Page 112 - Earth's Climate Past and Future

88 PART II • Tectonic-Scale Climate Change

The amount of thermal expansion of ocean ridges
varies through time in response to changes in the rates
of seafloor spreading, and the changes in the height of
the ridges in turn alter the volume of ocean basins and
their capacity to hold water (Figure 5-9). Ocean water is
displaced up onto the continents during times when
ridges spread rapidly and produce wide, high-elevation
(“fat”) profiles, but the sea withdraws from the conti-
nents during times when the ocean ridges spread more
slowly and produce narrower, low-elevation (“thin”)
profiles that displace less water.
The profiles (and volumes) of ocean ridges existing
in the past can be reconstructed based on a systematic
relationship observed in modern ocean basins. All ocean
ridges today have crests that lie at an average depth of
2500 m below the sea surface. Away from the crest, the
subsurface depth profiles of these ridges follow a simple
equation:

Ridge depth = 2500 m + 350 (crustal age) 1/2
(in meters) (at 0 age) (in Myr)
This equation describes a ridge crest that starts at an ini-
tial depth of 2500 m below the sea surface and gradually

High sea level

FIGURE 5-8 Marine limestone exposed on land Marine Ridge Smaller basin
limestone deposits that today form the coasts of southern Fast crest
England and northern France are evidence of higher sea levels spreading “Fat”
100 Myr ago. (Andrew Ward/Life File/PhotoDisc.) profile

and the weighed-down crust and added even more
A
weight that has caused still more subsidence. Obviously,
figuring out past changes in sea level is more difficult
than it might initially seem.
The evidence that sea levels in the Cretaceous Low sea level
(80–100 Myr ago) were higher by somewhere between
100 and 300 m than they are today has been attributed to Ridge
crest Larger basin
two groups of factors: (1) tectonically driven changes in
the volume of the ocean basins that altered their capacity Slow “Thin”
spreading
to hold water and (2) changes in the volume of water in profile
the ocean basins resulting from changes in climate.
B
Changes in the Volume of the Ocean Basins
1. Changes in the volume of ocean ridges Ocean FIGURE 5-9 Spreading rates and sea level Rates of
ridges owe their high elevations to unusual heating spreading at ocean ridges vary widely. (A) Fast spreading
from hot molten material located below the surface of creates wider ridge profiles that reduce the volume of the ocean
the ocean crust. Heating causes the rock in these basins and displace more water onto the continents. (B) Slow
regions to expand, and expansion of the rock causes the spreading produces narrower profiles that create larger ocean
surface of the ocean crust to rise. basins that can hold more seawater.