Geology Reference
In-Depth Information
Some scientists think the earth's mantle works very much like a lava lamp. Mantle rock
near the core of the earth is heated, becomes less dense, and begins to flow upward, dis-
placing the cooler, more dense mantle material near the crust, which sinks downward,
back toward the core. In these loops of rotating mantle material (or convection cells),
the heated mantle material that drives upward is called a
mantle plume.
In the mantle
plume model for driving plate tectonics, the rising of heated mantle rock initiates rota-
tion of a convection cell. As the mantle plume-driven convection cells move in a circular
motion, they pull the crust along with them, causing the plates to move.
Even though most scientists agree that mantle plumes and convection play an important
role in driving plate tectonics, there is still some debate on exactly how the mantle ma-
terial circulates and where the plumes originate. Some scientists propose that the
meso-
sphere
(the layer of the mantle closest to the core) and the
asthenosphere
(the part right
beneath the crust or
lithosphere
) each have a layer of convection cells that rotate in op-
posite directions. Others propose that the entire mantle convects like a lava lamp.
Tug of war: The slab-pull versus ridge-push mod-
el
The mantle plume model focuses on the upward motion of heated material to drive plate
motion, but we can also look at the motion from the other direction. Perhaps the sinking
of (relatively) cold, dense crustal plates actually drives plate motion. In the two models I
explain in this section — the slab-pull and ridge-push models — gravity plays a role
along with convection.
As I explain in Chapter 9, when a continental plate and oceanic plate meet, the oceanic
crust (which is more dense) is forced down
(subducted)
into the mantle beneath the less
dense plate of continental crust. The term
slab-pull
describes plate movements driven
by the sinking of oceanic crust (the “slab”) into the mantle, pulling the attached crustal
plate along behind it. The sinking slab is the downward arm of cooler, dense material in
a mantle convection cell. As it sinks, the heated mantle materials deeper in the earth are
forced upward, completing the convection cell's circular motion.
A related but opposite approach is described by the
ridge-push
model. In this model, the
heated mantle below a mid-ocean ridge (a plate boundary where new oceanic crust is
being created) slightly lifts the crustal plates as it wells up, erupting fresh lava (which
cools into basalt, creating a mid-ocean ridge).
This model emphasizes the role of gravity. It suggests that the slightly elevated position
of the mid-ocean ridges results in an outward and downward sliding movement (or
push) of the plates on the opposite sides of the ridge. The plates are pushed away from
the ridge, toward its opposite edge. In most cases, the opposite edge is a subduction
