6.4 One region on the Moon is covered with craters, while another is a smooth volcanic plane. Which is older? How much older? How do we know?
The crater-covered surface is close to the same age as the Moon is - about
4 billion years. The smooth surface is only one or two billion years
old. Collisions were much more frequent in the past, near the beginning
of the solar system, than they were later. (See figure 6.9)
6.10 If Venus is completely enshrouded in
clouds, how do we know as much as we do about its surface?
Magellan radar-mapped its surface.
6.17 Assume that the east coast of South America and the west coast of Africa are separated by 4,500 km. Say that GPS measurements indicate that the continents are now moving apart at a rate of 3.75 cm/yr. If you could assume that this rate has been constant over geological time, how long ago were these two continents joined together as part of a supercontinent?
A distance of 4.5 x 106 m divided by an expansion rate of 3.75 x 10-2 m/year, gives an expansion time of 1.2 x 108 years. So the continents would have been together 120 million years ago - during the Cretaceous, when the dinosaurs were around.