Okay so I have a feeling I am going to need to start at the beginning. In order to look at the mechanism of plate tectonics you need to know the structure of the earth. Okay now there are different ways to look at it, but I am going to try and take a lot of the jargon out of it. I am sure you have heard of the crust and the mantle. Those are differences based on composition but that’s not what we are going to be looking at and working with. First the favorite word to describe this way of splitting up the earth is rheology. Rheology is basically the way in which things deform. Think if it like if you were to push down with a finger on a cracker, jello, or some water. The cracker would break and crack, it would respond to that pressure in a brittle way, by faulting! The Jello would ooze out of the way, it’s not the best representative for plastic but, present it doesn’t bounce back, okay think of gak. Okay so the crust and the top part of the mantle break and fracture, deeper it flows a little more but overall once you get into a lot of plastic movement you are into the part of the mantle with another name. Ok so I don’t know how to get around these terms but the crust and upper mantle are the lithosphere and the mantle that is flowing is the asthenosphere.
Okay so you have the divisions in the earth, now we are on to how the plates move. You do know that you live on a plate right? The mashed potatoes are right over there, don’t you see them? Hehehe. Okay so you have these plates that are a little denser than the asthenosphere. Now you would think that then they would sink, flat out. But they are floating slightly, so this colder denser layer that includes the crust gets densest on the edges of the oceanic plates. Oceanic crust is all basalt and denser than continental, and in fact the only thing that WILL subduct. So you have this old dense oceanic crust and the edge starts to sink. Now the asthenosphere has to get out of the way for it to sink and it can’t move very quickly so this acts as a force against the sinking but not as large of a force as the one pulling the plate down. So this plate is sinking into the asthenosphere and as it sinks it pulls on the rest of the plate and pulls it down. Sometimes it starts to go down at a steeper angle and starts to pull back and push the asthenosphere behind it out of the way. This creates suction and pulls the plate above it that it is subducting against towards it. This creates stretching in the overriding plate and can lead to rifting or a break through of the asthenosphere behind the volcanic arc. You can see this in the western pacific.
Up until this point what I have been taught is that yes this is one of the forces driving plate tectonics but there is also mantle convection. But if you start to look at the forces and velocities required to move these plates with mantle convection they get too big. Also here is a good thought experiment. If you have a big honking continent sitting there in the middle of a plate that has oceanic crust on either side then you have this dip into the astheopshere. If you are driving that motion with mantle convection then that should act like a sail and propel that plate. But if you look at a graph of % continent vs. velocities, you see that, there is no such correlation of movement and amount of continent. There are several other diagrams like this that can be used to disprove mantle convection as the main driving force of plate tectonics but I feel that this one is the most telling. Also if you look at absolute velocities of plates against percentage subduction zone there is a correlation between more subduction zone boundaries and faster plate movement. This is the most telling one for the slab pull.
Like I said I have been of the school of thought that its kind of a combination but this argument using stress regimes force diagrams and relationships has never been presented to me. I had a “AH HA” moment and I really wanted to share this. I will most likely add more to this post as I have time, and find good references.
Fun idea: in order to show you how a plate sinks from one side, Wes my structure teacher said this can be seen in a bath with a dry washcloth. Place the dry washcloth on the surface of the water and watch one side slide in first. I am going to test this tonight. You know study me some geology. Geology is fun!
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