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Continental rocks have an average density of about 2.7 grams per cm (cubic centimetre),
and some are 4 billion years old. The rocks of the oceanic crust are younger (180 million years or
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less) and denser (about 3.0 grams per cm ) than continental rocks.
EARTH’S MANTLE. More than 82 % of Earth’s volume is contained in the mantle, a
solid, rocky shell that extends to a depth of about 2900 km. The boundary between the crust and
mantle represents a marked change in chemical composition. The dominant rock type in the
uppermost mantle is peridotite, which is richer in the metals magnesium and iron than the
minerals found in either the continental or oceanic crust.
The upper mantle extends from the crust–mantle boundary down to a depth of about 660
km. The upper mantle can be divided into two different parts. The top portion of the upper
mantle is part of the stiff lithosphere, and beneath that is the weaker asthenosphere. The
lithosphere (sphere of rock) consists of the entire crust and uppermost mantle and forms Earth’s
relatively cool, rigid outer shell. Averaging about 100 km in thickness, the lithosphere is more
than 250 km thick below the oldest portions of the continents. Beneath this stiff layer to a depth
of about 350 km lies a soft, comparatively weak layer known as the asthenosphere (weak
sphere). The top portion of the asthenosphere has a temperature/pressure regime that results in a
small amount of melting. Within this very weak zone, the lithosphere is mechanically detached
from the layer below. The result is that the lithosphere is able to move independently of the
asthenosphere.
It is important to emphasize that the strength of various Earth materials is a function of
both their composition and the temperature and pressure of their environment. The entire
lithosphere does not behave like a brittle solid similar to rocks found on the surface. Rather, the
rocks of the lithosphere get progressively hotter and weaker (more easily deformed) with
increasing depth. At the depth of the uppermost asthenosphere, the rocks are close enough to
their melting temperature that they are very easily deformed, and some melting may actually
occur. Thus, the uppermost asthenosphere is weak because it is near its melting point, just as hot
wax is weaker than cold wax. From 660 km deep to the top of the core, at a depth of 2900 km, is
the lower mantle. Because of an increase in pressure (caused by the weight of the rock above),
the mantle gradually strengthens with depth. Despite their strength however, the rocks within the
lower mantle are very hot and capable of very gradual flow.
EARTH’S CORE. The composition of the core is thought to be an iron–nickel alloy
with minor amounts of oxygen, silicon, and sulfur—elements that readily form compounds with
iron. At the extreme pressure found in the core, this iron-rich material has an average density of
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nearly 11 grams per cm and approaches 14 times the density of water at Earth’s centre. The core
is divided into two regions that exhibit very different mechanical strengths. The outer core is a
liquid layer 2270 km thick. It is the movement of metallic iron within this zone that generates
Earth’s magnetic field. The inner core is a sphere with a radius of 1216 km. Despite its higher
temperature, the iron in the inner core is solid due to the immense pressures that exist in the
centre of the planet.

Task 2. Build up a glossary to the most important terms used in the text.

Task 3. Answer the questions and do the tasks:
1. Сontrast the physical make-up of the asthenosphere and the lithosphere.
1. How do continental crust and oceanic crust differ?

Task 4. Look at Figure 11.1. Name the color that represents on the picture:
1 – inner core
2 – outer core
3 – lower mantle
4 – upper mantle
5 - crust