Large-scale earth structure - Introducing Tectonics, Rock Structures and Mountain Belts - page 6

largely of solid material with a com-
position similar to the igneous rock
peridotite . It is here that the processes
that mostly control what happens at
the Earth's surface originate, as we
shall see in the following chapter. The
innermost, approximately spherical,
region is called the core , and extends
to the centre of the Earth, at 6500 km

depth. The core is believed to consist
mostly of metallic iron, with some
lighter elements in addition, such as
nickel. In the outer core the metal is
in a molten state but the inner core is
solid. These constituents are thought to
have become molten at an early stage
in Earth's history and drained down
towards the centre, forming the core.

The nature of the crust
The crust varies in thickness from about
7 km beneath the oceans, to an average
of about 33 km in continental areas, and
reaches nearly 80 km in depth beneath
certain young mountain belts ( see
Figure 2.4). The composition of the crust
is known in some detail, since material
from the various depths of the crust is

X 10

Y Z

sea level

0

water

sediments 2.4

oceanic crust

Mohorovicic
discontinuity

3.0

10 M

2.8

continental
crust

20

2.8

km

mantle

>3.3

30

M

40

mantle

>3.3

M

~ level of equal weight

50

>3.3 mantle

Figure 2.4 Structure of the Earth's
crust. A. Average cross-sections
through thickened continental crust
(X); mean continental crust (Y); and
oceanic crust (Z); note that the extra
height of the continental sections
is compensated by their reduced
density so that the total weight of
each is equal at 50 km depth. B.
Schematic cross-section showing
how the crust varies in thickness;
differences in mantle thickness within
the lithosphere are compensated
by lateral flow within the weaker
asthenosphere.

A

20

sea level

0

oceanic crust

continental
crust

-20

Mohorovicic
discontinuity

km

-40

litho-
sphere

mantle

-60

-80

-100

zone of solid flow

asthenosphere

B