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Rift valley on
ridge crest

Trench Trench

Hot

Cold Cold

Core Mantle

Mid-oceanic ridge Volcano
Island arc Oceanic trench Oceanic trench
(volcanoes) Rift valley

Earthquakes

Continental crust
Oceanic crust
Magma Magma Earthquakes

Mantle

Cold seafloor Mantle Magma
and mantle rock

Hot
mantle Cold seafloor
rock and mantle rock

FIGURE 18.18 Not to scale. One idea about convection in the mantle has a convection cell circulating from the core to the lithosphere,
dragging the overlying lithosphere laterally away from the oceanic ridge.

and their boundaries are yet to be determined with certainty. (435 mi), which means that descending materials—parts of a
The major question that remains to be answered is, What subducted plate—must extend to that depth. This could mean
drives the plates, moving them apart, together, and by each that a convective cell might operate all the way down to the core-
other? One explanation is that slowly turning convective cells mantle boundary some 2,900 km (1,802 mi) below the surface.
in the plastic asthenosphere drive the plates (Figure 18.18). This presents another kind of problem because little is known
According to this hypothesis, hot mantle materials rise at the about the lower mantle and how it interacts with the upper
diverging boundaries. Some of the material escapes to form mantle. Theorizing without information is called speculation,
new crust, but most of it spreads out beneath the lithosphere. and that is the best that can be done with existing data. The
As it moves beneath the lithosphere, it drags the overlying full answer may include the role of heat and the role of grav-
plate with it. Eventually, it cools and sinks back inward under ity. Heat and gravity are important in a proposed mechanism
a sub duction zone. of plate motion called ridge-push (Figure 18.19). This idea has
There is uncertainty about the existence of convective cells a plate cooling and thickening as it moves away from a diver-
in the asthenosphere and their possible role because of a lack gent boundary. As it subsides, it cools asthenospheric mantle to
of clear evidence. Seismic data are not refined enough to show lithospheric mantle, forming a sloping boundary between the
convective cell movement beneath the lithosphere. In addition, lithosphere and the asthenosphere. The plate slides down this
deep-seated earthquakes occur to depths of about 700 km boundary.

468 CHAPTER 18 Plate Tectonics 18-14