Geology Reference
In-Depth Information
Figure 2.5. Seafloor depth versus square root of age for a selection of regions of
the sea floor. The same reference line is shown in each plot. After Marty and
Cazenave [10]. Copyright by Elsevier Science. Reprinted with permission.
There is a well-entrenched story that sea floor older than about 80 Ma approaches
a uniform depth. This story is a myth. It originated when the data coverage of the
sea floor was sparse, and it survives because people keep aggregating the data into a
single global distribution, thus missing important geographical information that is
evident in Figure 2.5. Further evidence for significant regional variation comes from
variations in the depth of the crests of mid-ocean ridges. These variations are not
clearly evident from Figure 2.4, although you can see that the ridge crest south of
Australia is unusually deep, and the crest of the East Pacific Rise seems to be unusu-
ally shallow. Profiles 25 and 26 in Figure 2.5 also exhibit low depths of ridge crests.
A more accurate description of the observations is that there are regional deviations
from the main depth-age distribution that have an amplitude of about 1 km.
It is worth emphasising how remarkable the main depth-age correlation of the sea
floor appears. Geologists working on the continental crust never see such regularity,
because the continental crust has been so thoroughly and pervasively worked over
by tectonic processes. Even for fluid dynamicists used to fluid convection, this is
a remarkably regular surface, not usually seen in convection systems. We will see
later that important insights can be gained from this simplicity.
