Part 3
One argument put forward in Kevin Mansfield’s theory is that some 70% of the Earth’s surface area is oceanic crust, none of which is more than 200 ma old. Mansfield seems to think it unlikely that this amount of crust could disappear “down a hole” within that time scale. However, a closer look at the situation reveals that this is possible.
Earth’s surface area = 5.1 x 10^8 km^2
Area covered by oceans is approximately 71% = 3.61 x 10^8 km^2
The oldest oceanic crust is 2 x 10^8 years old.
It seems reasonable to suggest that the ratio of oceanic crust to continental crust was not lower 2 x 10^8 years ago than it is today.
Recent estimates of the global subduction rate = 3 km^2 pa.
If this rate has been constant over the past 2 x 10^8 years, and as far as I am aware there is no evidence to suggest that the subduction rate has been significantly lower in the past, then there would have been time for 6 x 10^8 km^2 to have been subducted. This is a figure approaching twice the amount that would need to be subducted to achieve a complete change of oceanic crust in two hundred million years.
A point worth mentioning here is that if Mansfield’s theory is correct, and the current tectonic movements are just relict activity from the collision, it would be logical to reason that the rate of movement would be slowing with time. Thus, there would be scope for even greater turnover of oceanic crust than the above calculation indicates.
