory.
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From page 13... ...
All of these methods provide excellent and often distinct supplementary perspectives to corroborate or constrain theories based on data gathered with yet other methods. In a science for which the deepest mine penetrates barely 2 miles and the deepest hole less than 10, inferences assume greater importance because most measurements are necessarily indirect.
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From page 14... ...
Those models posit that temperature has a great effect on viscosity, but that would lead, he said, "to a planet completely covered with a surficial plate that has no dynamics at all," despite the convection beneath the surface, instead of the planet we actually observe. Thus the appealing theoretical success of mantle convection does not begin to explain much of the data.
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From page 15... ...
Since the lithospheric plates move across and above the asthenospheric points where a plume emerges from its long journey through the mantle, they get marked by a trail of volcanoes that leave a permanent record of the direction and speed of the plate itself. The first coherent theory of hot spots was proposed by the University of Toronto's I
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From page 16... ...
By positing a thinner plate in this region, McNutt can account quantitatively for the anomalous seafloor depths observed and qualitatively for the low velocities of seismic surface waves, the weak elastic strength of the lithosphere, and the region's high vulnerability to what is distinguished as hot-spot volcanism. The thinner plate hypothesis does not, however, account for other observed features, such as why the plates move faster and spread faster over the mantle in the Superswell region than anywhere else on the Earth.
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From page 17... ...
Seismic wave data show that the lithosphere beneath the Superswell has been thinned to about 75 kilometers, 40 percent less than the usual lithospheric thickness under the North Atlantic and North Pacific oceans. This thinner plate, she believes, is not the cause of the Superswell; rather it is the surficial manifestation of forces below, specifically a low-viscosity area under the plate and a plume of heat convecting through the mantle.
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From page 18... ...
To understand mantle convection, you have to take into account the fact that material melts to some extent." Dynamics in the Core and the Magnetic Field Looking deeper yet, beneath the mantle, scientists find that the material in the outer core is highly conductive and low in viscosity. The forces of convection moving through this region have somehow set up a continuously regenerating dynamo, a massive natural version of the man-made power systems up above that produce predictable magnetic fields.
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From page 19... ...
People are tackling it now on supercomputers and by approximation techniques." The magnetic field profile of the Earth shows a familiar dipole. The geomagnetic field actually observed, however, contains a number of other, more complex components.
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From page 20... ...
Stevenson cited recent analyses of data in ships' logs and marine almanacs suggesting that for almost 300 years the basic features of the geomagnetic field have remained fixed relative to the mantle, "indicating at least partial control of the field by mantle irregularities." That work has been conducted by another session participant, Jeremy Bloxham of the Department of Earth Sciences at Harvard University. Rather than taking a top-down, mantle-oriented approach, Bloxham has looked at the same complicated magnetic field inferred to exist at the CMB for implications about the dynamics of the deeper core.
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From page 21... ...
Bloxham said that one interpretation of the magnetic field data suggests that such a process is occurring in the outer core, with large convection rolls aligned parallel to the Earth's rotation axis then grazing the solid inner core. The observed, high-latitude, surface concentrations of magnetic flux would, according to this interpretation, correspond to the tops and bottoms of these convection rolls, while the regions of almost zero flux at the poles also fit the pattern predicted from such an effect.
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From page 22... ...
And the two Voyager spacecraft have yielded a wealth of data on more distant solar system neighbors Jupiter, Saturn, Uranus, and Neptune. Bloxham reported that "until 2 or 3 years ago, I think a discussion like this would have included the belief that all magnetic fields we are aware of align with the rotation axes of their planets.
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From page 23... ...
1985. The Earth's hot spots.
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From page 24... ...
Annual Review of Earth and Planetary Sciences 20:553-593. Jacobs, J.A.
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