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Class 9 Shaping of the Earth’s Surface: cheat sheet
CBSE Social Science. The Earth’s layers, plate tectonics and the three kinds of plate boundary, weathering and erosion, and landforms made by rivers, glaciers, wind, waves and groundwater.
Key ideas
- The Earth has a thin crust, a thick mantle, a fluid outer core and a solid inner core of iron and nickel.
- The lithosphere (crust and upper mantle) is broken into plates that move on the semi-molten asthenosphere, driven by convection currents in the mantle.
- Plates meet at convergent boundaries (the Himalaya), move apart at divergent ones (the Mid-Atlantic Ridge) and slide past at transform ones (the San Andreas Fault).
- Weathering breaks rock where it lies; erosion wears it away and carries it off.
- A river cuts V-shaped valleys and waterfalls in its upper course, makes meanders and oxbow lakes in its middle course, and builds a delta near its mouth.
- Glaciers carve U-shaped valleys and leave moraines; wind builds sand dunes; waves cut cliffs, caves, arches and stacks.
- Groundwater dissolving limestone makes karst landforms: caves, stalactites, stalagmites and sinkholes.
- Contouring, bunding and terracing hold soil and water on slopes.
Remember
| Crust thickness | 30–40 km under continents; 5–7 km under oceans |
| Seven major plates | Pacific, Eurasian, African, North American, South American, Indo-Australian, Antarctic |
| Ring of Fire | the belt round the Pacific with most earthquakes and volcanoes |
| Stalactite and stalagmite | hangs from a cave roof; rises from a cave floor |
| GLOF | Glacial Lake Outburst Flood |
| Sand dunes | barchan (crescent), longitudinal, star, parabolic |
Watch out
- Using weathering and erosion as the same word: weathering does not move the rock.
- Mixing up V-shaped valleys (rivers) and U-shaped valleys (glaciers).
- Swapping the boundaries: plates move apart at a divergent boundary and meet at a convergent one.
Worked example
The Gujarat earthquake was in 2001 and the Chamoli flood in 2021. How many years apart were they, and how many years before 2048 was the Chamoli flood?
Working: 2021 − 2001; 2048 − 2021
Answer: 20 years; 27 years
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