Ocean Forms - Ocean Worlds: The Story of Seas on Earth and Other Planets - page 63

On longer timescales, mid-ocean ridges have grown larger or
smaller as sea floor spreading has accelerated or decelerated globally
in response to variations in the deep currents within the mantle.
Larger ridges displace more water from the ocean basins to spill over
the continents, while smaller ridges allow water to settle back into
ocean basins that have become deeper and larger.
One has to try to resolve all of these short-term trends, and also to
try to distinguish them from those apparent sea level changes caused
by the land being tectonically raised or lowered. In any individual suc-
cession of rock strata, the patterns caused by purely local changes of
sea level caused by tectonic uplift or downwarp of a landmass can
look exactly the same as those caused by the global sea level rising
and falling.
To disentangle all of this, one needs to correlate the sea level signals
in strata around the world. This can help tell which sea level changes
take place simultaneously and so are global, and which take place in
one region but not in others and are due to sections of crust rising or
falling. To work out this kind of history a precise time framework is
needed, so that one does not match up a 100 million-year-old sea level
rise recorded in one place with a 110 million-year-old sea level rise in
another. With that kind of miscorrelation the end result is, geologic-
ally, gibberish.
Geologists are constantly aware of the gibberish factor, and of the
fact that geology as a science is blessed—or cursed—with a terrible
abundance of time. Therefore, they go to great lengths to pin down
time in rocks as precisely as possible, using any kind of evidence that
they can lay their hands on. This can range from the natural radio-
activity of volcanic crystals to subtle changes in the patterns of iso-
topes of various chemical elements. The oldest method, though, is still
in many ways the best and the most widely used in practice—the use
of fossil animals and plants, exploiting the evolution and extinction of