Question

The North American and European plates of the

Earth’s crust are drifting apart with a relative speed of
about 25 mm/yr. Take the speed as constant and find when
the rift between them started to open, to reach a current
width of 2.9 x 10^3 mi.

Answer 1

distance = speed/time

width/distance given is 2.9*10^3 miles
1 mile=1.6*10^6mm
total distance now is 2.9*1.6*10^9
speed is 25 mm/yr
plug the values,
therefore the rift takes approximately 185600000 years for a width of 2.9 x 10^3 mi.