Our Changing Earth — Class 7 Social Science

The earth's crust consists of several large and some small, rigid, irregularly-shaped plates which carry continents and the ocean floor. These are called Lithospheric Plates.

Key Facts about Lithospheric Plates:

• These plates move around very slowly - just a few millimetres each year
• The movement is caused by the flow of molten magma inside the earth
• The plates carry both continents and ocean floors
• When plates move towards each other, mountains are formed
• When plates move apart, ocean trenches and rift valleys are created

Types of Earth Movements:

The earth movements are divided on the basis of the forces which cause them:

• Endogenic forces: The forces which act in the interior of the earth (Endo = inside, genic = origin)
• Exogenic forces: The forces that work on the surface of the earth (Exo = outside, genic = origin)

Endogenic forces act in the interior of the earth. They sometimes produce sudden movements and at other times produce slow movements.

Sudden movements like earthquakes and volcanoes cause mass destruction over the surface of the earth.

Volcano:

A volcano is a vent (opening) in the earth's crust through which molten material erupts suddenly.

• The molten rock inside the earth is called magma
• When magma comes out on the surface, it is called lava
• Volcanic eruptions can be violent and destructive
• Volcanoes build up new landforms and can also destroy existing ones

When the lithospheric plates move, the surface of the earth vibrates. These vibrations can travel all round the earth. These vibrations are called earthquakes.

Key Terms:

• Focus: The place in the crust where the movement starts (the origin point underground)
• Epicentre: The place on the surface above the focus (the point on the ground directly above the focus)

Earthquake Prediction:

• Earthquakes cannot be predicted with certainty
• However, the impact can be minimised if we are prepared beforehand
• Some common local prediction methods include studying animal behaviour
• Fish in ponds get agitated before an earthquake
• Snakes come to the surface before an earthquake

Earthquake Preparedness - Where to take shelter:

Exogenic forces work on the surface of the earth (Exo = outside, genic = origin). The landscape is continuously worn away by two main processes:

1. Weathering:

• The breaking up of the rocks on the earth's surface
• It happens due to changes in temperature, water, and biological activity
• Weathering breaks rocks into smaller pieces without moving them

2. Erosion:

• The wearing away of the landscape by different agents
• Main agents of erosion: water, wind, and ice
• Erosion involves both breaking and transporting of rock material

Difference between Weathering and Erosion:

The running water in a river erodes the landscape and creates various landforms at different stages of its journey.

1. Waterfall:

• When the river tumbles at a steep angle over very hard rocks or down a steep valley side, it forms a waterfall
• Example: Niagara Falls, Angel Falls

2. Meanders:

• As the river enters the plain, it twists and turns forming large bends known as meanders
• Meanders develop because the river erodes its outer bank and deposits sediment on the inner bank

3. Oxbow Lake:

• Due to continuous erosion and deposition along the sides of the meander, an oxbow lake is created over time
• The meander loop gets cut off from the main river and forms a crescent-shaped lake

4. Floodplain and Levees:

• When the river overflows its banks, it leads to flooding of neighbouring areas
• The flooding deposits layers of fine soil and other material called sediments along its banks
• This leads to formation of a flat, fertile floodplain
• The raised banks along the river are called levees

5. Distributaries and Delta:

• As the river approaches the sea, the speed decreases and the river begins to break up
• The breaking of the river into a number of streams is called distributaries
• The river becomes so slow that it begins to deposit its load
• Each distributary forms its own mouth
• The collection of sediments from all the mouths forms a delta

The erosion and deposition of the sea waves gives rise to coastal landforms.

Erosional Landforms:

1. Sea Caves:

• Sea waves continuously strike at the rocks along the coast
• Cracks develop and become larger and wider over time
• Hollow-like caves are formed on the rocks - these are called sea caves

2. Sea Arches:

• Over time, sea caves become bigger and bigger
• Only the roof of the caves remains
• This results in formation of sea arches (arch-shaped rock structures)

3. Stacks:

• Further erosion breaks the roof of the sea arch
• Only the walls are left standing
• These wall-like features are called stacks

4. Sea Cliff:

• The steep rocky coast rising almost vertically above sea water is called a sea cliff

Depositional Landform:

5. Beaches:

• The sea waves deposit sediments along the shores forming beaches

A glacier is a large mass of ice continuously moving over land surface. Glaciers also erode the landscape by bulldozing soil and stones to expose the solid rock below.

Erosional Work of Glaciers:

• Glaciers carve out deep hollows in the mountains
• As the ice melts, these hollows get filled with water
• They become beautiful lakes in the mountains

Depositional Work of Glaciers:

• The material carried by the glacier includes rocks (big and small), sand, and silt
• When this material gets deposited, it forms glacial moraines
• Moraines are deposits of rock debris carried and left behind by glaciers

Wind is an active agent of erosion and deposition in the deserts.

Erosional Work of Wind:

1. Mushroom Rocks:

• In deserts, rocks are present in the shape of a mushroom - commonly called mushroom rocks
• Wind erodes the lower section of the rock more than the upper part
• Therefore, such rocks have a narrower base and wider top
• This is because wind-blown sand particles hit the lower part of the rock more

Depositional Work of Wind:

2. Sand Dunes:

• When the wind blows, it lifts and transports sand from one place to another
• When it stops blowing, the sand falls and gets deposited in low hill-like structures
• These are called sand dunes

3. Loess:

• When the grains of sand are very fine and light, the wind can carry them over very long distances
• When such fine sand is deposited in large areas, it is called loess
• Large deposits of loess are found in China