Chapter 3. Our Changing Earth

Movements of Earth
• lithosphere is broken into a number of plates called Lithospheric plates. They may be large or small, rigid & irregular shaped which carry continents and ocean floors.
• Due to circular movement of magma inside earth, these plates move around but very slowly –just a few millimetres each year.
• movement of plates is caused by endogenic forces [interior of earth] and exogenic forces [on surface/ exterior of earth].
• Endogenic forces may cause a sudden movement or slow movements [diastrophism].
• Earthquakes, volcanoes & landslides are examples of sudden movements which cause mass destruction.

• Due to movement of lithosphere, a vibration is produced which can travel all around earth and can cause an earthquake.
• place in crust where movement starts is known as focus.
• A place on earth’s surface above focus is known as epicentre.
• greatest damage is generally near epicentre and strength decreases as we move away from centre.
• There are three types of earthquake waves:
(1) P waves or longitudinal waves
(2) S waves or transverse waves
(3) • waves or surface waves

Measurement of Earthquake
• An earthquake is measured with a machine known as a seismograph.
• magnitude of earthquake is measured on Richter scale.
• An earthquake of 2.0 or less can be felt only a little. An earthquake over 5.0 can cause severe damage.
• A 6.0 or higher magnitude is considered very strong and 7.0 is classified as a major earthquake, examplea massive earthquake of 6.9 Richter scale hit Bhuj of Gujarat in 2001.
• Preparedness against Earthquake:
(1) Go to a safe spot for example under kitchen counter, table or desk.
(2) Stay away from fire places, area around chimneys, windows.
(3) Spread awareness among friends and family.
• landscape is continuously worn away by two processes – weathering and erosion.
• Weathering is breaking up of rocks on earth’s surface.
• Erosion is wearing away of landscape by different agents like water, wind & ice.

Work of River
Waterfalls: This is tumbling of river water at a steep angle over very hard rock or down a steep valley side. highest waterfall is Angel Falls of Venezuela in South America.
• other waterfalls are Niagara falls located on border between Canada and USA in North America and Victoria Falls on borders of Zambia and Zimbabwe in Africa.
• Meanders: As river enters plain it twists and turns forming large bends called meanders.
Ox-bow Lake: In due course of time meander loop cuts off from river and forms a cut-off lake, known as an ox-bow lake.
• Floodplain and Levees: When river overflows its banks, it deposits layers of fine soil and other materials known as sediments along its banks. This leads to formation of a flat fertile floodplain. raised banks are known as levees.
• Deltas and Distributaries: As river approaches sea, speed of flowing water decreases and river begins to break up into a number of streams known as distributaries. Each distributary forms its own mouth. collection of sediments from all mouths forms a delta.

Work of Wind
• Wind is an active agent of erosion and deposition in deserts.
• In deserts, rocks in shape of mushroom are found which are called mushroom rocks.
• When wind blows, it transportations sands from one place to another and creates a low hills like structure which is known as a sand dunes.
• very fine and light sand grains carried by wind and deposited over a large area are known as loess.

Work of Sea Wave
• Sea waves continuously strike at rocks which develops larger and wider cracks.
• Thus, hollow like caves are formed on rocks. They are known as sea caves.
• As these cavities become bigger and bigger only roof of caves remain and later only wall left that are known as sea arches and stacks respectively.
• steep rocky coast rising almost vertically above sea water is known as a sea cliff.
• sea waves deposit sediments along shores forming beaches. Stacks
• Glaciers carve out deep hollows on mountain surface. As ice melts they get filled up with water and become beautiful lakes in mountains.
• material carried by glacier such as rocks big and small, sand & silt gets deposited and forms glacial moraines.

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