Chapter Notes - Our Changing Earth - Notes | Study Social Studies (SST) Class 7 - Class 7

Overview

• The lithosphere is broken down into a number of plates known as the Lithosphere plates.
• The movement of these plates causes changes on the surface of the earth. The forces that act in the interior of the earth are called endogenic forces, while the forces that work on the surface of the earth are called exogenic forces.
• Sudden movements like earthquakes and volcanoes cause most destruction over the surface of the earth.
• A volcano is a vent (opening)in the earth’s crust through which molten material erupts suddenly.
• The vibration in the plates of the earth is called an earthquake.
• The place in the crust where the movement starts is called the focus.
• The place on the surface above the focus is called the epicentre.
• Although earthquakes cannot be predicted, the impact can certainly be minimized.  

Major Landforms  

• The landscapes are continuously worn away by two forces, weathering and erosion.  
• Weathering is the breaking down of the rocks on the earth's surface.  
• Erosion is the wearing away of the landscape by different agents like water, wind and ice.  

Work of a River

Work of a River

• When the river tumbles at a steep angle over hard rocks or down a steep valley side, it forms a waterfall.  
• As the river enters the plain, it twists and turns, forming large bends known as meanders.  
• At this point of time, the meander loop cuts off from the river and forms a cut-off called an ox-bow lake.  
• During flooding, the river deposits layer of fine soil and other materials called sediments along its banks. This leads to the formation of a flat fertile plain called floodplain.  
• The raised banks along the river are called levees. 
• The collection of sediments from all the mouths forms a delta.  

Work of Sea Waves

Work of Sea Waves

• The erosion and deposition of the sea waves give rise to coastal landforms.  
• Due to sea waves, hollow-like caves and formed on the rocks. They are called sea caves.  
• As cavities become bigger in size, only the roof of the caves remains, thus forming sea arches.  
• The erosion further breaks the roof and only walls are left. It is called stacks.  
• The steep rocky coast rising almost vertically above seawater is called a sea cliff.  

Work of Ice  

Work of Ice

• Glaciers are rivers of ice that erode the landscape by building soil and stones to expose the solid rocks below.  
• The material carried by the glaciers, such as big and small rocks, sand and silt gets deposited. These deposits form glacial moraines.  

Work of Wind

Work of Wind

• An active agent of erosion and deposition in the deserts is wind. It makes rocks in the shape of a mushroom, called mushroom rocks.
• When the wind stops blowing, the sand falls and get deposited in low hill like structures. These are called sand dunes.  
• When the grains of sand are very fine and light, the wind can carry it over long distances. When such sand is deposited in large areas, it is called loess.