Horvath et al.
e20200160-13
more closely at the composition and structure of the
crust, mantle and core.
The crust
: It is the outermost solid layer, and the only
one accessible by direct studies. It’s mainly made up of
basalts and granites, corresponding to the regions of the
oceanic and continental crust respectively. The overall
composition is roughly 2/3 of basalt (igneous rocks, or
solidified lava), 1/3 metamorphic rocks (i.e. rocks that
have been subjected to changes in pressure and tempera-
ture, thus changing their structure) and 1/3 sedimentary
rocks (produced by weathering and subsequent processes).
Over 90% of all rocks in the crust contain
silicates
. When
subjected to tensions, the crust deforms, and can fracture
beyond a certain critical threshold, thus giving origin
of the “shallow” earthquakes that originate there (see
below).
The mantle
: the mantle begins immediately below the
crust, and can be divided into upper and lower mantle.
The upper mantle is more rigid and forms, together
with the crust, the lithosphere. The main composition
throughout the mantle is of iron and magnesium silicates,
in particular the compounds olivine and pyroxene formed
under high pressure. The main differentiation in the
mantle comes from changes in the stability of certain
silicates. There is a transition zone (from 410-660 km),
and the lower mantle extends from there down to about
2900 km. The region between 2700 and 2900 km has
anomalous seismic wave propagation and is known by
geophysicists as the
D
00
layer.
The core
: It consists of two different regions separated
by a transition from liquid to solid phase. The liquid core
(2900 to 5150 km) is composed of nickel-iron. Convective
movements of the electrically charged material take place
there. The solid core, of little over 1200 km radius, where
iron and nickel have solidified has a crystalline structure
(the cherry core) and movements are not possible there.
The core temperature is high, certainly higher than 7000
K
, and it remains so due to residual heat from Earth’s
formation, the decay of radioactive elements that were in
the primordial nebula (thorium, nickel, titanium, etc.),
and the latent heat released by the material that solidifies
from the inside out, gradually increasing the solid core’s
size.
There are some controversies regarding the structure
and composition of the Earth, for example, the nature of
the most central region and the energy balance and core
temperatures. Of course, some changes from new research
are always possible. In addition to well-known tools such
as the study of seismic waves that bring structure infor-
mation when they cross the Earth, other methods are
developed to complement and assist in the study of our
planet. The structure and composition that have been
described above are shown to scale in Figure 13.
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