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Plate Tectonics 4
What Makes the Plates Move?
(And what exactly are the plates anyway?)
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Earthquake wave behavior (echoes, refractions, speed
changes, blockages) helps us recognize different layers
of different density within the Earth, even if we can’t see
them directly.
Probably solid
Fe/Ni like outer core.
Extreme pressure
Liquid mix of
keeps it solid.
Fe/Ni . Density is
right for metals.
Certain waves are
blocked here,
Indicating liquid state.
SAMPLES!
Density suggests
peridotite, though
not pure olivine.
The lithosphere includes the
crust and upper mantle (crust
only near ridges). It is brittle.
Another way of thinking about the interior
has little to do with composition, but
depends on the state of the material
(CORE)
Some geophysicists
contend that much of the
lower mantle is again brittle,
and call this the “lower
mantle”. This seems
unlikely as we will see.
?
The asthenosphere includes
the part of the upper mantle
below the lithosphere. It
behaves plastically. Its thickness
is not known.
Your book erroneously places a thickness on the asthenosphere. Indeed, it assigns different thicknesses to the layer in different
figures. In Fig. 2.28 it is said to be 250km providing a maximum depth for its base (assuming a 300km lithosphere, which is
pretty unusual) at 550km. In Fig. 22.9 its base is shown at ~750 km. Both of these things are probably wrong. It is not
inconceivable that the asthenosphere extends all the way to the core.
Ironically, the book acknowledges this possibility in
Fig. 22.4. Here the mantle is shown convecting
throughout, something not possible if the lower
mantle is a rigid, brittle solid, as the text claims.
Thus your text becomes a good example of what can
happen if you don’t think critically about what you are
saying or writing.
Images from Hewitt, et al., Conceptual Integrated Science.
The plates of the plate tectonic model are pieces of brittle lithosphere
(not just crust) that are not continuous with each other. These pieces
are free to move in different directions if given the impetus to do so,
and they do exactly that. We will return to the consequences, but for
now we need to consider what the impetus is: what force causes
them to move.
Several hypotheses have been proposed for a driving mechanism for
plate movements. All but one can be pretty easily falsified, some for a
variety of shortcomings, all because they simply cannot supply
enough force to move what they are hypothesized to move. We will
consider only one of these failed hypotheses here because your book
is evidently enamored with it. That hypothesis is called “slab pull”.
The hypothesis claims that the cold end of a plate (the slab)
descending into a trench pulls the entire plate away from the nearest
ridge.
The problems with slab-pull are numerous. I’ll start with the most serious criticisms, though any one is really sufficient
to call the hypothesis into question.
1) There are plates that are demonstrably moving that are not moving toward a trench. Indeed, it should be obvious that since one plate must
necessarily subduct beneath another plate that half (at least) of the plates do not subduct. Actually the proportion is greater than half. The
plate carrying India continues to push vigorously against the plate with the rest of Asia, even though there has not been a trench there for
millions of years. One or both of these plates is obviously moving and there is no reasonable slab to be moving them. To dig deeper
(deeper than I want you to follow, unless you’re really interested) the Himalaya are demonstrably higher than they should be – they are not
at isostatic equilibrium. We know this because their roots are far too shallow for their height. (See p. 646-647 for an explanation of
isostasy.) Thus the Himalaya (including the roots) must be being pushed up by something, not pulled down. Similarly, the Americas
cannot be moving westward, nor can Africa and Eurasia be moving eastward (at least one of which is clearly happening) because of slab
pull because there is no slab.
2) The lithosphere of a descending slab comprises about 5% basalt (at the top) and about 95% peridotite. The asthenosphere beneath is
100% peridotite. The lithospheric plate is nonetheless claimed to be denser because of the temperature difference between the slab and
the deeper asthenosphere. Among the various problems with this idea is the (apparent) assumption of zero heat flow from the
asthenosphere to the slab. Humans design things that they intend for rapid heat exchange with one very specific characteristic: thinness.
The narrow, flat tubes that the water that cools your car engine flow through in your radiator are thin (and thin-walled) specifically to move
heat quickly out of the water and into the air, where the fan can blow it away. In the opposite sense, the heating coils in your toaster are
very thin so they can rapidly absorb electrical energy and rapidly radiate it toward your bread. The idea that a thin slab of lithosphere will
stay cold, and keep its higher density long enough to make this work seems unlikely.
3) There is abundant evidence that new subduction zones form fairly often (geologically speaking), and a historical consideration of the
process makes obvious the need for them to do so at least occasionally. In the initial stages of formation of a subduction zone what puts
the slab into position so that it can then pull the plate “behind” it? That is, how does the process start? Obviously it can’t be started
by slab pull because (again) there’s initially no slab to do the pulling. It seems pointless to propose a driving mechanism for something if
that mechanism can’t begin the process. No matter how good a driver you are you’re going nowhere if you can’t crank your car.
4) The confusion in your book about the thickness of the asthenosphere is actually related to this slab pull hypothesis. (Figure 22.24 actually
includes it). Part of the slab pull argument is that it is a superior hypothesis to another hypothesis which requires a thick convecting
asthenosphere. Proponents of the slab pull hypothesis point out that if the asthenosphere is only a few hundred km thick (750km,
remember) and claim therefore that it cannot generate enough force to move the lithosphere. Consider this: the Pacific Plate is about
16,000 km across from the east Pacific Ride to the Japan Trench. The line below is a scale model of the plate including 750km of
subducting slab.
5) How likely do you think it is that that “ramp” will pull that chunk of lithosphere across a surface that your book claims flows about 1/10,000
the rate of the hour hand on your clock? The asthenosphere is plastic, not liquid: there is considerable friction between it and the overlying
lithosphere.
(CONTINUED)
5) Now consider this: if the bottom of the slab is at 750 km depth we presumably get the maximum possible force on
the “ramp”, given that the mantle below this depth is a rigid solid. But what happens when the slab has “pulled” its way
to this depth? Well, it has to stop because it cannot move the underlying lower mantle aside to go any deeper.
The slab that is supposed to have pulled 16,000 km of oceanic crust away from the ridge has lodged itself against a
rigid lower mantle only 750km down! How did that 1600km of crust (+the slab) get created and moved with so little
elbow room?
STOP
6) The fix for this is to hypothesize that the slab can go all the way to the core. There is some seismic wave behavior
that looks like it could represent a plate going that deep, or it could be something else entirely. But apparently if the
plate needs to go to the core then this is adequate evidence that it does so. (Can you hear the sarcasm in my voice?)
But how does it do that if the asthenosphere only goes down 750km? This is the conundrum: to allow the plate to go
that deep you also have to concede that the mantle is plastic to that depth. Once you do that then you have negated
your own argument against the alternate (and better) hypothesis of whole-mantle convection moving the plates. (The
deepest recorded earthquake, in a descending slab, occurred about 700km down.
7) If the force responsible for the plate’s motion was a pull from the trench end then the entire plate would be subjected
to tension. In fact, the maximum tension should be at the end being pulled. We see no evidence for tensional stress
anywhere in the plates except at the ridges – the end farthest from where the alleged pull is occurring.
Don’t believe everything you read in a textbook (or any other book). For that matter, don’t automatically disbelieve
something you read in a textbook (or any other book). Try to think things through critically because that’s the essence
of science and the essence of any other logical system.
With a couple of corrections Fig. 22.24 is a good model for the more likely driving mechanism. Note that if we disregard the
shallow asthenosphere and hypothesize convection of all, or at least a significant proportion, of the mantle (as the picture
obviously does) then the lithospheric plate movements relate to that convection.
Relatively hot mantle material rises beneath the ridges, just as hot air at the equator rises, and for the same reason: its lower
temperature makes it less dense that the material around it and it is capable of flowing in response to the resulting gravitational force.
But once it gets to the lithosphere two things happen: 1) it cannot go higher, and 2) it can begin to cool. Indeed, it loses a great deal
of heat to the volcanism at the ridge. It can’t sink back down because there’s more behind, pushing it on. What it can do is spread
out laterally in both directions. The resulting force splits the lithosphere and begins to pull (it’s a tensional force) it sideways in
opposite directions.
Notice that the friction between the lithosphere and asthenosphere is what causes the former to move. It is not something to be
overcome (as in the slab pull hypothesis) but the crux of the process of plate movement.
The main body of the plates does not experience that tensional force. The mantle material is spreading laterally so the plate simply
goes along as if on a conveyor belt. The force that moves them is like the one that moves an unpaddled canoe down a stream – no
part is being pulled or pushed any more than any other part – there is no net stress in either a tensional or compressional direction.
There is only momentum.
At a place where two laterally spreading convection currents meet they sink back toward the core. The reason is not just that they
have run into each other and can’t go up. The material has cooled adequately that it’s density is about as high as it can get this close
to the surface. As it sinks it displaces lower, hot mantle sideways toward the ascending parts of the convection cells.
The rigid lithosphere, however, cannot just sink like the asthenosphere does. It is too hard to flex freely downward that way, so even
though both plates are probably pulled downward, one piece rides over the top of the other piece and is not pulled as far downward.
The downwarping creates the trench and the attempts by both plates to stay near the surface creates the compressional stresses that
are characteristic of these plate margins. The melting of the descending plate feeds the volcanic arc that inevitably forms besided the
trench, on the upper plate.
(One last jab: it’s difficult to reconcile the idea of a plate that is cold enough to be dense with one that is hot enough to melt.)
When the plastic mantle reaches the much
denser core it again spreads laterally and
moves back toward the nearest place it can
re-ascend, completing the convection cell
There are a couple of further points to add to what the book says. If a trench develops away from a coast
(with oceanic crust on both plates) then the subducted plate will probably be the one closest to the ridge, or
the “faster” ridge if there are two involved. The equal density and thickness of the two pieces makes it a
toss-up as to which will wind up beneath, so things like slight differences in rate of motion are therefore the
controlling factor. If there is a continent at the edge of one plate then the continent will always remain on
the upper plate. Continents are too light and too thick to be subducted.
If there is continental crust on the edge of both plates then neither subducts. Instead they both fault and fold at the edges
as the plate behind them continues to compress them together. The folds buckle rocks upward and the faults shove large
pieces atop other pieces. These kinds of mountains (like the Himalaya) are the biggest of all mountains. They are not,
however, volcanic. Either the trench stops subducting lithosphere and melting it, or the crust becomes too thick for magma
to reach the surface, or both.
Continental Collision
The lithospheric plates that are moved around by the driving mechanism can move in three different ways with respect to
each other:
1) In opposite directions away from their mutual edge (a rift/ridge system)
2) In opposite directions toward each other (a trench/subduction zone or continental collision)
3) In opposite directions by sliding past each other (a strike-slip fault).
The first case creates what is called a divergent plate margin
The second creates a convergent plate margin
The last creates a transform margin.
(Note that two plates can move in a partly transform sense and also in either a partly divergent or convergent sense. If
one type of motion predominates (as it usually does) the margin is treated as that kind of margin. If both types of motion
are more or less equally apparent it can be called an oblique margin.
RIFT
volcanic
arc
trench
RIFT
Transform faults are always associated with
ridge/rifts and offset them. The form
because the plates are not flat pieces
sliding apart, but spherical surfaces (like an
eggshell) rotating apart. Some parts have
to move faster than other parts. Being thin
and brittle the plates break and the edge of
a faster one moves past the edge of a
slower one.
Notice that the small arrows that suggest
that the plates are moving in opposite
directions are not exactly right. The next
slide clarifies what actually happens. Both
of the plates are moving to the right but one
is moving faster than the other. The sense
of motion right at that place is as if they
moved in opposite directions.
RIFT
Both of the plates are moving to the right
because they are spreading outward from
the ridge. However, the upper one is
moving faster than the other, as indicated
by the bigger arrows.
Because of the differences in rate the
sense of motion right at that place is as if
they moved in opposite directions.
A good analogy is if you are facing
backwards in a car that is passing another
car the car you’re passing looks like it’s
going backwards – in the opposite direction
from yours – but of course it isn’t.
RIFT
Of course at the indicated point the
plates are literally moving in
opposite directions. This location is
between two ridge segments.