Exam practice

A mineral is homogeneous and has a definite chemical composition: every sample of it is made of the same element or compound, in the same proportions, wherever it is found. Quartz is always SiO₂ and hematite always Fe₂O₃. A substance with a fixed composition is a pure substance, so minerals are said to be pure chemicals.

In the Mesozoic era, forests of large conifers and other gymnosperms, such as pines, cycas, ginkgo, juniper and ephedra, replaced the fern forests and covered almost all the moist land. Cone-bearing (coniferous) plants were the main plants of the era, so it is called the era of conifers (or of gymnosperms).

At first the Earth was extremely hot and partly molten from collisions, with no solid crust, no oceans, and air with no oxygen. Life needs liquid water and a stable surface. It took hundreds of millions of years for the Earth to cool, for its crust and atmosphere to form, for rain to fill the oceans, and for simple chemicals to join into the large molecules of life (proteins and nucleic acids). Only then could the first simple cells appear.

A mineral is a natural, solid, pure substance with a definite chemical composition, usually crystalline, found in the Earth’s crust. Examples: quartz, mica, hematite, galena, gypsum, talc, gold, diamond. (In everyday use, any useful substance mined from the ground, such as coal, petroleum, limestone or marble, is also called a mineral.)

The hypotheses are: (a) the nebular hypothesis of Kant and Laplace, (b) Buffon’s planetesimal hypothesis, (c) the binary hypothesis of Chamberlin and Moulton, and (d) the tidal hypothesis of Jeans and Jeffreys.
The nebular hypothesis (Kant, 1755; improved by Laplace, 1796): Where the Solar System is now, there was a huge, flat cloud of gas and dust (a nebula) turning slowly. Gravity pulled its particles to the center, which grew and spun faster. The center became a hot, dense protostar; nuclear fusion began in its core and it became the Sun. The rest of the cloud flattened into a spinning disc around it. In the disc, particles gathered into lumps, which grew into the planets; the material around the planets became their moons, and the leftovers became asteroids, comets and meteoroids. (Laplace said the planets formed from rings thrown off the spinning Sun’s equator.) This explains why all the planets go round the Sun in nearly one plane and in the same direction.

• Agriculture, the work of most Nepalis, needs fertilizers and lime made from minerals such as phosphorite, gypsum and dolomite.
• Industry: limestone feeds the cement factories (Udayapur, Hetauda); silica, clay, magnesite and mica feed glass, ceramic and other industries, which give jobs.
• Construction: sand, gravel, stone, slate, marble and cement build houses, roads and bridges.
• Energy: coal, and petroleum and natural gas if they are found, could reduce what Nepal spends on imported fuel.
• Income: Nepal has more than 60 kinds of minerals (iron, copper, zinc, lead, gold, magnesite, uranium…). Mined and sold sustainably, they could earn money from exports, reduce imports and develop the country.

Minerals

• Metallic: ferrous (hematite, magnetite); non-ferrous (galena, bauxite); precious (gold, silver, platinum)
• Non-metallic: gems (diamond, emerald, sapphire); non-gems (mica, gypsum, talc, silica)
• Energy: coal, petroleum, natural gas, radioactive elements (uranium)

(Draw it as boxes joined by lines, as in Figure 11.1.)

The Archaeozoic era, the earliest part of the Cryptozoic eon, was unsuitable for life for most of its length. The Earth was extremely hot and kept melting under the impacts of huge rocks; there was no stable crust and at first no oceans; the air was made of volcanic gases such as carbon dioxide, ammonia, methane and sulphur dioxide, with no oxygen to breathe and no ozone layer to block the Sun’s ultraviolet rays. For example, a fish or a fern could not have survived even a minute: no water to live in, and nothing to breathe. Only near the end of this era, once the Earth had cooled and the oceans had formed, did the first simple cells appear.

Mammoths evolved in the Cenozoic era, the era of mammals (the first mammoths appeared about 5 million years ago; the woolly mammoth much later). Fish with lungs (lungfish) evolved in the Paleozoic era, about 400 million years ago.

• i.Possible reasons: not enough geological survey and research; lack of money, modern technology and trained people; steep, remote land without roads and electricity; deposits that are small or scattered; and weak rules and monitoring, so sand, stone and gravel are dug from rivers and hills carelessly, causing landslides and damaging rivers.
• ii.Iron, copper, coal and gold (panned from river sand) have been extracted traditionally, along with stone, slate, clay and sand. To use them sustainably: survey and map the deposits properly; use modern, safe mining methods; extract only what is needed, without wasting; study the environmental effects first, and fill and plant mined land afterwards; protect rivers and forests; train local people and share the income with the community; and recycle metals.

• i.In the Mesozoic era: dinosaurs appeared in the Triassic period and died out at the end of the Cretaceous period, 66 million years ago.
• ii.An asteroid about 10 km wide hit the Earth. Fires, and dust and soot in the air, blocked sunlight for months; plants died, food chains collapsed, and the climate changed suddenly. Huge volcanic eruptions in India added poisonous gases. The big animals, which needed a lot of food, could not survive.
• iii.An open question; a good answer could say: Giant plant-eaters would eat huge amounts of vegetation, clearing forests and competing with elephants, rhinos and cattle for food; big meat-eaters like tyrannosaurus would hunt large mammals, and humans could hardly have spread or farmed safely. Mammals might never have grown large, so humans might not have evolved at all. The food chains and the balance of nature would be completely different from today’s.

Their orbits come close to, or cross, the Earth’s orbit, so one could one day hit the Earth. An asteroid hits at many kilometers per second, so even a small one carries enormous energy: a 20-meter asteroid that burst over Chelyabinsk, Russia, in 2013 broke windows across a city and injured about 1,500 people, and a 10-km asteroid wiped out the dinosaurs. That is why astronomers search for and track them.

Each time a comet comes close to the Sun, the Sun’s heat turns some of its ice into gas, which streams away carrying dust, forming the coma and the tails. The Sun’s light and the solar wind push this material out into space, and it never comes back. A little is lost on every trip, so after many trips the comet has lost all its ice and gas and is left as a dark, rocky extinct comet.

Meteoroids enter the air at 11 to 72 km per second. The air in front of them is squeezed and heated so much that they become white-hot and vaporize, about 80–100 km up. Most meteoroids are tiny, like grains of sand, so they burn up completely long before they reach the ground. Only a few large ones leave a piece that lands as a meteorite.

Galaxies are the building blocks of the universe: almost all the stars, gas and dust are in them. They are so bright that we can see them at enormous distances, and since their light takes billions of years to reach us, we see distant galaxies as they were long ago, which shows us the universe’s history. The way galaxies move apart showed that the universe is expanding, the main evidence for the Big Bang; their motions revealed dark matter; and the black holes at their centers can be studied. A single planet or star can’t tell us any of this about the universe as a whole.

The universe is everything that exists: all of space and time, and all the matter and energy in them. It includes all the galaxies, stars, planets, moons, asteroids, comets, gas, dust, dark matter and light. The Earth and the Sun are tiny parts of it. It began about 13.8 billion years ago and is still expanding.

In 1927 Georges Lemaître worked out that the universe is expanding, and in 1929 Edwin Hubble showed that distant galaxies are moving away from us, the farther ones faster. If the universe is expanding, then long ago everything was packed together. The Big Bang theory says that about 13.8 billion years ago, all the matter and energy of the universe was in one extremely hot, dense, tiny state (Lemaître’s “primeval atom”), which began to expand very rapidly: the Big Bang. As it expanded it cooled: first subatomic particles formed, then atoms (mostly hydrogen and helium) and molecules; then gravity gathered the gas into stars, galaxies and nebulae, and much later into planets. The universe is still expanding. The theory is supported by the moving apart of galaxies, the faint afterglow of the Big Bang found in 1965, and the amounts of hydrogen and helium in the universe.

• They are small rocky or metallic bodies that go round the Sun: minor planets.
• Most lie in the asteroid belt between Mars and Jupiter; some (Trojans) share a planet’s orbit, and some (near-Earth asteroids) cross the Earth’s orbit.
• Most are irregular in shape, because their gravity is too weak to make them round.
• Their sizes range from a few meters to about 1,000 km (Ceres, about 940 km, is the largest).
• They are made of rock, minerals, metals such as iron and nickel, and clay.
• They have no tail, and they sometimes collide with one another or with planets.

A comet is a small, icy body of the Solar System that goes round the Sun in a long, stretched orbit and grows a bright tail when it comes near the Sun. Its parts, as in Figure 11.19:

• Nucleus: the solid center of ice, dust and rock, a few kilometers across.
• Coma: the glowing cloud of gas and dust around the nucleus, formed when the Sun’s heat turns its ice into gas.
• Tails: gas and dust pushed away from the Sun by sunlight and the solar wind. The ion tail is thin, straight and bluish, made of charged gas; the dust tail is broad, curved and whitish, made of dust reflecting sunlight. Both point away from the Sun.

(Draw the nucleus as a dot, the coma as a hazy circle round it, and the two tails streaming away from the Sun, the ion tail straight and the dust tail curved.)

Meteoroids are small pieces of rock and metal, from dust grains to boulders, moving through the Solar System; most are crumbs of comets and asteroids. When the Earth’s gravity pulls one into the atmosphere, it enters at 11 to 72 km per second. The air in front of it is squeezed and heated so violently that the meteoroid becomes white-hot and burns up, about 80–100 km above the ground, leaving a bright streak of light across the sky. That streak is a meteor, or shooting star.

Meteor shower: a night when many meteors are seen, all seeming to come from one point in the sky. It happens when the Earth passes through the stream of dust left along a comet’s orbit; the grains burn up as meteors high in the air. Showers come on the same dates every year, such as the Geminids in December, and are harmless.

Observable universe: the part of the universe from which light has had time to reach us since the universe began, 13.8 billion years ago. It is a sphere around us about 93 billion light-years across (because space has stretched while the light traveled). We cannot see anything beyond it, however powerful our telescopes.

• i.They are constellations: groups of stars that make recognizable patterns. Each has a name, often of an animal, a person or an object (a bear, a hunter, a swan, a winged horse, twins). Their stars are at very different distances from us and only appear together. They keep the same shapes for thousands of years. Through the night they all turn around the Pole Star (in the Little Dipper’s handle), and different ones are seen in the evening in different seasons. Some, like Aries, Taurus, Gemini, Cancer and Leo, are zodiac constellations, on the Sun’s path.
• ii.They are a map of the sky. Astronomers use them to say where anything is found (“a comet in Leo”, “a meteor shower from Gemini”), and to find and name stars, planets, galaxies and nebulae. Their positions tell the time of night and the season, and they show directions: the Big Dipper’s pointer stars lead to the Pole Star, which marks north. Learning them is the first step in observing the sky.