Solid-Earth Sciences and Society (1993) / Chapter Skim
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2 Understanding Our Active Planet
2 Understanding Our Active Planet
Pages 47-90
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From page 47... ...
Since their adoption ofthis expanded too} kit for investigating the implications of plate tectonics, earth scientists have made unprecedented progress. The Earth began over 4.5 billion years ago with the accretion of material orbiting around the Sun, supplemented by the capture of other bodies from intersecting orbits.
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From page 48... ...
Magmas and other fluids that move through the crust have the potential of becoming significant sources of minerals and energy. Along subduction zones, hydrated crust and water-saturated ocean-bottom sediments descend into the interior beneath a mantle wedge that extends over the sinking plate.
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From page 49... ...
This pattern manifests itself at the surface as spreading centers and subduction zones, where vast slabs of lithosphere can be seismically traced along descending arms of the convecting system. The convection, which governs plate dynamics, may be limited to an outer layer of the mantle, possibly complementing another convection system delivering energy and material through an inner mantle.
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From page 50... ...
Evidence of plate tectonics is restricted to Earth, despite rigorous surveys of similar planets and planet-like bodies. The mantle convection that results in plate tectonics originated soon after establishment ofthe Earth-Moon system 4.5 billion years ago.
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From page 51... ...
The earliest history of the Earth has been destroyed by plate tectonic processes. Early Earth history must be deduced from analyses of lunar and meteorite material and from exploration of other planets.
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From page 52... ...
With no apparent plate tectonics, heat may have to escape from deep within Venus in other ways. The comparative study has just begun, and it should be greatly enhanced by continuing analyses of data from the Magellan project.
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From page 53... ...
This is the process by which heat is most effectively transported from the deep interiors of planets With this realization, it was no longer possible to view earthquakes, volcanic activity, mountain belts, sedimentary basins, or the general division between oceans and continents as isolated surficial phenomena. Temperature variations within the Earth control the convection that ultimately produces the magnetic field, surface topography, and active geology.
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From page 54... ...
54 SOLID-EARTH SCIENCES AND SOCIETY Upper Mantle Transitior Zone 14 <~ .( .
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From page 55... ...
The instability of these boundary layers ultimately produces crustal deformation through the forces of subduction and plumes. Thus, seismic tomography in principle can map out the underlying motions that drive plate tectonics at the surface.
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From page 56... ...
Plate tectonics is the surface manifestation of mantle convection. The rigid plates are the uppermost thermal boundary layers of mantle convection.
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From page 57... ...
To understand the magnetic field on a geological time scale requires the equivalent of understanding long-term climatic patterns in the flow of the outer core. One outstanding question is why the magnetic field varies with time.
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From page 58... ...
The San Andreas Fault is called a strike-slip fault because here the plates are sliding past each other horizontally. Along other major plate boundaries the crust is either spreading apart, as in ocean ridges
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From page 59... ...
Geological Survey. Or continental rifts, or is converging, as in subduction zones and continental collisions.
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From page 60... ...
Study of the faulting involved in deep earthquakes sheds light on the process of subduction and on the fundamental nature of mantle convection. Unclerstancling Earthquakes While seismologists have made progress in characterizing the global distribution of earthquake activity and the types of faulting involved, a basic understanding of the physics of earthquake rupture is still lacking.
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From page 61... ...
In fact, these measurements have directly confirmed the theory of plate tectonics, by resolving relative plate motions, and these measurements offer the potential of detecting departures from the motions predicted by rigid plate models (Figure 2.5~. As geodetic measurement precision and global coverage increase, it will become possible to directly monitor surface deformation, which is an important component of the dynamic Earth system.
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From page 62... ...
The patterns of flow in the mantle, however, may be controlled or determined by events outside the mantle, such as enhanced heat flow from the core or changes in the motion of the rigid surface plates. The realization that the core, mantle, and crust interact to determine the surface characteristics is an outgrowth of the plate tectonics paradigm.
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From page 63... ...
The magma may ascend along grain boundaries like liquid flowing through a porous solid At the confining pressures within the Earth, a liquid cannot move unless replaced by some other material; gaps do not remain within rocks of the interior Liquid must be kneaded, or squeezed, out of the solid component of magma-producing rock This occurs through recrystallization that fills gaps left behind as the liquid travels upward along grain boundaries This process allows the liquid to coalesce into accumulations of increasing size At some stage the pressure from accumulating magma forms veins and cracks in the overlying rock The flow of magma through rock shares many similarities with the movement of other fluids, such as groundwater and petroleum A better understanding of the physical processes of fluid flow through the Earth could provide answers to a wide variety of problems in the earth sciences Extensive geophysical observations of Hawaiian eruptions carried out by the Volcano Observatory of the U S Geological Survey have produced tremendous improvements in our understanding of magma transport and storage (Figure 2 6)
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From page 65... ...
Recent speculation suggests that large submarine eruptions along ocean ridges may alter ocean temperatures, establishing the El Nino condition with its subsequent implications for climatic variations. Further understanding will come from global monitoring of volcanism, most likely through satellite-based remote sensing and a better theoretical grasp of the relationships dictating climatic response to heat and mass transfer from the Earth's interior to the hydrosphere and atmosphere.
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From page 66... ...
Ocean Spreading Centers Ocean basins open along axes where the crust is torn by the force of plates moving away from each other. Basins spread along centers where partial melting of the shallow mantle generates basalt that ascends toward the surface and is solidified as ocean crust (Figure 2.8~.
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From page 67... ...
Intraplate Volcanism: Hot Spots and Oceanic Plateaus Enormous amounts of volcanic material erupt at hot spots such as Hawaii. Unlike that at spreading centers, however, the rise of mantle beneath hot spots is not a passive response to movement of the overlying plate.
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From page 68... ...
Integrated models that incorporate trace element and isotopic data, such as the high proportion of 3He compared to 4He at hot spots, address previously intractable questions. Numerical modeling and experimental simulations of mantle convection suggest that plumes arise from unstable boundary zones between convective layers.
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From page 69... ...
Oceanic plateaus may record sporadic and episodic events in mantle convection, so closer scrutiny could help to answer some questions about time dependence of mantle processes. Plate Kinematics The creation of new ocean crust at spreading centers is an essential feature of the plate tectonic theory.
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From page 70... ...
If these arcs swept together, the underlying intrusions could eventually form the huge batholiths that form the roots of cordilleran mountain ranges characterizing ocean-continent convergent plate boundaries. This is the process going on today in Southeast Asia, especially the Philippines and' Indonesia.
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From page 71... ...
As with many areas of the earth sciences, understanding the causes and consequences of subduction zone volcanism and the history of the assembly of the continents from volcanic arcs will result from integration of the increasingly detailed data provided by geological, geochemical, and geophysical observations with powerful numerical models. CONTINENTAL STRUCTURE AND EVOLUTION The other terrestrial planets lack the surface division into continental and oceanic crust that is a distinctive feature of the Earth.
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From page 72... ...
This evidence for continental roots suggests that the mineralogical constitution, and hence bulk chemical composition, of the upper mantle beneath ancient continental crust differs from that of the surrounding mantle. One explanation for the origin of this distinct mantle beneath continents is that it represents the residue left behind when partial melts were removed to form overlying crust.
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From page 73... ...
This information will open a new window into the internal workings of the crust and upper mantle and will help to test our current views on the workings of plate tectonics. Improvements in analytical instruments have recently produced a highly significant advance.
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From page 74... ...
The concept of converging tectonic plates that contract the crust and produce mountains fits well with our understanding of plate tectonic theory. Within the past decade researchers have emphasized another process that actively deforms vast tracts of continental crust.
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From page 75... ...
Soon afterward came the realization that plate tectonic processes represented the dominant way in which the Earth dissipates its internal heat. The next question addressed the length of time that plate tectonics has played a major role in earth processes.
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From page 76... ...
This is the origin of the San Andreas Fault dominance of plate tectonics during that inter- system in California and the neighboring Basin and val 5 percent of earth history is essentially Range province (Figure 2.14~. proven.
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From page 77... ...
Early transform faulting was west of the present-day San Andreas Fault and presumably separated young oceanic rocks of the Pacific plate Strom rocks of the North American plate Over time, the transform faulting has stepped eastward, and virtually all of the present fault is now in rocks of the North American plate Figure from USGS Professional Paper 1515 77
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From page 78... ...
When a river carries very large quantities of sediment, extensive deltas develop even if they flow directly into the open ocean as in the Amazon system. During certain periods of earth history, when ocean spreading centers were very numerous and broad, water was displaced from ocean basins, and high seas flooded extensive tracts of low-lying continental crust, creating inland seas.
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From page 79... ...
Collapse of mountain ranges by extension may be as general and as significant a part of their history as their construction by shortening. The kinds of integrated geological and geophysical approaches applied in western North America suggest that the mountain belts of the Alps and the Mediterranean region are uncoupled from the underlying mantle and are moving independently.
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From page 80... ...
We do not yet know whether these temporal and geographic differences record stochastic operation of plate tectonic processes or whether they reflect systematic changes in the Earth's behavior in space and time. The central part of North America with perhaps 75 percent of the present continental area was put together long ago.
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From page 81... ...
Determining long-term feedback relations between the elements of mantle convection, including plate formation and subduction as well as mantle plumes, and the elements of surface geology, including sea-level change, continental assemblies, and continental rupture, is a major interdisciplinary challenge. Regional geology, mantle geophysics, and geochemistry need to be used together in a new way.
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From page 82... ...
The recent discovery of an isotope of beryllium in arc volcanoes, but in no other volcanic system on Earth, provides clear evidence that even the upper few meters of sediment on the ocean floor are transported to a depth of at least 150 Len in subduction zones, the depths at which the processes of debydration and perhaps partial melting initiate the generation of arc magmas. There is a direct link with cycles in the atmosphere and hydrosphere.
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From page 83... ...
These materials are injected into the mantle in subduction zones; are carried along by the general mantle circulation, during which time they partially mix with surrounding mantle; and eventually return to the surface beneath ocean ridges and in mantle plumes. Estimates of the time necessary to travel from subduction zone to ridge are on the order of 1.7 billion years.
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From page 84... ...
The research areas are interrelated. There is continuity between the processes occurring at the coremantle boundary, mantle convection, the physical deformation of the lithosphere and mountain building in the crust, and the geochemical transfer of material from mantle to crust.
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From page 85... ...
B C II. Geochemical and Biogeochemical Cycles · Evolution of the crust and its relationship to the mantle · Fluxes along the global rift system · Fluxes at convergent plate margins · Mathematical modeling in geochemistry · Mineral-water interface geochemistry · Role of pore fluids in active tectonic processes · Magma generation and migration · Oceanic lithosphere generation and accretion · Architecture and history of continental rift valleys · Sedimentary basins and continental margins · Continental-scale modeling · Recrystallization and metasomatism of the lithospheric mantle and lower crustal metamorphism · Thermal structure, physical nature, and thickness of the continental crust · The lithosphere at convergent plate boundaries · Tectonic and metamorphic history of mountain ranges · Quantitative understanding of earthquake rupture · Rates of recent geological processes · Real-time plate movements and near-surface deformations · Geological predictions · Modern geological maps · Origin of the magnetic field · Core-mantle boundary · Imaging the Earth's interior · Experimental determination of phase equilibria and the physical properties of Earth III.
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From page 86... ...
Patching together diverse results from three decades of geophysical surveys and geochemical analyses has led to the formulation of a model for the detailed structural and magmatic segmentation of mid-ocean ridge spreading centers. Role of Pore Fluids in Active Tectonic Processes The fluid state may be most important in understanding the mechanics of deformation, including earthquakes, and insight can be gained from laboratory experiments of rock fracture and rock friction with fluids present.
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From page 87... ...
ospl~ere at Convergent Plate Boundaries Multidisciplinary studies of this environment are essential for understanding mantle dynamics and geochemistry as well as the chemical differentiation of the Earth. Tectonic and Metamorphic History of Mountain Ranges A new dimension, time, has recently been added to the depth-temperature framework provided by experimental petrology.
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From page 88... ...
This transport is part of the major geochemical cycles of the Earth. FACILITIES, EQUIPMENT, AND DATA BASES The earth sciences offer special opportunities for the development and application of new technologies, as exemplified by the instrumentation that has recently been created for use in the field and in the
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From page 89... ...
The Rote of Continental Scientific Drilling in Modern Earth Sciences Scientific Rationale and Plan for the 1990's, 151 pp. Interagency Coordinating Group for Continental Scientific Drilling (1991~.
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From page 90... ...
USGS (1990~. The San Andreas Fault System, California, U.S.
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