How can slab-pull and ridge-push contribute to the movement of a plate at the same time?
This is the text book:
Oceanic crust is created at a mid-ocean ridge by igneous material being extruded through the ridge by mantle convection at constructive plate margins. The creation of new material here is continuous over millions of years. This continual pushing of material through the ridge effectively pushes the plate along.
The mantle convection is also involved in slab-pull which is caused by the subducting oceanic plate that has been thrusted beneath another plate at a destructive plate margin. The weight of the plate gradually pulls it deeper into the mantle, and combined with ridge-push it is a major part of the tectonic plate theory.
In laymans terms, please?
Its really not that hard to understand, let me see if I can simplify it for you. Scientists were trying to sell this idea that the continents were drifting in the early part of the 20th century, but they could not explain the mechanism, or what caused it. In the middle of World War II, the sea-floor spreading you mentioned was discovered, along with the trenches, which is where the old ocean floor is being subducted back into the mantle. So its like this: imagine you are trying to slide a very heavy box across the floor. It is too heavy to move by yourself, so you enlist the help of a friend. You tie a rope around the box, which you pull on. Your friend pushes on the box from behind. Together, your combined efforts are enough to make the box slide across the floor. That is all the book is trying to explain, that new ocean floor is "pushing" the old ocean floor from behind, while the old ocean floor is being "pulled" back down into the mantle. Between the both of them, that section of the ocean floor moves; slowly, mind you, but it does move.