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0 30' 60 90' 120 150 180 -150 -120 -90' -60 -30' 0' Figure 2.2. The global distribution of both shallow and deep seismicity for well-located earthquakes with magnitude > 5.1. The shallow seismicity closely delineates plate boundaries. Based on Engdahl et al. (1998). Figure 2.2. The global distribution of both shallow and deep seismicity for well-located earthquakes with magnitude > 5.1. The shallow seismicity closely delineates plate boundaries. Based on Engdahl et al. (1998). 0 30 60 90...

Info

Note For plate abbreviations, see Table 2.3. PH, Philippine plate. Note For plate abbreviations, see Table 2.3. PH, Philippine plate. fraction of plate circumference occupied by ridges and trenches versus plate velocity for each plate (Forsyth and Uyeda, 1975). According to Figures 2.32b and d, plate velocity is sensitive to the fraction of the plate occupied by continental crust and to the length of subducting slabs attached to the plate. In general, continents occupy slow-moving plates, while...

Special Effects in Mantle Convection

Here we summarize special physical effects that give mantle convection its unique character. The often subtle interplay between these effects may hold the key to the further understanding of mantle convection. 15.7.1 Solid-state Phase Transformations From depths of 220-660 km there are increases in density and density discontinuities that require deviations from the adiabatic behavior of an iso-mineralic mantle this is the transition zone. The behavior of the transition zone can be attributed...

Model of Coupled Core Mantle Thermal Evolution

While the simple models of the previous section are adequate for demonstrating many of the important aspects of the Earth's evolution, more elaborate models are needed if, for example, the evolution of the core is to be included. We now consider a more complex model developed by Stevenson et al. (1983) with a coupled core and mantle. Thermal evolution Figure 13.6. Urey ratio versus time in the thermal history calculation of Figure 13.4. Figure 13.6. Urey ratio versus time in the thermal history...

Form of Upwelling

There is direct surface evidence for two forms of mantle upwelling flows associated with accretional plate margins, which we also refer to as spreading centers and mid-ocean ridges, and localized upwellings beneath volcanic hot spots, which we refer to as mantle plumes. In addition, seismic tomography provides some evidence for larger-scale upwellings at depth. First we discuss the character of upwellings beneath spreading centers, the mid-ocean ridges. The central question on their role in...

Isotope Systematics of Ocean Island Basalts

So far we have concentrated on the crustal and depleted mantle reservoirs. We now turn to the isotope systematics of ocean island basalts (OIB). Unlike MORB, the OIB have considerable isotopic variability. Interpretations of this variability require an identification of the sources of OIB. It is clear that OIB cannot come from the nearly homogeneous upper mantle reservoir that is the source of MORB. The ocean islands where OIB are found are hot spots that are attributed to partial melting in...

Spherically Averaged Earth Structure

Spherical Zone The Earth

The determination of elastic parameters and density throughout the Earth using observations of seismic waves and other constraints is the prototype inverse problem in geophysics. Like many inverse problems, it is formally nonunique and suffers from practical difficulties such as incomplete sampling and errors in the data. In spite of this, it is remarkable how much is now known about mantle structure, and this is particularly true of spherically averaged properties. The major divisions of the...

The Wilson Cycle and the Time Dependence of Plate Tectonics

Wilson (1966) proposed that continental drift is cyclic. In particular he proposed that oceans open and close this is now known as the Wilson cycle and was based on the opening and closing of the Atlantic Ocean. The Wilson cycle, in its simplest form, is illustrated in Figure 2.34. Question 2.15 How are accretional plate margins formed The first step in the Wilson cycle, illustrated in Figure 2.34, is the breakup of a continent. This occurs on continental rift zones. Present examples are the...

O

Pattern of global heat flux variations complete to spherical harmonic degree 12. After Pollack et al. 1993 . For a color version of this figure, see plate section. 0 30' 60' 90' 120' 150' 160 -150' -120' -B0 -60' -30' 0' 0 30' 60' 90' 120' 150' 160 -150' -120' -B0 -60' -30' 0' 0' 30' 60 90' 120' 150 ISO' -150' -120 -90' -60' -30 0 Figure 2.9. Global distribution of volcanoes active in the Quaternary. 0' 30' 60 90' 120' 150 ISO' -150' -120 -90' -60' -30 0 Figure 2.9. Global...

Why Are Island Arcs Arcs

Island Arc Basalts

Question 2.6 Why do subduction zones have arcuate structures One of the striking features of subduction zones is their arcuate structure in map view or planform. Subduction zones are made up of a sequence of arc structures with a clear planform curvature this is the origin of the term island arc. A good example is the Aleutian arc, shown in Figures 2.1 and 2.5. Just as accretionary margins are characterized by their orthogonal ridge-transform geometry, subduction zones are characterized by...

P

Illustration of Euler's theorem. Plate B is moving counterclockwise relative to plate A. The motion is defined by the angular velocity m about the pole of rotation P. Double lines are ridge segments, and arrows denote direction of motion on transform faults. data include 277 spreading rate determinations based on magnetic anomalies. An example of the magnetic profiles for the spreading boundary between the Cocos and Pacific plates is given in Figure 2.30. The NUVEL-1 model also...

Dip of Subduction Zones

Question 2.3 What determines the subduction dip angle Since the gravitational body force on the subducted lithosphere is downward, it would be expected that the subduction dip angle would tend toward 90 . In fact, as shown in Figure 2.19, the typical dip angle for a subduction zone is near 45 . One explanation is that the oceanic lithosphere is foundering and the trench is migrating oceanward. In this case the dip angle is determined by the flow kinematics Hager and O'Connell, 1981 . While this...