Summary
Chapter 3 'Our Changing Earth' explains how lithospheric plates move a few millimetres each year due to molten magma, triggering endogenic forces such as earthquakes and volcanoes, while exogenic forces — rivers, sea waves, glaciers, and wind — continuously erode and deposit material to build new landforms.
Lithospheric plates move slowly — just a few millimetres each year — because of circular movement of molten magma inside the earth. This movement generates endogenic forces: sudden forces like earthquakes and volcanoes, and diastrophic forces that build mountains. Earthquakes are measured on the Richter scale using a seismograph; the Bhuj earthquake of 26 January 2001 measured 6.9. Exogenic forces act on the surface through erosion and deposition. Rivers form waterfalls, meanders, ox-bow lakes, floodplains, levees, and deltas. Sea waves create caves, arches, stacks, and cliffs, and deposit beaches. Glaciers carve deep hollows and leave glacial moraines. Wind shapes mushroom rocks and deposits sand dunes; fine sand spread over large areas is called loess, with large deposits found in China.
Key points & formulas
- 01Lithospheric plates move a few millimetres each year because molten magma inside the earth moves in a circular manner.
- 02Endogenic forces originate inside the earth — sudden forces include earthquakes and volcanoes; diastrophic forces build mountains.
- 03Exogenic forces act on the earth's surface through erosion (by water, wind, and ice) and subsequent deposition to create landforms.
- 04An earthquake's point of origin in the crust is the focus; the point directly above it on the surface is the epicentre. The three types of earthquake waves are P waves (longitudinal), S waves (transverse), and L waves (surface).
- 05Earthquakes are measured using a seismograph; magnitude is read on the Richter scale — over 5.0 causes damage, 6.0+ is very strong, and 7.0 is a major earthquake.
- 06Rivers form waterfalls over hard rocks, meanders on plains, ox-bow lakes from cut-off meanders, floodplains and levees from flood deposits, and deltas at the sea through distributaries.
- 07Sea waves erode rocks into sea caves, then sea arches, and finally stacks; they deposit sediments along shores to form beaches.
- 08Glaciers (rivers of ice) carve deep hollows that fill with water to form mountain lakes, and deposit rocks, sand, and silt as glacial moraines. Wind creates mushroom rocks in deserts, deposits sand as dunes, and spreads fine sand over large areas as loess.
Frequently asked questions
01Why do lithospheric plates move?
Lithospheric plates move because of the movement of molten magma inside the earth. The magma moves in a circular manner, and this movement slowly pushes the plates — just a few millimetres each year.
02What are endogenic and exogenic forces?
Endogenic forces are forces that act in the interior of the earth, such as earthquakes and volcanoes. Exogenic forces are forces that work on the surface of the earth, such as erosion and deposition by rivers, wind, sea waves, and glaciers.
03What is a volcano?
A volcano is a vent (opening) in the earth's crust through which molten material erupts suddenly.
04What is the difference between the focus and the epicentre of an earthquake?
The focus is the place in the crust where the earthquake movement starts — it is the origin of seismic energy. The epicentre is the place on the surface directly above the focus. Greatest damage usually occurs closest to the epicentre.
05What are the three types of earthquake waves?
The three types are P waves (longitudinal waves), S waves (transverse waves), and L waves (surface waves).
06How is the magnitude of an earthquake measured?
Earthquakes are measured using a machine called a seismograph. The magnitude is read on the Richter scale. An earthquake of 2.0 or less is felt only a little; over 5.0 can cause damage; 6.0 or higher is considered very strong; and 7.0 is classified as a major earthquake.
07What happened during the Bhuj earthquake?
A massive earthquake measuring 6.9 on the Richter scale hit Bhuj Town on 26th January 2001. It caused widespread destruction — phone lines, water pipelines, and power transmission lines were knocked out, buildings collapsed, fires broke out, and a state of emergency was declared.
08How are ox-bow lakes formed?
As a river flows through a plain, it forms large bends called meanders. Due to continuous erosion and deposition along the sides of the meander, the ends of the loop come closer together. Eventually the meander loop cuts off from the river and forms a cut-off lake called an ox-bow lake.
09How are floodplains and levees formed?
When a river overflows its banks, it floods the neighbouring areas and deposits layers of fine soil and other material called sediments. This creates a flat, fertile floodplain. The raised banks formed by these deposits are called levees.
10How do sea waves form sea caves, sea arches, and stacks?
Sea waves continuously strike rocks along the coast, causing cracks that widen over time into hollow caves called sea caves. As the caves grow, only their roofs remain, forming sea arches. Further erosion breaks the roof away, leaving behind wall-like features called stacks.
11What are glacial moraines?
Glacial moraines are deposits of material — rocks (big and small), sand, and silt — that are carried and deposited by glaciers as they move and melt.
12Why do mushroom rocks form in deserts?
In deserts, wind erodes the lower section of rocks more than the upper part. This gives the rocks a narrower base and a wider top, making them look like mushrooms.
13What is loess and where are large deposits found?
When wind carries very fine, light sand grains over long distances and deposits them over large areas, the resulting deposit is called loess. Large deposits of loess are found in China.
14Is the NCERT Class 7 Our Changing Earth chapter available as a free PDF without sign-up?
Yes — the official NCERT PDF of Our Environment Chapter 3 is available free on cbseprepmaster.com with no sign-up required.
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