Physical geography for CDS

Physical geography rewards understanding more than any other GK block. If you know why the west coasts of continents in the subtropics have cold currents and deserts, you don't need to memorise five pairs; you can work them out. CDS asks both kinds of question: straight recall (which layer has the ozone) and reasoning dressed as recall (why a region gets rain in winter).

The earth's interior and rocks

We know the outer core is liquid because S-waves (secondary, transverse) cannot pass through liquids, while P-waves (primary, longitudinal) pass through solids, liquids and gases. The crust's most abundant element is oxygen, then silicon; the most abundant metal is aluminium.

Earth's movements and seasons

• Rotation, from west to east, takes about 24 hours and causes day and night.
• Revolution takes about 365¼ days. The axis is tilted about 23.5° from the perpendicular to the orbit, and that tilt, not the distance from the Sun, causes seasons.
• The earth is nearest the Sun (perihelion) in early January and farthest (aphelion) in early July. The Northern Hemisphere has winter at perihelion, which proves distance is not the cause.

Longitude and time

The earth turns 360° in 24 hours, so 15° of longitude = 1 hour, and 1° = 4 minutes. Places east of Greenwich are ahead in time.

Worked example. India's standard meridian is 82.5° E. Time difference from Greenwich = 82.5 × 4 = 330 minutes = 5 hours 30 minutes, which is why IST is GMT + 5:30.

Worked example. When it is noon at Greenwich, what is the local time at 45° W? 45 × 4 = 180 minutes = 3 hours behind, so 9 a.m.

The International Date Line roughly follows the 180° meridian, with bends to avoid splitting countries. Crossing it westward, you add a day.

The atmosphere

Nitrogen is about 78% of dry air, oxygen about 21%, argon about 0.9% and carbon dioxide about 0.04%.

Pressure belts and planetary winds

Winds blow from high to low pressure: the trade winds from the subtropical highs to the equator, the westerlies from the subtropical highs towards the subpolar lows, and the polar easterlies from the poles. The Coriolis force deflects winds to the right in the Northern Hemisphere and to the left in the Southern (Ferrel's law). That is why the trade winds are north-easterly in the north. In the Southern Hemisphere, the westerlies blow over open ocean and are so strong they are called the Roaring Forties.

Tropical cyclones have regional names: hurricanes in the Atlantic and north-east Pacific, typhoons in the north-west Pacific, and cyclones in the Indian Ocean.

Landforms

For earthquakes, the focus (hypocentre) is the point of origin inside the earth; the epicentre is the point on the surface directly above it. The Richter scale measures magnitude (energy released); the Mercalli scale measures intensity (effects felt). Most active volcanoes lie on the Pacific Ring of Fire.

Ocean currents

The pattern follows the winds. In low latitudes, currents run warm along the east coasts of continents and cold along the west coasts. Cold currents cool and stabilise the air above them, so the adjoining coasts are dry: the Humboldt current and the Atacama, the Benguela and the Namib.

Where warm and cold currents meet, plankton flourishes and fog forms. The Grand Banks off Newfoundland (Gulf Stream and Labrador) and the waters off Japan (Kuroshio and Oyashio) are rich fishing grounds.

Tides

Tides are caused mainly by the Moon's gravity, with a smaller pull from the Sun. Spring tides (the greatest range) occur when the Sun, Moon and Earth are in line, at new moon and full moon. Neap tides (the smallest range) occur when the Sun and Moon are at right angles, at the first and third quarters.

Practice MCQs

1. Which seismic waves cannot pass through the outer core? (a) P-waves only (b) S-waves only (c) Both P- and S-waves (d) Neither P- nor S-waves
2. Marble is formed by the metamorphism of (a) limestone (b) shale (c) sandstone (d) granite
3. The main cause of seasons is (a) the earth's distance from the Sun (b) ocean currents (c) rotation (d) the tilt of the earth's axis
4. If it is 12 noon at Greenwich, local time at 60° E is (a) 8 a.m. (b) 4 p.m. (c) 3 p.m. (d) 6 p.m.
5. The ozone layer lies mainly in the (a) troposphere (b) stratosphere (c) mesosphere (d) thermosphere
6. The Chinook is a (a) cold wind of France (b) dusty wind of West Africa (c) hot wind of India (d) warm wind east of the Rockies
7. Winds in the Northern Hemisphere are deflected to the right because of (a) Coriolis force (b) pressure gradient (c) friction (d) gravity
8. Which current is responsible for the dryness of the Atacama coast? (a) Gulf Stream (b) Benguela (c) Humboldt (d) Kuroshio
9. Stalactites hang from (a) the cave floor (b) glacier snouts (c) river beds (d) the cave ceiling
10. Neap tides occur at (a) new moon (b) full moon (c) quarter moons (d) equinoxes

Answers:

1. (b) S-waves cannot travel through liquid.
2. (a) Limestone becomes marble.
3. (d) The axial tilt; the earth is nearest the Sun in January.
4. (b) 60 × 4 = 240 minutes = 4 hours ahead.
5. (b) The stratosphere.
6. (d) A warm, dry wind on the eastern slopes of the Rockies.
7. (a) The Coriolis force, due to the earth's rotation.
8. (c) The cold Humboldt current off Peru and Chile.
9. (d) Stalactites hang from the ceiling; stalagmites rise from the floor.
10. (c) The Sun and Moon pull at right angles.

Revise this

Apply these ideas region by region in Indian geography and world geography.

• I can name the layers of the earth and the two discontinuities.
• I can give the three rock types with two examples each and four metamorphic pairs.
• I can explain seasons using tilt, and fill the solstice–equinox table.
• I can convert longitude difference to time in both directions.
• I know the atmospheric layers and one fact about each.
• I can draw the pressure belts and planetary winds from memory.
• I can list five warm and five cold currents and explain the west-coast desert pattern.
• I can explain spring and neap tides.