Geography

Chapter 3: Our Changing Earth

Lithospheric plates

  • The earth’s crust consists of several large and some small, rigid, irregularly shaped plates (slabs) which carry continents and the ocean floor. These plates are Lithospheric plates or Tectonic plates.

  • These plates are regions of Earth’s crust and upper mantle that are fractured into plates that move across a deeper plasticine mantle – just a few millimetres each year.

Earth Movements

  • Plates at our planet’s surface move because of the intense heat in the Earth’s core that causes molten rock in the mantle layer to move.

  • It moves in a pattern called convection cell that forms, when warm material rises, cools, and eventually sinks down. As the cooled material sinks down, it is warmed and rises again.

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

A Volcano

  • A volcano is a landform (usually a mountain) where molten rock erupts through the surface of the planet.

  • In simple terms a volcano is a mountain that opens downward to a pool of molten rock (magma) below the surface of the earth. It is a hole in the Earth from which molten rock and gas erupt.

An Earthquake

  • When the Lithospheric plates move, the surface of the earth vibrates. The vibrations can travel all round 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 epicentre.

  • Vibrations travel outwards from the epicentre as waves.

  • Greatest damage is usually closest to the epicentre and the strength of the earthquake decreases away from the centre.

Types of Earthquake Waves

  • There are three types of earthquake waves:

1. P waves or longitudinal waves

2. S waves or transverse waves

3. L waves or surface waves

Earthquake Preparedness & Prediction

  • Although earthquakes cannot be predicted, the impact can certainly be minimised if we are prepared before-hand.

  • Some common earthquake prediction methods adopted locally by people include studying animal behaviour; fish in the ponds get agitated, snakes come to the surface.

The Richter Scale

  • An earthquake is measured with a machine called a seismograph. The magnitude of the earthquake is measured on the Richter scale.

  • An earthquake of 2.0 or less can be felt only a little. An earthquake over 5.0 can cause damage from things falling.

  • A 6.0 or higher magnitude is considered very strong and 7.0 is classified as a major earthquake.

Weathering and Erosion

  • 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.

  • 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.

  • This process of erosion and deposition create different landforms on the surface of the earth.

Work of a River

  • Rivers are powerful and dynamic geological agents. The water flowing through a stream performs three kinds of geologic works as erosion, transportation and deposition.

  • Hence, a river is considered as one of the geological agents on earth. The flowing water has the force, velocity and power to generate electricity.

  • The running water in the river erodes the landscape. When the river tumbles at steep angle over very hard rocks or down a steep valley side it forms a waterfall.

  • The highest waterfall is Angel Falls of Venezuela in South America.

  • The other waterfalls are Niagara falls located on the border between Canada and USA in North America and Victoria Falls on the borders of Zambia and Zimbabwe in Africa.

    River in a flood plain

  • As the river enters the plain it twists and turns forming large bends known as meanders.

  • In due course of time the meander loop cuts off from the river and forms a cut-off lake, also called an ox-bow lake.

  • At times the river overflows its banks. This leads to the flooding of the neighbouring 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 work of sea water is performed by several marine agents like sea waves, oceanic currents, tidal waves and tsunamis but the sea waves are most powerful and effective erosive agent of coastal areas. 
  • Sea waves are defined as undulations of sea water characterized by well-developed crests and troughs

  • A sea cave, also known as a littoral cave, is a type of cave formed primarily by the wave action of the sea. The primary process involved is erosion.

  • 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 wallsare left. These wall like features are called stacks.

  • The steep rocky coast rising almost vertically above sea water is called sea cliff. The sea waves deposit sediments along the shores forming beaches.

Work of 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 there. 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

  • 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.

  • 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 are very fine and light, the wind can carry it over very long distances. When such sand is deposited in large areas, it is called loess.
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