Exam practice

crust

5000 (over 5,000 °C, about as hot as the surface of the Sun)

autumn, winter and spring

weathering

Sun

Mercury

Venus

deposition (the soil laid down in the new place; blowing it away is erosion)

True. Neptune is the eighth planet, about 30 times as far from the Sun as the Earth.

True. The Earth’s orbit is an ellipse, though very nearly a circle.

False. The Moon goes round the Earth in about 27 days (about 29½ days from one full moon to the next). It is the Earth that takes about 365 days to go round the Sun.

False. Uranus and Neptune, much farther from the Sun, are colder than Saturn. The coldest air measured is on Uranus.

False. The crust is the thinnest layer. The mantle, about 2,900 km thick, is the thickest.

The inner core lies under all the other layers of the Earth. Their weight presses on it with an enormous pressure, millions of times that of the air. This pressure squeezes the iron and nickel so tightly that they can’t melt, even at over 5,000 °C. So the inner core is solid.

Soil carried away by water or wind doesn’t vanish: it has to be laid down somewhere when the water or wind slows down. So whenever soil is eroded from one place, it is deposited in another. A river that cuts its banks in the hills lays the soil down on the plains.

The Sun is a star: a huge ball of very hot gas that makes enormous amounts of energy in its center (about 15 million °C) and gives it out as heat and light. The Earth gets almost all its heat and light from it: it warms the land, sea and air, drives the winds, rain and rivers, and lets plants make food. Other sources, like the Earth’s inner heat, are tiny beside it.

Soil is the loose top layer of the land. It is made of tiny pieces of rock and minerals, organic matter (humus from rotted plants and animals), and air and water in the spaces between them. It forms from rock by weathering, and it is where plants grow.

Soil forms from rock by weathering, the breaking up of rocks and minerals on the Earth’s surface, mainly by water, air and the climate, with help from living things:

• a.Rivers carry stones that knock together and wear into pebbles, sand and fine grains.
• b.Heating by day and cooling at night make rocks swell and shrink until they crack and break into small pieces.
• c.Water seeps into cracks; when it freezes it swells and splits the rock, and it slowly dissolves some minerals.
• d.Plant roots grow into cracks and push them wider until the rock crumbles.
• e.Strong wind throws sand against rock and wears its surface into fine particles.

Then plants grow, die and rot, and worms and tiny living things mix the humus in with the rock particles. Over hundreds of years, this mixture becomes soil.

• a.Almost all our food grows in soil; without fertile soil there would be no crops, fruit or fodder.
• b.Soil forms very slowly: 1 cm of it can take hundreds of years, but a flood can wash it away in a day.
• c.Soil is home to countless living things, and it holds water for plants.
• d.Losing soil causes landslides and floods, and silt fills rivers and dams.

So we must protect it from erosion and pollution.

The axis is the imaginary line through the center of the Earth that joins the North Pole and the South Pole. The Earth spins on it once in about 24 hours. The orbit is the path along which the Earth goes round the Sun, once in about 365 days.

By the condition of the rock and what it is made of, the inside of the Earth has three layers:

• a.Crust: the outermost, thinnest layer, 5–70 km thick, of rocks and soil (silicate minerals). We live on it.
• b.Mantle: the thickest layer, about 2,900 km, of hot rock rich in iron and magnesium; about 1,000 to nearly 4,000 °C; solid, but it creeps very slowly.
• c.Core: the innermost part, of iron and nickel. The outer core, about 2,200 km thick and 4,000 to 5,000 °C, is liquid. The inner core, about 1,200 km in radius and over 5,000 °C, is solid because of the enormous pressure.

The Earth’s axis leans at 66.5° to the plane of its orbit. Because of this:

• a.Day and night are not always equal, except at the equator: in summer the days are longer, in winter the nights are longer.
• b.The seasons change through the year, and they are different in the north and the south at the same time.
• c.The Sun’s rays do not always fall straight down on the same place: they fall directly in summer and at a slant in winter.

How: The Earth goes round the Sun once a year with its axis tilted (66.5° to the plane of its orbit), always leaning the same way in space. For half the year the Northern Hemisphere leans towards the Sun: the Sun’s rays fall directly on it and the days are long, so it is hot (summer). For the other half it leans away: the rays fall at a slant and spread out, and the days are short, so it is cold (winter). In between come spring and autumn.

When: in Nepal’s months, spring is Chaitra and Baisakh, summer Jestha to Bhadra, autumn Ashwin and Kartik, and winter Mangsir to Falgun. The longest day is about 21 June (early Asar), and the shortest about 22 December (early Poush).

Where: all over the Earth except near the equator, where it is warm all year; the Southern Hemisphere has its seasons the other way round.

Soil that is worn away in one place is always laid down in another, so erosion and deposition go together.

• a.In the monsoon, rivers like the Koshi and the Narayani cut soil from their banks and slopes in the hills (erosion) and leave it as sand and silt on the plains of the Terai (deposition).
• b.A landslide strips the soil off a hillside (erosion) and piles it at the foot of the slope or in the river below (deposition).
• c.Wind blows dry soil off a bare field (erosion) and drops it as dust or sand where it slows down (deposition).

Almost all our food comes from the soil, directly or through animals.

• a.Rice, wheat, maize, millet, lentils, vegetables and fruit grow in soil. Their roots take water and minerals (such as nitrogen and potassium) from it, and it holds them up.
• b.Milk, eggs and meat come from animals that eat grass, fodder and grain grown in soil.
• c.Fertile soil, rich in humus, gives good harvests; eroded or polluted soil gives poor crops, and harmful chemicals in soil can get into our food.

So good soil means good food, and protecting soil protects our food.

• a.Use less chemical fertilizer and pesticide; use compost, dung manure and natural pest control instead.
• b.Treat factory waste and chemicals to make them harmless before letting them out.
• c.Use fewer plastic bags and plastic things; reuse and recycle; carry a cloth bag.
• d.Sort waste: compost what rots, and send the rest for recycling instead of dumping or burning it.
• e.Make people aware of the harm that pollution does to soil.

Polythene bags don’t rot: thrown away, they stay in the soil for hundreds of years, breaking only into tiny bits. In the soil they block water and air from reaching roots and soil life, harm the living things that keep the soil fertile, and make plants grow poorly. Bags also block drains and ditches, so rainwater floods and washes soil away. So banning polythene bags, and using cloth or paper bags instead, prevents soil pollution and helps conserve the soil.

The solar system is the Sun’s family:

• a.The Sun: a medium-sized star at the center, a huge ball of hot gas that gives out heat and light. Its gravity holds the family together.
• b.Planets: eight bodies with no light of their own that go round the Sun: Mercury, Venus, Earth, Mars (small and rocky), and Jupiter, Saturn, Uranus, Neptune (giants of gas and liquid).
• c.Satellites: bodies that go round planets, such as the Moon, the Earth’s only natural satellite.
• d.Dwarf planets and asteroids: smaller rocky bodies going round the Sun, such as Pluto and Ceres, and many asteroids between Mars and Jupiter.
• e.Comets: balls of ice and dust in long elliptical orbits that grow tails near the Sun.
• f.Meteoroids: small bits of rock that, burning up in our air, are seen as meteors; pieces that land are meteorites.