Geology Reference
In-Depth Information
directly accessible at the surface some-
where because of tectonic movements.
There is a great difference between
oceanic and continental crust;
oceanic
crust
is composed almost entirely of
the volcanic rock basalt, whereas con-
tinental rocks are extremely varied in
composition, including the whole range
of igneous rocks together with all the
different types of rock derived from
them. However, the average composi-
tion has been estimated to be similar
to a mixture of granite and basalt.
The present-day coastlines are arbi-
trary, in the sense that sea level fluctu-
ates through time in response to various
geological processes. The formation or
removal of ice sheets and the uplift or
depression of land masses cause either
the retreat or advance of the shorelines
across the continental margins. The
true margin of the continents may be
taken as a line about halfway down the
continental slope, where the nature
of the underlying crust changes from
continental type to oceanic type (Figure
2.5). Measured in this way, the 'conti-
nents' would occupy around 40% of the
Earth's surface area rather than less than
the 30% that currently consists of land.
Looking in more detail at the Earth's
surface, it becomes apparent that a large
proportion of it consists of either plains
or plateaux on land, or the deep-ocean
floor; these show little variation in relief
until interrupted by the extreme eleva-
tions and depressions of the mountain
ranges and deep sea trenches, which
occupy only about 3% of the surface
area. Between these two dominant
levels is a region of intermediate depths,
amounting to perhaps 15% of the total
area, representing the continental
slopes and the ocean ridges. The reason
10
highest mountain
2
continent
ocean
5
average
height
6
7
ocean ridge and
continental slope
sea level
0
km
average depth
-5
continental crust
oceanic crust
-10
deepest ocean
20
40
60
80
100
% surface area
Figure 2.5
The hypsographic curve. This graphical representation shows the variation in surface
height (and depth) in terms of the proportion of surface area occupied; there are two large parts of
the graph corresponding respectively to continental plains and deep ocean, with a transitional part
representing the continental slopes and ocean ridges; areas of extreme relief (mountain ranges and
ocean trenches) occupy a very small part of the total.
for this distribution, as we shall see, is
that the zones of high relief result from
localised tectonic instabilities, whereas
the areas of low relief are more stable.
The differences between continental
and oceanic areas are due to differences
in the composition, and consequently
the density, of the underlying crust, as
shown in Figure 2.4.
Continental crust
,
with a composition corresponding to
a mixture of granite and basalt, has a
mean density of around 2.8, whereas
oceanic crust
, composed largely of
basalt, has a mean density of around
2.9. Moreover, continental crust has
a mean thickness of about 33 km and
oceanic crust is much thinner, averaging
only about 7 km. This explains their dif-
ference in mean height (or depth) with
respect to sea level. Because the Earth is
in a state of approximate gravitational
balance (termed
isostasy
), the weight of
any particular sector is similar to that of
any other. Consequently, the less dense
continents must attain a higher level
than the denser oceans if they are to
have the same gravitational effect (i.e.
have the same weight). In other words,
we can imagine the continents as being
more 'buoyant' than the oceans, as if
they were icebergs floating on the sea.
Although the mantle underlying the
crust is made up of solid rock (approxi-
mating to
peridotite
in composition),
it is able to flow in the solid state at a
very slow rate of centimetres per year,
enabling it to gradually adjust to the
pressure of gravitational differences
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