Context - Mantle Convection for Geologists - page 9

Table 2.1. Defining characteristics of crust and lithosphere.

Characteristic Continental Oceanic

Crust
Defining property low seismic velocity low seismic velocity

Reason composition composition

Average composition andesitic basaltic

Average thickness 35 to 40 km 7 km

2700 kg/m 3 2900 kg/m 3

Average density

Lithosphere
Defining property

strength strength

Reason low temperature low temperature

Thickness 100 to > 200 km 0 to 100 km

a striking feature, not observed on any other body in the solar system. Hess [6]
argued that it is due to the combined action of seafloor spreading, which sweeps
continental material together, and subaerial erosion, which planes the continental
surfaces down to sea level.
The next most prominent feature of the Earth's topography is the system of mid-
ocean ridges, which form a continuous network within the ocean basins. These
stand 2-3 km above the deeper parts of the ocean basins, which are 5-6 km below
sea level. They will give us important information about mantle convection. Not
very visible in this image are the deep ocean trenches, bordering the Pacific basin,
Indonesia and a few other places. These extend to depths of around 10 km below
sea level. Various plateaus and mountain belts are visible on the continents, and
various plateaus and chains of seamounts are visible in the ocean basins. A broad
swell in the sea floor is visible around Hawaii and the chain of seamounts extending
northwest from Hawaii, in the mid-North Pacific. This, and a few features like it,
will also tell us something important about mantle convection.
There is, in the seafloor topography, a surprising regularity that is not obvious
just from a map like Figure 2.4. It is that seafloor depth correlates strongly with
the age of the sea floor. More specifically, it correlates with the square root of the
seafloor age. This is illustrated in Figure 2.5. Obviously there are deviations from
the correlation, but overall the seafloor age is the main predictor of seafloor depth.
There is also some regularity in the deviations from the main correlation in
Figure 2.5. The deviations are mainly positive, and they occur mainly on older
sea floor. In other words, there is a tendency for older sea floor to be shallower
than the correlation predicts. This tendency is not universal, however. For example,
profiles 1 and 2 extend, with only minor deviations, to ages of 175 Ma and 100 Ma,
respectively.