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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. 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. From a depth of
660 km 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
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density of nearly 11 g/cm and approaches 14 times the density of water at Earth’s
center. 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
having 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 center of the planet.

Task 2. Pay special attention to the pronunciation of the following word-
combinations, transcribe them: chemical composition, peridotite, magnesium,
lithosphere, asthenosphere, oxygen, silicon.

Test yourself

Task: Tell whether the sentences are true or false. Correct the false ones.
1. Geologists divide rocks into three major groups: igneous, sedimentary, and
metamorphic.
2. Crystallization may occur only beneath the surface of Earth.
3. Weathering is the process when the day-in and day-out influences of the
atmosphere slowly disintegrate and decompose rocks.
4. Lithification is a term meaning “conversion from rock.”
5. When metamorphic rock is subjected to additional pressure changes or to still
higher temperatures, it will melt, creating magma.
6. The upper crust has an average composition of a granitic rock called granodiorite.
7. The dominant rock type in the uppermost mantle is granodiorite.
8. The upper mantle can be divided into three different parts.
9. The composition of the core is thought to be an iron alloy with minor amounts of
oxygen, silicon, and sulphur.
10. The core is divided into two regions that exhibit very different mechanical
strengths.

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