Page 113 - Earth's Climate Past and Future

CHAPTER 5 • Greenhouse Climate 89

Through time, ridge profiles may vary on a globally
1 averaged basis. At times like the present, when globally
Depth below sea level (km) 3 averaged rates of seafloor spreading are relatively slow,
2
mean ridge profiles are relatively thin, and little water is
displaced onto the continents. At times when average
4
have been relatively wide and more water would have
5 spreading rates were faster, mean ridge profiles would
60 40 20 0 20 40 60 been pushed up onto the continents. To use the ridge
Age of ocean crust (Myr)
depth equation to calculate spreading rates at any time
in the past requires resetting the ages of the ridge crests
FIGURE 5-10 Subsidence of ocean ridges with time All
ocean ridges show the same average profile of age (time to zero for the time being examined and recalculating
since formation) versus depth. Heat elevates the ridge crests the past ages of the ridge flanks as deviations from this
to a depth of 2500 m below the ocean surface, a level high adjusted “zero” age.
above the rest of the seafloor. As the crust spreads away Mean spreading rates from 100 to 80 Myr ago are
from the ridge crest and ages, it cools and contracts, rapidly generally thought to have been higher than they are
at first and then more slowly. The crust eventually reaches a today, but the amount is highly uncertain. One reason
stable depth of more than 5000 m below sea level. for this uncertainty is that the rate of spreading 80 Myr
(Adapted from J. G. Sclater et al., “The Depth of the Ocean ago is not known for the former (now destroyed) Tethys
Through the Neogene,” Geological Society of America Memoir seaway in the tropics. In addition, estimated spreading
163 [1985]: 1–19.) rates for the few preserved areas of ocean crust that
formed 80 to 125 Myr ago have recently been revised
downward. Until recently, faster spreading rates were
deepens with age away from the ridge crest as the heated thought to have increased global sea level by well over
rock cools and contracts (Figure 5-10). The seafloor 200 m between 80 and 100 Myr ago. Newer estimates
ages that were used to derive this relationship were tend to be only half as large, and some question now
obtained from the paleomagnetic age data examined in exists about whether spreading rates have changed at all
Chapter 4. in the last 175 Myr.
Ridge crests initially stand high above the rest of the 2. Collision of continents Most plate tectonic move-
seafloor because of anomalously strong heating associ- ments do not change the net area of either the oceans or
ated with formation of new crust from molten magma. the continents: creation of new ocean crust at ocean ridge
Ocean ridge elevations initially subside rapidly while crests is balanced by destruction of ocean crust subduct-
moving away from the crest because of rapid heat loss, ing into trenches, leaving the area of the ocean basins
but later subsidence is more gradual as the rate of heat constant. However, collision of continents does alter the
loss slows on the lower ridge flanks. By 60 Myr after area of the ocean basins and also affects sea level.
they form, the crust and upper mantle have lost most of Because continental crust is low in density, two col-
their excess heat, and the ridge elevations have reached liding continents tend to float near Earth’s surface
a nearly stable depth of 5500 m (see Figure 5-10). Local rather than be pushed or pulled deep down into Earth’s
variations in depth of a few hundred meters occur at mantle (Figure 5-11). In the region where they collide,
ridge crests and down the ridge flanks as a result of continental crust thickens from its normal value of
small-scale tectonic irregularities, but the mean values 30 km to about twice that amount. This process builds a
of ocean ridge depths follow the equation remarkably high plateau that rises well above sea level and at the
well throughout the world’s oceans. same time thickens the subsurface low-density “root” of
Paleomagnetic evidence from today’s ocean the plateau down to 60 or 70 km below Earth’s surface.
shows that different ridges spread at different rates (see In the upper 15 km of Earth’s crust, the thickening that
Figure 4-17). Because all ridge depths are constant with creates the plateau occurs by movements along faults
age (as shown by the preceding equation), crust of a that cause thin slivers of crust to shear off and stack up
given age (and a particular depth below sea level) will on top of each other. Below a depth of 15 km, thicken-
have been carried much farther from the ridge crest in a ing occurs when slow flow causes rock layers to be
given amount of time in fast-spreading areas like the squeezed and folded.
South Pacific than in the slow-spreading ones like the Because collision drives two continents together to
North Atlantic. Fast spreading gives the Pacific ridge a form a plateau with a double-thick crust, this thickening
“fatter” elevation profile than that of the Atlantic (see must result in a net loss in the area of continental crust.
Figure 5-9), and the wider Pacific ridge profile displaces To a first approximation, the area of plateau across
more water for each kilometer of its length than does which the crust doubles in thickness should equal the
the narrow Atlantic ridge. net loss of area of continental crust. This decrease in