Ocean Forms - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 64

different fossil species as unique time-markers in rock strata. This
works magnificently, but only for a little over the last half-billion years
since large, complex, hard-shelled or bony multicellular animals
appeared on Earth (see Chapter 6)—and that is not much more than
10 per cent of Earth's history.
Then, as our uncertain starting point, there are those hints of early,
more capacious oceans, and of frequently flooded continents too,
some 3 billion years ago—given that there seems to be an abundance
of basalt that has erupted underwater in continental settings (such
basalt develops particular structures: the 'pillow' shapes we noted at
Ballantrae).
It may be, therefore, that the oceans have diminished, perhaps
halved in volume, since their beginning. Size is not everything
though—quality matters too. We have other aspects of the oceans to
consider, such as: what makes them salty?