Origin and Evolution of Earth Research Questions for a Changing Planet (2008) / Chapter Skim
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2 Earth's Interior
2 Earth's Interior
Pages 35-70
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From page 35... ...
When the planet is and convection over very long timescales also play key geologically active, evidence of that activity is reflected roles in studies of internal dynamics. However, despite in the nature of its surface and atmosphere and perhaps continuing advances, we still cannot uniquely describe the existence of a magnetic field.
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From page 36... ...
Sources of this surface heat the atmosphere and ultimately to space, and also carry flow include the slow cooling of the mantle and core cold surface rocks down to great depths. Unresolved over the history of the planet; heating produced by issues concerning mantle convection arise from unradioactive decay of U, Th, and K; and minor sources certainties about material properties at high pressures such as tidal heating.
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From page 37... ...
EARTH'S INTERIOR 37 FIGURE 2.1 Cutaway view of Earth's interior showing major layers (oceanic and continental crust, upper mantle, lower mantle, outer core, inner core) and features (mantle plumes, subduction zones, midocean ridges, convection currents, magnetic field)
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From page 38... ...
. The seafloor of the western Pacific Ocean in mantle plumes, which are cylindrical upwellings of contains enormous volcanic mountain ranges, which hot (and hence low viscosity)
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From page 39... ...
this mean that thermal plumes do not exist in the Although there is good geological evidence that lower mantle or that the seismic resolution is still too mantle plumes exist, seismological evidence for the low to make them out? Seismic data suggest that the existence of narrow, hot, cylindrical upwellings in large low-seismic-velocity regions near the base of the the lower mantle is only equivocal.
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From page 40... ...
In general, mantle models mantle flow, and mantle plumes can have both thermal based on geochemistry suggest that mantle convection and chemical components to their buoyancy (Davaille, occurs in two layers, whereas most geophysical evidence 1999; Farnetani and Samuel, 2003)
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From page 41... ...
midocean ridges have long been considered evidence that the lower mantle (whence mantle plumes presum- Seismic interpretation. The most direct observations ably come)
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From page 42... ...
Among the challenges involve mineral phase transformations that tend to of simulating mantle convection are the strong depenimpede flow through the transition depth. However, dence of viscosity on temperature and composition, seismological data also show some subducted slabs ex- mineralogical heterogeneity in the mantle on both large tending to depths greater than 1,000 km (Figure 2.5)
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From page 43... ...
As the core cools, it solidifies that even with phase transitions inhibiting flow and from the bottom up, so we deduce that the solid inner a viscosity increase in the lower mantle, it is plausible core is growing and the liquid outer core is shrinking. that large-scale transport of material between the The inner core–outer core boundary must have a upper and lower mantle does occur.
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From page 44... ...
of reversals. Moreover, there is evidence that Earth's How long the inner core has existed, its rate of magnetic field is older than 2 billion years (Tarduno growth, and why the core has not fully solidified are et al., 2007)
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From page 45... ...
At the core-mantle boundary we infer there is midocean ridges, and subduction. The mantle may also mainly heat exchange, but there is tantalizing evidence exchange material with Earth's outer core.
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From page 46... ...
The balance between hydrothermal acidity and river alkalinity is affected strongly by the mantle convection system of which the ocean ridge system is a part. The chemically altered oceanic crust is returned to the deep mantle in subduction zones, which allows the hydrothermal systems to affect the chemical composition of the entire mantle.
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From page 47... ...
An essential aspect of melting in most planetary interiors is that mantle must be moving upward to melt. On Earth this happens at midocean ridges, mantle plumes, and subduction zones.
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From page 48... ...
Although mod- and magma, the role of water in melting, the effect of est in number, subduction zone volcanoes represent melting on the viscosity of partially molten rock, and nearly all of the explosive volcanoes (Question 9) and the distribution of volatile elements between solids the mechanism by which much of the continental crust and liquids.
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From page 49... ...
For example, we do not know how much of Earth's past volcanism was produced by mantle plumes and how much by plate tectonics, or why there were short periods of intense volcanic activity that could have changed the ocean basins, continents, and even global climate. In addition, we still have only FIGURE 2.9 Inferred features at the core-mantle boundary hints about how subduction zones work and how the (CMB)
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From page 50... ...
The most the even more basic questions of why Earth has plate common type of divergent boundary occurs at the tectonics in the first place and how closely it is related midocean ridge system, which takes the expression of to other unique aspects of Earth -- the abundant water, a 40,000-km-long submarine mountain range that rises the continent-ocean elevation dichotomy, the existence about 2.5 km above the average ocean floor (Figure of life. We do not know whether it is possible to have 2.11a)
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From page 51... ...
ments are concentrated in the continental crust, and as Complexities in the plate model arise from dif- a consequence the deep parts of the continental crust
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From page 52... ...
The different aging and cooling of the plate as it moves away from behaviors of oceanic and continental crust influence the the ridge. The oceanic crust is relatively young because nature of plate boundaries.
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From page 53... ...
Additional small contributions to plate ture of mantle rocks. Computer models can produce movement come from topography on the base of the a convection pattern that looks more or less like plate plates, but very little comes directly from upwelling of tectonics if this low-viscosity zone exists in the upper mantle beneath the midocean ridges.
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From page 54... ...
the continental crust been preserved at Earth's surface for billions of years, allowing land life to evolve as it What Causes New Plate Boundaries to Form?
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From page 55... ...
Volcanism also occurs within continents, mantle plumes in the evolution of continental crust is and the lower part of the crust can itself melt and feed a fundamental unresolved issue, one that becomes more volcanoes in continental margin subduction zones like urgent and less tractable when considering the oldest the Andes and in continental collision zones like the continental crust. Whether all continental crust has Alps.
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From page 56... ...
The processes by which oceanic crust is created (at Continental plates consist not only of crust but also midocean ridges) and destroyed (at subduction zones)
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From page 57... ...
that they can be extended to the early Earth. Thus Most of the continental area has an elevation just a few the volume of the continental crust through time, the hundred meters above sea level (Figure 2.11a)
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From page 58... ...
of rocks deep in the continental crust and in the upper mantle beneath the mountains. As Figure 2.15 implies, deep crustal rock flows laterally when pressure is decreased by erosion.
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From page 59... ...
Establishing continental crust nor the process of destroying it and the criteria for plate tectonic convection is a fundamen- returning it to the mantle is well understood. Nor do we tal research goal for geologists and doing so will require know whether the continents were smaller or larger in better models for rock deformation properties and im- Earth's past or whether the processes that produce and proved approaches to representing those properties in shape them were the same.
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From page 60... ...
cally change a mineral's behavior. High-pressure mineral transformations, and their Fortunately, the surge of interest in understanding dependence on temperature, allow us to estimate the Earth materials at the atomic level has been accom- temperature of the deep Earth and provide constraints panied by rapid development of new tools, including on how mantle convection works.
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From page 61... ...
These phase transformations are so drastic some 100-GPa pressure) that exhibits intriguing and that they can influence mantle convection; a phase highly variable seismological features (see Question 4)
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From page 62... ...
At high pressures characteristic of Earth's deep mantle the spins pair (called the low-spin or LS state) , the atomic magnetic moments vanish, and iron-bearing minerals are nonmagnetic.
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From page 63... ...
Over Earth's long history the outer core, it would also mean that chemical exchange repeated processes of melting, melt ascent due to buoy- across the boundary would be much more effective than ancy, and eruption onto the surface have completely if the mantle is solid; this would change the way we rearranged many of its chemical elements. This process think about the origin of chemical heterogeneity in the of planetary differentiation, making chemically distinct mantle (Question 4)
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From page 64... ...
We know of two potential reservoirs of water: mantle minerals and rocks, just as radiologists need hydrous phases, such as clays that contain predictable to know how bone and other types of tissue transmit amounts of water within their crystal structures, and X-rays. The changes in seismic wave velocity through nominally anhydrous phases, such as olivine (the most different structures in the deep Earth are small -- about abundant mineral in the upper mantle)
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From page 65... ...
Dark- and light-blue polyhedra are SiO6 and AlO6 for their role as a unique reactive surface area, and they coordination environments, respectively; red spheres are oxygen also help us understand how minerals form, since all atoms; and the green sphere is a hydrogen atom. The solubil minerals start out as nanophases in the form of small ity of water in this mineral reaches a few percent at conditions typical of the shallow lower mantle.
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From page 66... ...
. Laboratory analogs are likely The behavior of faults raises many scale-related to be accurate for some aspects of mantle convection, fundamental questions: How are earthquakes (large but they have limits.
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From page 67... ...
For example, as magma forms behavior of the mantle illustrates the importance of by melting inside Earth, it juxtaposes relatively fluid time in the material properties of large domains. The magma with mineral crystals that are essentially rigid.
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From page 68... ...
. Reprinted by permission from Macmillan Publishers Ltd.: Nature, copyright 2004.
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From page 69... ...
Earth materials present a challenge to understand ing because of their complex chemical composition and the high pressures and temperatures of planetary inte riors. The long timescales of geological processes also create difficulties because some of the critical processes that affect planetary evolution take place so slowly that they cannot be simulated in the laboratory and because they may be caused by mechanisms that are not imporFIGURE 2.25 Simulation of the distribution of melt (as mea- tant or even perceptible at laboratory timescales.
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