Internal and External Structure of the Earth

Internal and External Structure of the Earth

In the very beginning of earth’s history this planet was a giant, red hot, gaseous ball. Gradually this gaseous sphere began to cool and solidified forming a thin crust on its surface. The earth consists of three main layers-the core. the mantle and the crust. The core is the inner part of the earth, the crust is the outer part and between them is the mantle. The crust of the earth is composed of a great variety of rocks. Enduring processes of the landforms of the earth are influenced by form of rocks and minerals. The surface of the earth is always changeable. This changing nature has two faces. One occurs suddenly and the other is a slow process. In this article we shall discuss about internal and external structure of the earth, types of rocks, slow and abrupt change of the earth surface and different types of landforms.

Internal and External Structure of the Earth

At the end of this article you will be able to :

• Describe the internal structure of the earth.

• Describe the external structure of the earth.

• Describe the landform change of the earth surface.

• Explain the reason of the abrupt change of the earth surface.

• Analyze the result and reason of earthquake, tsunami and volcano.

• Explain the result and reason of slow change of the earth surface.

• Explain the rivers path way.

• Discuss the landforms created by rivers.

• Describe the characteristics of the main landforms of the earth.

• Analyze the impact of tsunami occurred in the past on the basis of data.

Internal Structure of the Earth

In the very beginning earth was a hot, gaseous ball. From that condition it became cool and solidified. During that period the heavy metals concentrated at the centre and comparatively lighter materials sequentially concentrated around that heavy core according to its density and weight. These different layers called Spheres. The upper most sphere is called Lithosphere. The upper part of lithosphere is known as Earth Crust.

The earth's crust : The hard stratum of the rock covering the outer shell of the whole earth is known as the earth's crust (fig. 4.1). The thickness of the earth's crust is the lowest in comparison to the other interior layers of the earth. On an average, it is about 20 kilometres. The thickness below the earth's crust on an average is about 35 kilometres known as SIAL which is composed of Silicon (Si) and of Aluminium (AL). On the other hand, the earth's crust beneath the ocean is composed of basalt which is heavier than that of Sial layer and its major elements are Silicon (Si) and Magnesium (Mg) and is generally known as SIMA. The external structures such as Mountains, Plateau, and Plains are visible. The temperature beneath the earth's crust is increased by 30° Celsius for each kilometre towards its centre.

Cross Section of Earth's Structure

Barysphere : The stratum beneath the earth's crust which is about 2,885 kilometres thick is called the Barysphere. Barysphere mainly consists of Basalt rock. This sphere has silica, magnesium, iron, carbon, and other minerals. Barysphere is divided into two parts.

(a) Upper Barysphere which is extended up to 700 kilometres. This stratum mainly composed with silicate minerals enriched with iron and magnesium.

(b) Lower Barysphere mainly consists of minerals of iron oxide, magnesium oxide and silicon dioxide.

Centrosphere : Centrosphere is situated just after the Barysphere. It is extended from the lower parts of the Barysphere up to the centre of the earth. The thickness of this layer is about 3,486 kilometres. It is known from the seismic waves that the Centrosphere has a liquid cover which is about 2,270 kilometres thick and also has a hard inner-part which is about 1,216 kilometres thick. The scientists believe that iron, nickel, mercury and lead exist among the elements of Centrosphere. But the two major elements of Centrosphere are iron (Fe) and Nickel (Ni).

Rocks and Minerals

Earth's crust is formed with rocks. Rocks are composed of different types of minerals. Mineral is the compound form of some basic elements mixed by natural process. Minerals are natural inorganic elements which have distinct chemical composition, physical and chemical characteristics. Though generally minerals are formed by two or more elements but some minerals can be formed with only one basic element. Such as, diamond, gold, copper, silver, mercury and sulphur. Rock forming minerals have different characteristics and most of the rocks are composed with two or more minerals. In that sense minerals and rocks are the same things. For example, limestone which is a sedimentary rock also known as calcite is a mineral.

Difference between rocks and minerals : Minerals are homogenous inorganic matter and the rocks are heterogeneous materials. Minerals are solids and crystallized, though some rocks are solid but are not crystallized. Minerals have specific chemical composition but rocks have no such thing. The characteristics of minerals are determined by the nature of their constructing elements. Whereas, the nature of rocks are regulated by the nature of the minerals that compose them.

Classification of Rocks

The materials of the earth crust or lithosphere are generally called Rock. Every solid and fluid material which formed the earth surface is rock. Such as pebbles, sandstone, granite, clay, chalks etc. According to formation of rocks can be classified into three major parts: (1) Igneous rocks, (2) Sedimentary rocks, (3) Metamorphic rocks.

(1) Igneous rocks : At the beginning the Earth was a hot gaseous spheroid. This gaseous spheroid was transformed into liquid by radiating heat. heat. After radiating more heat its upper part became cool and solidified. Rocks which became solidified from liquefied state are called Igneous Rocks. As the igneous rocks form at the very beginning of earth's origin it is also called the Primary Rocks. There are no layers in this rock type. So, it is also called Unstratified Rock. There is no fossil in this rock type. The characteristics of igneous rocks are : (a) granular or crystalline rocks, (b) do not have strata, ( c) hard and less fragile, ( d) do not contain fossils, ( e) comparatively heavy.

Often volcanism and earthquake create fracture at the weak part of the earth surface. Then the hot molten lava comes out from the interior of the earth and forms igneous rocks. In this way Basalt and Granite are created. Igneous rocks can be divided into two parts : (a) Extrusive igneous rocks, (b) Intrusive igneous rocks.

(a) Extrusive igneous rocks : The hot liquid material in the interior of the earth which is called Magma finds its way to the surface of the earth through the crack of the earth's outer skin. It is called Lava. The lava after cooling becomes extrusive Igneous rocks. The granules of the igneous rocks are refined and have deep colour. The examples of extrusive igneous rocks are Basalt, Rhyolite, Andesite etc.

(b) Intrusive igneous rocks: When the rising magmas during a volcanic activity do not reach the earth's surface rather they are cooled and solidified below the surface of the earth, the resultant igneous rocks are called Intrusive Igneous Rocks. The granules of these rocks are coarse and have light colour. Granite, gabbro, dolerite, laccolith, batholith, dike and sill are the examples of intrusive igneous rocks (fig. 4.2).

Igneous Rocks

(2) Sedimentary rocks : Sedimentary rocks are formed due to aggregation and composition of sediments. Due to influence of rain, wind, glacier, heat, wave etc. igneous rocks are eroded and disintegrated and transformed into gravel, mud, sand   and dust. These eroded rock debris are transported by stream, wind and glacier and deposited in lowlands, lakes, ponds, basins, rivers and seas. So, sediments and debris are regularly deposited by layers. Continuous sedimentation increases the weight and pressure and different layers are consolidated and compacted to form sedimentary rock. Sedimentary rocks cover 5 percent area of total earth's crust. However, 75 percent of the continental land mass is sedimentary rock. This is called Sedimentary Rocks as this form from sediments or debris (fig. 4.3).

Sedimentary Rocks

Sedimentary rocks are also called as Stratified or Layered Rocks because these rocks have different layers or strata of different types of sediments. Sedimentary rock can be formed by mechanical, organic or chemical processes. Sandstone, coal, shale, limestone, claystone, kaolin are the examples of sedimentary rocks. As coal and mineral oil originated from organic body these are also called Organic Rocks. In many sedimentary rocks different types of fossils of plants and animals are found

Characteristics of sedimentary rocks : Sedimentary rocks are stratified, soft and light, easy to erode. Fossils can be seen in sedimentary rocks. This rock is porous.

(3) Metamorphic rocks : Metamorphic rocks form deep in the earth where high temperature, great pressure, and chemical reactions cause one type of rock to change into another type of rock. The rocks are partially melted and the chemicals within them are rearranged so that the final rock is very different from the original rock. One very common metamorphic rock is marble. Marble is formed when heat and pressure are applied to limestone for many thousands of years. Some other examples of metamorphic rocks are gneiss, slate, schist, and quartzite.

Characteristics of metamorphic rocks :

(a) Classified by texture and composition.

(b) Rarely have fossils.

(c) May have alternate bands of light and dark minerals.

(d) May be composed of only one mineral, example marble and quartzite.

(e) May have layers of visible crystals.

(f) Usually made of mineral crystals of different sizes.

(g) Rarely have pores or openings.

(h) May have bent or curved foliation.

Changing Process of the Earth Surface

The earth's crust is always changing. Different landforming process bring about this change. Landforming processes are the activities through which the changes are caused naturally in the formation of the land. For example, a river is building a flood plain through deposition. Therefore, the depositional activities of a river is a landforming process. The landforming processes are done with the help of different natural forces such as the gravitational forces, interior temperature, and the solar bring changes on the earth's crust either slowly or rapidly. Generally, energy. The different landforming processes with the help of the aforesaid agents external force like solar energy brings about a slow change in the formation of land. The changes brought about on earth's surface through longer period of time, is known as slow changes. The slow changes are caused in two processes, i.e; denudation and deposition.

Landforming Process Flowchart

Gradual change : This is the opposite of rapid change where changes occur due to natural forces like solar energy, wind, rain, stream, glacier etc. Generally slow process occur over a huge area.

Rapid change : The inner part of the earth is still hot and molten. When there is a difference in temperature and pressure within this portion there causes earthquakes. Due to the earthquakes most of the changes occur. Rapid change of the earth surface mainly occurs due to Earthquakes, Volcanoes, and Tsunami.


An earthquake is a shaking of the ground caused by the sudden breaking and shifting of rocks beneath the earth surface. The shaking could last seconds or minutes, and there may be several earthquakes over a period of time.

Causes of Earthquakes

The scientist have determined various reasons of earthquake by examining different types of geological events. The causes of earthquakes may be divided into three main groups (i) surface causes, (ii) volcanic causes and (iii) tectonic causes.

(i) Surface causes : Great explosions, landslides, slips on steep coasts, dashing of sea waves, avalanches, railway trains, heavy trucks, some large engineering projects cause minor tremors. Some of them are man-made, others are natural.

(ii) Volcanic causes : Volcanic eruptions produce earthquakes. Earthquakes may precede, accompany and frequently follow volcanic eruptions. They are caused by sudden violent displacements of lava within or beneath the conduit of the volcano.

(iii) Tectonic causes : Structural disturbances resulting in the relative displacements of the parts of the lithosphere is the main cause of this type of earthquake. Most of the disastrous earthquakes belong to this category and occur in areas of great faults and fractures. Sudden yielding to strain produced on the rocks of accumulating stress causes displacements especially along old fault zones known as great transform faults.

Earthquakes often occur on the ocean floor. This produces large sea waves known as tsunami that produces devastating effects on the sea coasts. Recently, the tsunami produced by the earthquake near the Sumatra coast affected distant places like Sri Lanka and South India and even African coast.

Effects of Earthquakes

Earthquake is accepted as a natural calamity. Most of the times, it causes great loss of lives and wealth. Generally, it may be mentioned that the devastative nature of earthquakes brings the following damages and changes in the earth's crust.

(i) Faults, cracks or landslides occur in the earth's crust due to the earthquakes. The course of the rivers is also changed. For example, the bed of the Brahmaputra River was raised and the river changed its original course and started flowing through the channel of the Jamuna due to earthquake of Assam in 1787.

(ii) Earthquakes can cause severe and widespread damage to weak buildings or structures, or to those located on ground subject to fault breakage, strong shaking, or landsliding. The slip (movement) on the fault may break the surface of the Earth, offsetting roads and tearing apart buildings or pipelines built across the fault. Such damage can be spectacular, but it is limited to the vicinity of the fault.

(iii) Most damage results from strong shaking during the passage of seismic waves, which spread out from the fault over a large region. Shaking may be severe enough and long enough to collapse weak buildings, overturn furniture, topple water heaters and storage tanks, and collapse unsafe dams. These effects can result in further damage through fires resulting from broken gas pipeline and fallen electric wires, the loss of water to fight fires because of broken water lines, oil spills caused by failure of storage tanks, and flooding resulting from dam failure. Shaking can also cause landslides. These in turn can damage buildings, roads, and pipelines built on slide areas or downhill from them.


Tsunami is a Japanese term that means 'harbor wave.' Tsunamis are giant waves caused by earthquakes or volcanic eruptions under the sea. Tsunamis have very long wave lengths. Crest to crest they measure between 10 and 500 km and they travel through the ocean at more than 700 km/h. Sometimes there appears to be just one wave but often there are multiple waves travelling a few minutes apart. Most tsunamis are caused by submarine earthquakes but not all submarine earthquakes cause tsunamis. Movement on the fault must have a vertical component that generates sufficient displacement to set a tsunami running. Earthquakes, landslides, volcanic eruptions, explosions, and even the impact of cosmic bodies, such as meteorites, can generate tsunamis. Tsunamis can savagely attack coastlines, causing devastating property damage and loss of life. An under sea earthquake in the Indian Ocean on 26th December 2004 produced a tsunami that caused one of the biggest natural disasters in modem history. Over 200,000 people are known to have lost their lives.


A volcano is an opening in the crust of the Earth through which molten rock known as magma and gases from the interior of the Earth reach the surface. The definition of volcano can include the mountain like structures that may be created from the material ejected at the volcano. Magma erupting from a volcano is called Lava and is the material which builds up the cone surrounding the vent (fig. 4.4). 


This opening allows hot ash, molten rock and gases to blow off from the underground spaces. Volcanoes generally acquire shape of a mountain. Volcanoes are usually found at places where tectonic plates get converged or diverged. But their existence at a particular place depends upon many environmental and geological factors.

Reasons of Volcanic Eruption

Volcanic activity frequently occurs at the boundaries of the Earth's tectonic plates which are a series of large blocks moving between each other. The movement of these plates plays a significant role in the type of volcano formed, which influences its shape.

Types of Volcano

On the basis of eruption there are three types of volcanoes :

1. Active volcano : The volcanoes which are erupting or have erupted recently are known as active volcano. Mauna Loa and Mauna Keya of Hawaii are the best examples.

2. Dormant volcano : These volcanoes show no signs of activity. They have not erupted for a long time. A dormant volcano can be active again. Fujiama of Japan is a dormant volcano.

3. Extinct volcano : These volcanoes have remained dormant for many years and all volcanic activities have stopped. Kohisultan of Iran is an extinct volcano.

Volcanoes Classified on the Basis of Shape and Size

1. Shield volcano : Shield volcanoes are dome shaped mountains with broad bases and gentle slopes, commonly great in size. Shield volcanoes, formed when large flows of lava spread rapidly from central vents or rows of vents. Mauna Loa of Hawaii island is a shield volcano.

2. Strato volcano : It is formed as layers or strata of ash and lava from the erupting volcano. Most strato volcanoes are irregularly shaped mountains formed by material ejected from both central and other vents on the sides of the mountain.

3. Cinder cone volcano : Small volcanoes are called Cinder cone volcanoes. They are formed as a result of repeated small explosions of gas rich magma, which ejects small chunks of lava and ash onto small area around vent. The average size of cinder cone is about 800 metres wide base, and 100 metres high. Paracuting of Mexico is an example of cinder cone volcano.

Effects of Volcanic Eruption : Due to volcanic eruptions many changes occur on the earth's crust. On the other hand, volcanos have some good impacts.Various dimensions of volcanoes have been discussed below :

(1) Sometimes the molten materials are ejected from the volcano accumulated in the surrounding places and form a plateau. Black soil plateau of South India is formed from volcanic lava.

(2) There are many volcanoes in the ocean floor. Lava ejected from these volcanoes produces island. These are known as high island. Hawaii archipelago in the Pacific ocean is a volcanic island.

(3) Volcanic eruption can create a deep trench in the surface of the earth. In 1883 a deep trench developed due to volcanic eruption between Java and Sumatra.

(4) A lake is formed in the crater of an extinct volcano. Mount Adakama of Alaska, Kosegaina of Nicaragua are this type of lakes.

(5) Lava flowing for a long time from the volcanic eruption forms mountain which is known as the volcanic mountain. Vesuvius of Italy is a volcanic mountain.

(6) A volcanic plateau is produced by volcanic activity. Lava plateau is produced by numerous successive eruptions through numerous vents.

Volcanic eruption destroys villages, cities, agricultural land, and everything. In 1879 AD eruption from Vesuvius mountain destroyed two cities of Italy Pompeii and Herculeum. Volcanoes not only destroy the cities and agricultural lands but also increase the fertility of the soil. Layer of lava produces fertile black soil which is good for producing cotton. Many mineral substances flow with lava which is useful for the industrial activity.

Cause and Effect of Slow Changes of the Earth's Surface

Sudden changes occur on the earth's surface and produce three types of landform. These are : mountains, plateaus and plains.These landforms gradually change through natural processes like solar heat, temperature, wind and rain and transform into a different landform. This process of change is known as slow change. In this process, solar heat, wind and rain erodes the surface layer of the earth very slowly. As a result, rocks of the upper layer of the surface break down. Rocks are eroded, transported and again this process takes place and new layer of rock is broken down. In this way, natural processes erode the surface. There are four processes through which the earth surface is changing. These are :

a. Weathering and Erosion

b. Transportation

c. Denudation

d. Deposition

a. Weathering and Erosion : Weathering is the process of decomposing the rocks. Rock particles are loosened, chemically or mechanically and stays in place is called Weathering. When the rock particle is moved by some flowing agent such as air, water and ice it is called Erosion.

b. Transportation : Transportation is the movement of solid particles, typically due to a combination of the force of gravity acting on the sediment and the movement of the rivers, wind and glacier.

c. Denudation: Denudation is the long term process of wearing of the earth's surface leading to a reduction in elevation and relief of landforms and landscapes. Denudation can involve the removal of both solid particles and dissolved materials.

d. Deposition : Deposition is the geological process by which material is added to the landform or land mass. Eroded particles from wind, river and glacier are deposited to a new place.

The natural processes through which gradual change in the landform occurs are wind, rainfall, river and glacier. Erosional work of these agents are discussed below :

Wind : Air contains oxygen, carbon dioxide and water vapour. The chemical process of these three elements break down the rocks. Erosional work of the wind is more prominent in the desert. Desert areas are dry, rainless and are without vegetation coverage. Due to less vegetation coverage in the desert area the soil is not strongly compacted. The solar heat in the day time and radiation at night make the rocks expand and contract. As a result, the rocks prone to become erosion. Wind blows the loose rock particles from one region to another and slowly change the landform.

Rainfall : Rain water flowing on the surface of the land erodes it gradually in a large scale. Flowing rain water erodes the rocks partially and loosened eroded rocks are expanded. In high rainfall area the soils of cultivated lands are easily transported. In mountainous areas hard rock lies on the clay layer in a slanting position. When rainfall enters the cracks of the clayey layer of the mountain and erodes the hard rocks they become unstable and eventually fall down as landslide. In this way landforms change with the passage of time.

Glacier : Glacier is also an agent of change in cold region. The rocks underneath the passage of the glacier are eroded while it moves. When glacier moves through the valley in the mountains the rocks are broken into smaller pieces and moved away farther along its path. If there are cracks in the mountain sides water enters in those cracks, freeze and loosen the rocks and the pressure of glacier erodes the rocks from the sides of the mountain. This is the gradual process of change of the landform by glacier.

River : The natural process that is changing the earth's surface regularly is the river. When the river flows through the mountainous region the velocity of the current erodes the rocks into smaller pieces in the river bed and the sides of the mountain walls. In the mountainous stage of the river the work of the river is only erosion. When the river enters the plains it works both as an erosion and deposition agent. In the river course where the river flows over soft rocks it makes its way on it by erosion. Eroded materials are deposited in the course of the river and finally it reaches the sea.

Definition of river : In the high mountain, plateau or highland where rainfall, spring, glacier or melted waters from the glacier flow downwards due to gravitational force in a fixed course is called a River. The river flows in a course through plains and reaches at ocean or lake. The origin of the river is called the Source of river. The place where the river reaches the ocean or lake called the Mouth of the river. The extended area along the Mouth of the river is called the Estuary.

River course : River is important in the life of the people. All ancient civilizations flourished beside the river. At that time river was the only medium through which people could move around.

Doab : Land between two active rivers is known as doab.

Confluence: When two or more rivers meet together at a place is known as confluence.

Tributary : A stream or river that flows into a main stream or a lake is called a Tributary. A tributary does not flow directly into a sea or ocean.

Distributary : A stream that branches off and flows away from a main stream channel. They are common in the delta areas in the lower course of the river.

River valley : A valley formed by flowing river is called by River valley.

River bed : The river bottom through which river flows is called the River bed.

River basin : The vast regions having branches of rivers through which water flows from its origin and fall into ocean and lake are called River basin.

Learn more about - 

👉👉 Life Cycle of a River

👉👉 External Structure of the Earth

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