if new crust is constNTLY BEING CREATED AT MID-OCEAN RIDGES SO WHY IS THE OLDEST OCEANIC CRUST OLNY ABOUT50 M?

Answer 1

You sort of answered your own question here... The reason the oldest oceanic crust is only about 50 million years old (actually the oldest oceanic crust is more like a couple hundred million years old) is because it is constantly being created and destroyed.

Convection currents inside the mantle/asthenosphere cause hot molten rock to rise up to mid-ocean ridges where it punches a hole in the surface. Once this hot and relatively less dense rock reaches the surface it starts to cool, and moves away from the ridge as new hotter rock is pushed up to replace it. This is partially responsible for the motion of oceanic crust.

The main mechanism for the movement of the oceanic crust is the gravitational pull of the plate itself as it rises at the mid-ocean ridge, and sinks at subduction zones -- the reason it sinks is it has become cooler and more dense compared to the asthenosphere below.

So, basically... the reason the oceanic crust gets destroyed in a relatively short time is because over time it becomes more dense than the asthenosphere below it, and sinks back into the hot inner earth where it is mostly destroyed.

Answer 2

Because just as the crust is being created at the mid ocean ridge, it's also being subducted under the moving continents.

Answer 3

Because as an oceanic plate moves, it slowly shifts the "new" crust to the outer edges of the plate. Eventually, that crust is subducted (meaning it is pushed under) a continental plate.