Why does mantle convection occur
Ridge Push (fig. 2) has been considered in twodifferent manners, as a body force and as a boundary force. As a body force,ridge push has been attributed to the cooling and thickening ofthe oceanic lithosphere with age (McKenzie, 1968; McKenzie, 1969; Richards,1992; Vigny et al., 1992). This type of force can be thought of as createdby the horizontal pressure gradient attributable to the cooling and thickeningof the oceanic lithosphere, and calculated as this force integrated overthe area of the oceanic portion of a given plate (Lister, 1975). In thisrespect, Ridge Push can be considered a body force, rather than a boundaryforce acting over the oceanic part of a plate (Wilson, 1993). When makingsuch a calculation however, one must take into account that oceanic lithosphereolder than 90 Ma is no longer cooling significantly, and therefore notcontributing to the effective ridge push force (Ziegler, 1993). The alternative,Ridge Push as a boundary force, is caused by the "gravitywedging" effect ( Bott, 1993). This effect results from warm, buoyantmantle upwelling beneath the ridge crest which causes a topography-inducedhorizontal pressure gradient. Here the force would be acting as a boundaryforce at the edge of the lithospheric plate, proportional to the lengthof the ridge, and not as a body force over the entire oceanic portion ofthe plate. In both of the above cases Ridge Push would be amplified, byas much as a factor of two when hot spot activity is centered on a spreadingridge axis (Ziegler, 1993). This is important when considering the effectsof ridge push as a cumulative force acting on all the plates, and mustbe taken into account in any net force calculations.
from its interior to the surface through mantle convection.
To the question, "What drives plate tectonics?" wehave presented two options: (1) mantle convection, and (2) lithosphericplate boundary and body forces. It is in the opinion of this author thatit is the plates themselves that are the dominant source of force involvedin the absolute movement of the lithospheric plates over the surface ofthe Earth. The strong correlations between observed tectonic stress andabsolute plate motions shown by the World Stress Map Project point directlyto the present lithospheric stress fields being dominated by the individualplate boundary and body forces (Zoback et al., 1989, Zoback, 1992). Theseobservations, along with the Net Torque Model, allow us to begin to puta coherent story together in terms of the relative magnitudes of differentPDF's. Although the slab forces (slab pull and collisional resistance)dominate the other PDF's, their equal and opposite nature allows ridgepush to be the most important observable plate driving force.
Globalseismic tomography is used to interpret the presence of ancient subductedslabs, locate the source of hotspots, and model convection patterns inthe mantle.
How do convection currents work in the mantle? | …
Using a combination of the known tectonic stresses along with a quantifiedforce relationship, we should be able to devise an accurate account ofall the active forces involved in today's plate motion. In addition, weshould better understand the magnitude scale for the different categoriesof mantle and plate forces.
How do convection currents work in the mantle
WHAT Drives Plate Tectonics ???? This question has been the subjectof intense debate ever since the plate tectonic theory was first ecceptedby the geologic community in the late 1960's. The major concern is whethermantle convection and the activity of mantle plumes dominate the drivingforces of plate motion, or whether surface boundary and plate forces, suchas slab pull and ridge push provide the most important forces. The argumentis basically whether the plates are passively riding along on the top ofa mantle convection cell, or whether the plates themselves the active drivers,dragging along with them the mantle below.
Many scientists think that mantle convection does not ..
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