Our Changing Earth – Major Landforms – Science Class 7

Our Changing Earth: The lithosphere is broken into a number of plates known as the Lithospheric plates. You will be surprised to know that these plates move around very slowly – just a few millimeters each year. This is because of the movement of the molten magma inside the earth.

Let us do an activity to show, how molten magma moves inside the earth and produces our changing earth

  • Take a small colored paper pellet and put it in a beaker half-filled with water.
  • Place the beaker on a tripod stand and heat it. As the water warms up, you will observe that the paper pellet is moving upward along with the warm layers of water and then sinks back along with the cooler layers of water. It is interesting to note that the molten magma inside the earth moves in a similar manner.

The movement of these plates causes changes on the surface of the earth. The earth’s movements are divided on the basis of the forces which cause them.

The forces which act in the interior of the earth are called Endogenic forces and the forces that work on the surface of the earth are called Exogenic forces. Endogenic forces 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. Similarly, when the Lithospheric plates move, the surface of the earth vibrates.

The vibrations can travel all around the earth. These vibrations are called earthquakes. The place in the crust where the Movement starts is called the focus. The place on the surface above the focus is called the epicenter. Vibrations travel outwards from the epicenter as waves.

The greatest damage is usually closest to the epicenter and the earthquake’s strength decreases away from the center. Although earthquakes cannot be predicted, the impact can certainly be minimized if we are prepared beforehand. Some common earthquake prediction methods adopted locally by people include studying animal behavior; fish in the ponds get agitated, snakes come to the surface.

MAJOR LANDFORMS

The landscape is being continuously worn away by two processes – weathering and erosion. Weathering is the breaking up of the rocks on the earth’s surface. Whereas, Erosion is the wearing away of the landscape by different agents like water, wind, and ice. The eroded material is carried away or transported by water, wind, etc., and eventually deposited.

As a result, This process of erosion and deposition creates different landforms on the surface of the earth.

Work of a River

Now let us see various agents of weathering and erosion in nature that result in our changing earth. The running water in the river erodes the landscape. When the river tumbles at a steep angle over very hard rocks or down a steep valley side it forms a waterfall.

As the river enters the plain it twists and turns to form large bends known as meanders. Due to continuous erosion and deposition along the sides of the meander, the ends of the meander loop come closer and closer.

In due course of time, the meander loop cuts off from the river and forms a cut-off lake, called an ox-bow lake. At times the river overflows its banks. This leads to the flooding of the neighboring areas. As it floods, it deposits layers of fine soil and other material called sediments along its banks. This leads to the formation of a flat fertile floodplain.

The raised banks are called levees. As the river approaches the sea, the speed of the flowing water decreases, and the river begins to break up into a number of streams 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.

Work of sea waves

The erosion and deposition of the sea waves give rise to coastal landforms. Seawolves continuously strike at the rocks. Cracks develop. Over time they become larger and wider. Thus, hollow caves are formed on the rocks. They are called sea caves.

As these cavities become bigger and bigger only the roof of the caves remain, thus forming sea arches. Further, erosion breaks the roof and only walls are left. These wall-like features are called stacks. The steep rocky coast rising almost vertically above seawater is called a sea cliff. The sea waves deposit sediments along the shores forming beaches

Work of Ice

Let us know how our changing earth is the result of the work of the ice. Glaciers are “rivers” of ice which too erode the landscape by bulldozing soil and stones to expose the solid rock below. Glaciers carve out deep hollows. As the ice melts they get filled up with water and become beautiful lakes in the mountains. The material carried by the glacier such as rocks big and small, sand and silt gets deposited. These deposits form glacial moraines.

Work of wind

Have you ever visited a desert? Try to collect some pictures of sand dunes. An active agent of erosion and deposition in the deserts is wind. In deserts, you can see rocks in the shape of a mushroom, commonly called mushroom rocks. Winds erode the lower section of the rock more than the upper part. Therefore, such rocks have a narrower base and wider top.

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

Read More: Natural Vegetation and Wildlife – Science Class 7

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