Geoscience Reference
In-Depth Information
0
Figure 9.2.
The
bathymetry of the regions
of a passive continental
margin such as the
margins of the Atlantic
Ocean. Slopes are
approximate; the vertical
exaggeration is
100-200 : 1.
5
CONTINENTAL
SHELF
CONTINENTAL RISE
CONTINENTAL
SLOPE
OCEAN BASIN OR
ABYSSAL PLAIN
part of the plates. For this reason, mid-ocean ridges are often called spreading
centres. The examples in Chapter 2 show that present-day spreading rates of
the mid-ocean ridges vary between approximately 0.5 and 10 cm yr
−
1
. Spreading
rates are generally quoted as half the plate-separation rate. For example, the North
American and Eurasian plates are separating at approximately 2 cm yr
−
1
,sothe
Mid-Atlantic Ridge is said to be spreading at a rate of 1 cm yr
−
1
. The fastest
spreading ridge today is that portion of the East Pacific Rise between the Nazca
and Pacific plates; its rate is almost 10 cm yr
−
1
.Itisoften helpful to consider
ridges in fast-, intermediate- and slow-spreading categories (see Table 9.3 later).
The East Pacific Rise (half-rate 5-10 cm yr
−
1
)isafast-spreading ridge, the Juan
de Fuca Ridge (half-rate 2.5-3.3 cm yr
−
1
)isanintermediate-spreading ridge, the
Mid-Atlantic Ridge (half-rate 1.2-2.5 cm yr
−
1
)isaslow-spreading ridge and the
very slow-spreading ridges are the Southwest Indian Ridge and the Arctic ridges
(half-rates
1cmyr
−
1
).
Bathymetric profiles across the Mid-Atlantic Ridge, Southwest Indian Ridge
and Pacific-Antarctic Ridge are shown in Fig. 9.1. The relation between the mean
bathymetric depth and its age is discussed in detail in Section 7.5 and presented
in Table 7.5. Depth increases linearly with the square root of age over young
oceanic crust. This holds for all ridges irrespective of their spreading rate. The
East Pacific Rise is wider than the more slowly spreading Mid-Atlantic Ridge
because the width of any ridge is proportional to its spreading rate. The detailed
cross sections of the axial regions of these three ridges reveal more differences
among them. These are discussed in Section 9.4.1.
<
Ocean basins
Sedimentation occurs throughout the oceans. As a result of this and the subsi-
dence of the seabed (due to cooling and contraction of the plates), over time
the rugged, faulted topography of the mid-ocean ridges is buried under sedi-
ments. The almost flat regions of the seabed, thousands of kilometres in width
and some 5-6 km below sea level, are often called ocean basins or abyssal plains.
Isolated volcanic islands occur in most of the oceans, often in chains (e.g., the
