CRUST REPLACEMENT

Growth rate of 2 - 4 cm per year and a length of 60,000 km means that new crust is created at a rate of Crust - (2-4 cm per year) x 60,000 km = (2-3x10**[-5] km) x 60,000 km = 1.2-1.8 square kilometers per year

How long does it take to replace all of the oceanic crust?

The total surface area of the Earth is

Area = 4 x pi x R**2 = 4 x pi x (6,400 km)**2

===> Area = 5.1x10**8 square km.

The oceans cover 55 % of the Earth's crust and so the time required to replace all of the oceanic crust is

time = 0.55 x 5.1 x 10**8 sq km / 1.2-1.8 sq km per year

===> time = 160,000,000-240,000,000 years

That is, the entire oceanic crust of the Earth is replaced every few hundred million years.