prize9-3, Exercise 2: Distance moved by a plate

Working: 2.5 cm per year x 100,000,000 years = 250,000,000 cm = 2,500 km (250,000,000 cm / 100,000 cm per km).

Meaning: 2,500 km is a large fraction of the width of the present Atlantic, which is a few thousand kilometres, depending on where it is measured (check). So a slow movement, steady over a very long time, is enough to open an ocean. The Atlantic is generally dated as beginning to open about 180 million years ago (check), consistent with this scale.

Care: real plate speeds vary (roughly 1 to 10 cm a year) and may not have stayed constant, so this is an order-of-magnitude check.