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ScienceWeek
EARTH SCIENCE: ON THE BUILDING OF MOUNTAINS
The following points are made by Peter Molnar (Nature 2003 426:612):
1) The endless debate over the relative importance of nature and nurture in child development has its equivalent in geomorphology. In this case, the argument is about the roles of tectonics and climate in mountain erosion. Tectonics (nature) sets the initial conditions by raising Earth's surface and, where active, renewing topography. Climate (nurture) shapes the surface into its various forms through its effect on glaciers and rivers. Recent reports (1-4) take the argument forward by isolating and evaluating the importance of certain climatic and tectonic factors in erosion.
2) As well as offering individual insights, these papers show how lack of resolution in the debate is impelling research in geomorphology away from observation and towards theory. Moreover, in different ways, they remind us that understanding the physical processes that govern the most pervasive of erosive forces, river erosion, requires a geological approach to provide data way back in time. Although there is a long tradition of measuring sediment transport by rivers, only infrequently will that record include the full range of floods that have shaped the landscape. Flooding obeys a power-law distribution(5), and the largest floods have the largest effect on landscape. But only in rare regions are we likely to have witnessed them -- hence the need for an approach that will produce a record that includes several such extreme events.
3) To quantify erosion rates, four research groups(1-4) apply thermochronometric methods to rock now exposed at the surface. Temperature increases with depth in the Earth and, at high temperatures, noble gases emitted in radioactive decay diffuse away; defects (tracks) in crystals, produced by the charged particles expelled in nuclear fission, anneal. By measuring the concentrations of such gases or tracks, one can date when the sample cooled below a temperature at which diffusion or annealing is slow. With an assumed temperature gradient in the Earth, the result is an estimate of the average exhumation rate, which in these cases equals the rate that material above the sampled rock has been eroded. Such estimates apply to periods as short as 500,000 years to as long as several million years. But in all cases they span several glacial and interglacial cycles, and so smooth the effects of large climatic changes.
4) Reiners et al.(1) present the simplest result: erosion rates averaged over the past few million years in the North Cascade Mountains of Washington state correlate with the present-day distribution of rainfall. Precipitation and erosion rates vary by an order of magnitude across the range, with rapid erosion having occurred where rain falls most today. As virtually all of the rock exposed in the mountains must have lain below sea level a few million years ago, they deduce that rock moved up relative to sea level despite the absence of any tectonic process. Because of isostasy (Archimedes' principle applied to the Earth's crust immersed in its more dense mantle)(1), removal of a mass of rock from the Earth's surface will be compensated by the rise above sea level of approximately 80-85% of that mass.
References (abridged):
1. Reiners, P. W., Ehlers, T. A., Mitchell, S. G. & Montgomery, D. R. Nature 426, 645-647 (2003)
2. Burbank, D. W. et al. Nature 426, 652-655 (2003)
3. Dadson, S. J. et al. Nature 426, 648-651 (2003)
4. Wobus, C. W., Hodges, K. V. & Whipple, K. X. Geology 31, 861-864 (2003)
5. Turcotte, D. L. & Greene, L. Stochastic Hydrol. Hydraul. 7, 33-40 (1993)
Nature http://www.nature.com/nature
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ON MANTLE PLUMES AND MOUNTAINS
The following points are made by J.B. Murphy et al (American Scientist 1999 87:146):
1) Theories of mountain building were revolutionized in the 1960s by plate tectonic theory. The horizontal motions of rigid plates of material above a pliable mantle helped explain the creation and destruction of oceans, the generation of mountain belts and sedimentary basins, the distribution of volcanic and earthquake activity, and the locations of ore, oil, and gas deposits. But plate-tectonic theorists have had difficulty accounting for many of the geological details of southwestern North America, including the uplifting of the Rocky Mountains, the extent of the Basin and Range region of Nevada, Utah, and Arizona, and the extensive volcanic deposits of the Columbia Plateau.
2) Established methods of mountain building all depend either directly or indirectly on subduction zones. Three methods are currently recognized:
a) A subduction zone may lead directly to mountains formed by ascending *magma and heat, as is the case in the Andes.
b) The subduction process may also transport *microcontinental fragments to the continental margin, where they accrete as "*terranes", a process that has added significantly to the North American West Coast over the past 400 million years.
c) When an ocean is consumed by colliding continents, as in the example of India and Asia, spectacular mountain building can result.
3) Mantle plumes produce island chains such as the Hawaiian Islands. The plume remains relatively stationary while the oceanic plate moves over it. Plumes are thought to rise all the way from the core-mantle boundary, 2,900 kilometers below the Earth's surface, in relatively narrow columns. On reaching the base of the lithosphere, a plume spreads out, underplating a large area of lithosphere, causing it to heat and be bowed upward.
4) Hot spots, the surface manifestations of mantle plumes, are widely distributed around the Earth, although the exact number is controversial. Hot spots are essentially stationary relative to the faster-moving plates. No modern ocean could be consumed at a subduction zone without a plate margin encountering a hot spot, so the interaction between subduction zones and hot spots must be common throughout geological time.
5) Most of the mountain building activity on the western margin of North America over the past 300 million years represents episodes of *magmatism and deformation associated with microcontinent collisions. The Sonoma, Nevada, and Sevier mountain-building events are examples. The Rocky Mountain (Laramide) orogeny, however, is distinctive because it is characterized by a lack of magmatism coupled with widespread deformation in the continental interior.
6) The authors propose that an additional (fourth) method of mountain building has been largely overlooked and may help explain not only the Laramide Orogeny but also other unusual geological features of the southwestern US. Their model involves the interplay of the horizontal motions of traditional subduction-related mountain-building processes with vertical plumes of hot mantle ascending from thousands of kilometers below the Earth's surface. The authors suggest that "together these mechanisms may offer a convincing explanation for what long has been a geologically puzzling part of the world and may lead to better understanding of mountain building worldwide."
American Scientist http://www.americanscientist.org
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Notes:
magma: In general, any molten mass of rock.
microcontinental fragments: In general, any fragment or remnant of continental crust up to approximately the size of Madagascar (Malagasy).
terranes: (terrains) In general, a terrane is any region of crust with well-defined margins which differs significantly in apparent tectonic evolution from neighboring regions.
magmatism: In general, the development and movement of magma within the Earth.
ScienceWeek http://scienceweek.com
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