How fast and in what direction were plates moving at the time? Flashcards
How fast and in what direction were plates moving at the time?
Reconstructing the relative speed and direction of motion of the lithospheric plates for geological ages older than the oldest ocean floor remains challenging. But for the last ~200 million years surely we can use this beautiful map of the magnetic age of the oceans to work this out
How would you figure out the direction and speed of motion of the Pacific plate between the eastern and western boundaries of the blue box?
Speed would be relative to the age of the rock
We can see that the Pacific Plate forms at the East Pacific rise and moves from the east to the west, and the strike of the transform faults gives us a good idea of relative direction of motion
We can see that the eastern and western boundary ages are about 120 and 40 million years i.e., it took 80 million years to cover the distance between east and west. If we now measure this distance and divide it by 80 million years, then we get the average plate speed in this time interval
- QUESTION: The present-day velocity of the major plates is approximately constant, a state that is known as a ‘dynamic equilibrium’, which requires what?
o Driving forces>inhibiting (resisting) forces
o Driving forces< inhibiting (resisting) forces
o Driving forces = inhibiting (resisting) forces
Correct answer is Driving forces = inhibiting (resisting) forces
In order to maintain the speed of the plate you need to have just as much motion to overcome the friction of the plates against each other.
The plates are in dynamic equilibrium, which means the driving forces are enough to balance the resisting forces and to keep the plates moving
There are respectively three main driving stresses and five main resisting stresses
What are they key stresses resisting plate motion?
(1) Emerging plate rigidity at the mid-ocean ridge, plate needs to ‘break off’ by normal faulting before it can move.
(2) Viscous drag of mantle rocks on all surfaces an sides of the moving ocean lithosphere and subducting slab. Happens on under and over part of plate. As the slab moves downwards, it will be coupled to the mantle rocks and drag the mantle rocks down with it- slab suction (force is needed)
(3) Vertical bending of the oceanic lithosphere to force it into subduction (imagine trying to bend a ruler or wooden stick- you will apply force/stress to do so). Force is required to bend the subducting plate (another resisting force)
(4) friction with the overriding plate is another resisting force but only happens when the subducting and overriding plate come into contact
(5) Viscous deformation of mantle rocks at the leading edge of the subducting slab, causing stress whose magnitude is usually many times higher than that of any of the other resisting stresses
Driving stress: How physically does the “ridge push”?
At mid-ocean ridges, magma injection exerts an upward push and maintains anomalously hot and therefore less dense rocks, so that the ridge is elevated above the surrounding ocean floor.
The lithosphere then cools and becomes denser away from the ridge and sinks isostatically into the asthenosphere as forced by gravity. This exerts a substantial push in the direction of plate motion and thus drives the plate away from the ridge, making space for new magma injections.
Driving stresses are even larger (and potentially several times larger) if a mantle plume is present at the ridge, e.g. AFAR in East Africa or in Iceland which sits on the Mid-Atlantic Ridge. This is because the extra heat elevates the ridge above the ocean floor. The more it is elevated, the steeper the gradient is going to be and the hotter the ridge will be which will cause it to cool and contract more will then sink into the mantle more and cause more push.
Although ridge push is a strong force, it exerts only about a quarter of the pull at subduction zones
Plates attached to subducting plates therefore generally tend to move faster than those without
Driving stress: How physically does the “slab pull
Density increases cause a pull caused by gravity
Older and denser lithosphere will have a greater pull on it by gravity
First, we can see that the temperature of the subducting slab is much lower than that of the surrounding mantle. It will therefore be relatively dense and heavy and thus be anomalously strongly ‘pulled down’ by gravity
Gravitational force has a greater effect on the colder, denser slab
Second, we can see that phase change from less dense olivine to denser spinel happens at a shallower depth in the subducting slab than in the surrounding mantle. This is because it is already anomalously dense and under relatively elevated pressure
What is slab suction?
If the subduction zone is mature then its lower slab will have suffered various stresses for a long time. It will then have become ‘weak’ and its mechanical resilience and thus is more likely to tear and break off the subducting slab
Once it becomes detached the strong gravitational pull (see reasons on previous slide) will cause it to accelerate and sink through the lower mantle, and possible all the way to the core-mantle boundary
As it sinks through the mantle, viscous coupling is hypothesized to cause regional-scale mantle convection that might increase the driving stress on the plate that is still attached to the remainder of the subducting slab
What is trench suction?
Important: don’t confuse trench suction with slab suction, they are two completely different things
Trench suction acts on the overriding plate, not the subducting plate
We have already seen, for example, that the overriding South American plate moves fast into the trench
It is not yet clear what causes trench suction and, as always, there may be different reason for it at different subduction zones
What is the first idea for trench suction?
convection in the mantle wedge could be caused by viscous coupling of the rocks in it with the down going slab
This would drive at least the leading edge of the overriding plate into the trench, and especially so if back arc spreading occurs and a smaller silver of arc lithosphere is split from the overriding plate
What is the second idea for trench suction?
If the subducting slab rolls back over time then this would cause the trench to widen and get deeper. The overriding plate would therefore experience much less resistance. It would then move to close the gap, as supported (potentially) by rollback-enhanced convection in the mantle wedge
