ICSE Class 6 Geography Chapter 4 Earths Structure

Unit 2 - Structure of the Earth

Chapter Four

Earth's Structure

Syllabus

Earth's Structure: Core, mantle, crust - meaning, extent and their composition.

Our knowledge about the interior of the earth comes not from excavations and explorations of the interior parts of the earth but from the study of seismic waves, materials thrown up by volcanoes and the evidence from the theories of the origin of the earth. In respect of seismic or the earthquake waves, only primary waves (P-waves) reach the surface of the earth. The secondary waves (S-waves) do not pass through liquids. The scientific instruments which record these waves have enabled scientists to know about the density, pressure and temperature of the interior of the earth.

The molten material thrown up by volcanoes rises to the surface of the earth from the magma chamber deep inside the earth. From the study of this material, scientists have come to know about the materials that constitute the interior of the earth. Keeping in mind the theories of the origin of the earth, scientists study meteorites which fall on the surface of the earth. The earth is said to be constituted of the same materials as the meteorites. The heat given out by the earth comes from radioactive elements deep inside the earth. The study of this heat in interior areas helps us to estimate the temperature of the interior of the earth.

Chemical Composition

The Lithosphere or the outer most crust of the earth is the thinnest layer.

It is made up of rocks with large content of silicates, feldspar, mica etc.

The Lithosphere can be divided into two broad divisions: the SIAL and SIMA.

Sial appears to float above Sima because it has less density.

The name Sial is taken from Silica (Si) and Aluminium (Al). Sima is named after Silica (Si) and Magnesium (Mg).

Earth's Structure

The earth is made up of several concentric layers. The outer layer is the earth's crust - the lithosphere. Parts of the earth's crust that are immersed under the oceans and seas form the hydrosphere. Extending upwards, the earth is enveloped by a blanket of gases which make up the atmosphere (Figure 4.4).

The Crust or Lithosphere

The crust is called Lithosphere because it is a sphere of solid rocks (lithos meaning stones or rocks). Crust is only 1% of earth's mass and contains all known life in the universe.

The crust is a relatively thin layer of rocks with a thickness of 60 km below high mountains and just 5-10 km below the oceans.

Considering the size of the earth, the crust is a relatively thin layer.

The temperature of the upper part of the crust has been estimated at 0°C but it gradually starts rising towards the interior.

The weight and pressure of rocks also starts increasing with depth.

Mohorovicic Discontinuity: This discontinuity between the Crust and the Mantle was discovered by a Yugoslavian scientist Andreja Mohorovicic in 1909. It is simply known as Moho Discontinuity.

It forms the boundary between the crust and the mantle.

Like the crust it does not exist at a uniform depth. It is found at about 8 km beneath the oceans and about 32 km beneath the continents.

It has the characteristics of both the upper layer (the crust) and its bottom layer (the Mantle).

The Mantle

This layer lies below the crust.

Its average thickness is 2900 km and makes up 84 per cent of the earth's total volume.

The mantle is divided into two parts: the upper mantle and the lower mantle. The upper mantle extends from the Moho Discontinuity to a depth of 700 km. It is hotter and makes up the asthenosphere. Lower mantle extends from 700 km to 2900 km.

Lower mantle is hotter and denser than the upper mantle.

At the depth of around 100-410 km, the mantle is partially molten and is known as asthenosphere. Lower mantle is solid due to high pressure.

As the pressure increases towards the interior of the earth, the density also increases.

The temperature varies between about 1100°C is around 3,700°C.

The Inner Core

It is the innermost part lying at the centre of the earth.

The core is the densest part of the earth.

It is also divided into outer and inner core.

The entire core is about 7000 km in diameter.

The temperature range from about 4400°C to about 6000°C.

It is generally believed, that the core is made of iron and nickel which is responsible for earth's magnetism. The magnetic field is oriented towards North and South Poles.

The outer state of the inner core is in high density and unique which have liquids and molten materials. The inner core is in which displaces in the inner core which supports it in a molten state.

Gutenbert Discontinuity: The boundary between the mantle and the core or the mantle core interface is known as the Gutenberg Discontinuity.

It begins at the depth of 2800 km.

Here there is an abrupt change in the seismic waves (generated by earthquakes or explosions) that travel through the earth. Primary seismic waves (P waves) decrease in velocity while secondary seismic waves (S waves) disappear completely.

It is dense probably due to a greater percentage of iron.

Teacher's Note

Understanding Earth's layers helps us appreciate how the ground beneath our feet is structured and why earthquakes and volcanoes occur, phenomena we experience in our daily lives.

Crust or Lithosphere

The crust is called Lithosphere because it is a sphere of solid rocks (lithos meaning stones or rocks). Crust is only 1% of earth's mass and contains all known life in the universe.

The crust is a relatively thin layer of rocks with a thickness of 60 km below high mountains and just 5-10 km below the oceans.

Considering the size of the earth, the crust is a relatively thin layer.

The temperature of the upper part of the crust has been estimated at 0°C but it gradually starts rising towards the interior.

The weight and pressure of rocks also starts increasing with depth.

Mohorovicic Discontinuity: This discontinuity between the Crust and the Mantle was discovered by a Yugoslavian scientist Andreja Mohorovicic in 1909. It is simply known as Moho Discontinuity.

It forms the boundary between the crust and the mantle.

Like the crust it does not exist at a uniform depth. It is found at about 8 km beneath the oceans and about 32 km beneath the continents.

It has the characteristics of both the upper layer (the crust) and its bottom layer (the Mantle). Because of the continuous changes in the earth's crust due to forces of nature, the thickness of the lithosphere is very fragile compared to the nature and composition of the earth. The Theory of Continental Drift put forward by Wegener in the early 20th century and the Theory of Plate Tectonics put forth in 1960s suggest that the earth's crust undergoes structural changes. Besides, the lateral movements give the earth a new shape though very slowly over millions of years.

Teacher's Note

The lithosphere is where we live and build our cities, so understanding its thin, fragile nature reminds us to respect geological processes that shape our environment.

Terms to Remember

Magma Chamber: Hollow spaces between rocks where magma collects deep inside the earth and remains in a fluid state.

Seismic waves: Earthquake waves that are in the form of P-waves, S-waves and L-waves.

Density: Ratio of mass to volume for a given substance.

Teacher's Note

These key terms form the vocabulary of earth science, much like knowing the names of parts helps us understand how a machine works.

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