Earth's Structure
ICSE Class 9 · Geography · Chapter 4
What this chapter covers
- The layered interior of the earth
- The crust: meaning
- The crust: extent
- The crust: composition
- The mantle: meaning
- The mantle: extent
- The mantle: composition
- The core: meaning
- The core: extent
- The core: composition
- Comparing the crust, mantle and core
Key points
- The earth's interior has three main layers: crust, mantle and core, separated by boundaries called discontinuities.
- Temperature, pressure and density all increase with depth; the earth's average density is about 5.5 g/cm3.
- The interior is known indirectly from mines, volcanic material, earthquake waves, the earth's density and meteorites.
- The crust is the thin, solid outermost layer: about 5 to 10 km thick under oceans and 30 to 70 km under continents; about 1 % of the earth's volume.
- The crust is made mainly of oxygen, silicon and aluminium; its upper continental layer is sial (granite, density about 2.7 g/cm3) and its lower layer and the ocean floor are sima (basalt, about 3.0 g/cm3).
- The Conrad discontinuity separates sial and sima; the Mohorovicic discontinuity (Moho) separates the crust and the mantle.
- The mantle extends from the Moho to about 2,900 km, forms about 84 % of the earth's volume, and is made of silicates of magnesium and iron such as peridotite.
- The lithosphere is the rigid crust plus the uppermost mantle; the asthenosphere below it is soft, partly molten and the source of magma.
- The Repetti discontinuity (about 700 km) divides the upper and lower mantle; the Gutenberg discontinuity (about 2,900 km) separates the mantle from the core.
- The core extends from about 2,900 km to the centre at about 6,371 km, is made mainly of iron and nickel (nife), and has a density of about 10 to 13 g/cm3.
- The outer core (2,900 to 5,150 km) is liquid and produces the earth's magnetic field; the inner core (5,150 km to the centre) is solid because of immense pressure.
- The Lehmann discontinuity (about 5,150 km) separates the outer and inner core.
Questions answered
Name the three main layers of the earth from the surface to the centre.
Crust, mantle and core.
Define the crust and state its thickness under the continents and under the oceans.
The crust is the thin, solid, outermost layer of the earth on which the continents and ocean floors lie. It is about 30 to 35 km thick on average under the continents (up to about 70 km under the Himalayas) and only about 5 to 10 km thick under the oceans.
What do the terms sial and sima stand for? Give the main rock of each.
Sial stands for silicon and aluminium; its main rock is granite. Sima stands for silicon and magnesium; its main rock is basalt.
Why is sial absent from the ocean floor?
Oceanic crust is made only of the heavier sima layer of basalt. The light granitic sial forms the continents, so it is found only under the land masses and not under the deep oceans.
Name the two most abundant elements of the crust and the most abundant metal in it.
Oxygen and silicon are the two most abundant elements; aluminium is the most abundant metal.
Define the mantle and state its extent.
The mantle is the thick middle layer of the earth, between the crust and the core, made of hot, dense rock rich in magnesium and iron. It extends from the Moho (about 35 km under continents and 5 to 10 km under oceans) down to about 2,900 km.
What is the asthenosphere? Give two reasons why it is important.
The asthenosphere is the soft, partly molten zone of the upper mantle just below the lithosphere, roughly 100 to 400 km deep. It is important because the rigid plates of the lithosphere slide over it, and because it is the main source of magma for volcanoes.
Distinguish between the lithosphere and the crust.
The crust is only the outermost rock layer, ending at the Moho. The lithosphere is the rigid outer shell made of the crust together with the uppermost solid part of the mantle, about 100 km thick on average.
Past board exam questions
ICSE 2019 sample paper · 2 marks
What is the composition of the Earth’s Crust?
Show answer
The crust is the thin outermost layer of the Earth made of solid rocks. Its upper part, the continental crust, is called SIAL (mainly silica and aluminium; lighter granitic rocks). Below it and under the ocean floors lies SIMA (mainly silica and magnesium; heavier basaltic rocks). The most common elements are oxygen, silicon, aluminium and iron.