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Volcanoes
A volcano—often, but not always, a cone-shaped mountain—is an opening in the Earth’s crust through which molten rock, called magma erupts. When a volcano erupts and hurls out its red-hot rock, this is one of the most awesome events of nature. It happens at a hole, crack or weak point in the solid rocks of the Earth’s crust. Melted rock called magma from deep...
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Hot-spot volcanoes
Many volcanoes form near the edges of tectonic plates—but not all of them. Hot-spot volcanoes are found above mantle plumes, areas beneath the Earth's crust where magma from the mantle wells up. In some places, often far from the plate edges, the magma forces its way through cracks in the crust and erupts on to the ocean floor or in the centre of continents. These...
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San Andreas Fault
The San Andreas Fault stretches more than 1200 kilometres (750 miles) along the coast of California. It forms the boundary between two plates, the Pacific plate and the North American plate, that are sliding past one another in opposite directions. The jerky movements result in constant, tiny shocks. Occasionally, pressure builds up over the years and is released in a...
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Caves
Caves are underground holes in the rock. Some caves open up when the ground splits, as in an earthquake. Some are eroded by waves hurling stones and pebbles at a cliff. But most are made in limestone, a kind of sedimentary rock, by a chemical process. Rainwater naturally contains tiny amounts of acid. It trickles into cracks and reacts with the rock made of soluble...
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Coastlines
Coastlines are where ocean or sea meets land. Some coastlines are shaped by the action of waves crashing against them, wearing away the rocks. Along others, sand and shingle are piled up in beaches or mudflats. The coast is a continuing battle between sea and land. Sometimes the sea “loses” as shingle, sand or mud piles up and the land grows. In other places, the...
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Deserts
A desert is an area with very low rainfall, usually with less than 25 centimetres (10 inches) of rain or snow a year. Other than at oases, places where underground water comes to the surface, there are few signs of life. Deserts may be hot all year round, as in the Sahara of Africa, or always cold, as in Greenland or Antarctica. The Gobi Desert of Central Asia is hot...
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Glaciers
A glacier is a moving mass of ice. Some glaciers snake down mountain valleys, while others such as the ice sheets of Greenland or Antarctica are so huge and thick they almost totally cover the land. Although it is solid, ice can flow down slopes and around bends, although much more slowly than a river—often less than a metre (3 ft 3 inches) per day. Glaciers occur...
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Limestone landscapes
Limestone, a type of sedimentary rock, consists of the remains of tiny sea creatures that lived millions of years ago. It is made of calcium carbonate, a substance that dissolves easily in water. Because of this, limestone landscapes, also known as karst topography (from the landscape around the Kras Plateau in Slovenia), have a number of distinctive features. These...
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Rivers
Rivers are natural channels that carry rain, melted ice and snow downhill from mountains and uplands to lowlands, lakes and seas. The world’s longest rivers are the Nile in Africa and the Amazon in South America, both about 6600 kilometres (4100 miles) long. Rivers shape the land as they flow over rocks of varying hardness, widening and deepening their valleys by...
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Weathering and erosion
Over the millions of years of geological time, great mountain ranges have been forced up—then disappeared. Colliding tectonic plates, faulting and other earth movements have created these mountains. What has happened to them? Most have been ground down by the processes of weathering (attack by rainwater, frost and temperature change) and erosion (the removal of rock...
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Folds and faults
Sliding plates and drifting continents are responsible for some of the Earth’s major landscape features. If a large slab or plate of the Earth’s surface is gradually squeezed, the solid rock slowly wrinkles and crumples. Its layers become wavy folds. When, in other places, rocks are stretched or bent they crack or split along weak points. These cracks are known as...
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Great Rift Valley
The biggest gash in the Earth’s surface on land is the Great Rift Valley. This series of rifts runs from the eastern Mediterranean southeast through the Dead Sea and Red Sea, then south across East Africa through Lake Turkana. It divides around Lake Victoria to continue south to Lake Tanganyika and Lake Malawi. The entire valley system is some 5000 kilometres (3000...
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Mountains
In those regions where tectonic plates push against one another, the land ‘‘folds up” like a rug on the floor. This process, called orogeny, is how many mountain ranges, over periods of millions of years, are formed. The great mountain ranges of the world, including the Andes in South America, the Rockies in North America, the Alps in Europe and the Himalayas in...
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Rift valley lake: Lake Baikal
Millions of years ago, Asia split apart and the floor of the Baikal valley dropped away along a series of giant faults, cracks in the Earth’s crust. The bedrock plunges to a depth of at least 9 kilometres (nearly 6 miles), the deepest point on any of the continents. For many millions of years, parts of the lake bottom remained dry, but eventually all of the basin...
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Black smokers
Snaking across the ocean floor is a vast undersea mountain chain known as the Mid-Oceanic Ridge. Here, the Earth’s crust is gradually spreading apart and magma, hot molten rock from beneath the crust, rises to the surface of the seabed. In some places along the Ridge, water seeping down into the rocks is heated by the magma. It shoots up through cracks in the ocean...
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Ocean floor
Much of the ocean seems completely empty: a great expanse of featureless water, dotted with the occasional island. But beneath the waves it is a different picture. Between 4000 and 5000 metres (13,000–16,500 feet) down lies a dramatic landscape of vast abyssal plains, jagged mountains, erupting volcanoes and deep canyons. The undersea world amounts to nearly...
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Tides
Sea levels rise and fall twice a day. This pattern is known as the tides. The tidal range is the difference between high and low tide. Seas that are almost surrounded by land, such as the Mediterranean Sea, have a very small tidal range. Tides are caused by the the combined gravitational pull of the Moon and the Sun and the rotation of the Earth. Most coastlines have...
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Waves
As wind moves across the surface of the ocean, the water turns over and over in circles, forming waves. The stronger the wind and the wider the expanse of ocean it has blown across, the greater the waves. The wave itself moves along, but the water in a wave does not. It spins around in the same place and makes the water below turn, too. In mid-ocean, large waves...
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Atmosphere
The atmosphere is an “ocean of air” that surrounds our planet. A mixture of gases, mainly nitrogen (four-fifths) and oxygen (one-fifth), it gets thinner or less dense with height and fades away completely about 800 kilometres (500 miles) above the ground. Here the atmosphere ends and the nothingness of space begins. Besides providing oxygen for us to breathe, the...
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Earth's magnetic field
The Earth has its own magnetism—an invisible field of magnetic force all around us. Too weak to notice in daily life, the magnetic field affects iron-based materials and other magnets. We can detect it using a magnetic compass. The compass needle is a long, thin magnet that lines itself up with Earth’s magnetism to point north-south. This helps us to read maps and...
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Planet Earth
The Earth is a spinning ball of rock and metal. It is one of eight planets that orbit, or circle, our nearest star, the Sun. Its surface is made up of oceans and land masses called continents. A layer of air called the atmosphere surrounds the whole planet. The Earth’s outer layer, the crust, is a thin, rocky shell. Beneath the crust lies the mantle, a thick layer...
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Igneous rocks
Igneous rocks are formed when magma, molten rock from beneath the Earth’s crust, rises, cools and solidifies. Intrusive igneous rock, such as granite, is formed when magma becomes solid in cracks or chambers beneath the ground. Extrusive igneous rock, such as basalt, is formed when the magma solidifies above the ground quite quickly. This happens when magma seeps...
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Metamorphic rocks
Metamorphic rocks are formed when rocks are subjected to such great pressure and heat that their mineral composition is altered. The original rocks may be sedimentary, igneous or metamorphic rocks. Marble, slate, gneiss and quartzite are all examples of metamorphic rocks. Some minerals in rocks undergoing metamorphism may be transformed into gemstones.
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Rocks
Rock is the hard material that makes up the Earth’s crust, its hard “shell”. Rock lies beneath all natural soil layers—and everything else: city streets, a sandy beach, even the ocean. Of course, bare rock lies at the surface itself in many places, such as at the tops of mountains or on sea cliffs. Rocks are stony masses made up from minerals. Minerals, in...
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Sedimentary rocks
Sedimentary rocks, such as sandstone and mudstone, are made from sand, gravel, mud and other fragments of rock that result from erosion. Limestone and chalk, also types of sedimentary rock, is made of the remains of tiny marine plants and animals that lived millions of years ago. The fragments settle in layers in lakes, rivers and seas. As more layers settle on top of...
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Geological time
The Earth is 4.6 billion years old. This extremely long stretch of time is difficult to imagine, so events in the Earth’s history are therefore measured in geological time, spans of millions of years. A “recent” event in geological time might be, for example, the current Ice Ages that peaked about 20,000 years ago when ice caps extended over the continents of...
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Climate change
Climates change over time. In the past, the Earth has been both hotter and cooler than it is now. During the Age of Dinosaurs 100 million years ago, there were no polar ice caps and tropical forests grew in temperate lands. More recently, 20,000 years ago, ice caps extended from the North Pole as far as northern Europe during the Ice Ages. Between about 1550 and 1800...
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Climate
Weather varies from day to day around the world. Over a longer period, especially many years, each region has a regular pattern of rain, wind, temperature and other weather features. This long-term pattern of weather is called climate. A region's climate is the result of a number of factors: its latitude, which affects the amount of heat it receives from the Sun; its...
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Clouds
When water vapour cools, it turns back into liquid water. This is called condensation. Water droplets form around tiny particles in the air, such as sea salt and dust. Millions of these droplets gather together to form clouds. In the highest clouds, the water freezes into ice crystals. Clouds form at different heights and have different shapes. There are a variety of...
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Tropical cyclones
Tropical cyclones are massive spinning storms with strong winds (120 km/h or 75 mph and above) that form over the ocean in tropical areas. These storms are referred to as tropical cyclones only in the Indian Ocean and south Pacific. In the northwestern Pacific Ocean they are called typhoons and in the Atlantic Ocean and northeastern Pacific they are known as...
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Monsoons
Billions of people depend on the arrival of the monsoon every summer to the parched lands of southern Asia. Monsoons are winds that reverse direction each year. This happens when there are great differences in temperature between the oceans and the land. Cool, dry winds are replaced in the summer by warm, moist winds from the Indian Ocean. Heavy rains then arrive.
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Storms
There are many different kinds of storm. Most involve severe, violent weather with regions of high winds and heavy rain, and perhaps a sudden change in temperature. Some have thunder and lightning. They move across sea and land and may cause great damage and loss of life. Powerful winds in tornadoes or hurricanes blow down buildings and bridges and toss cars and...
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Tornadoes
The most destructive storms on the planet, tornadoes are twisting columns of air, swirling at up to 400 km/h (250 mph). A tornado is even more powerful than a hurricane, although the damage it causes usually only affects a narrow strip of land. Its base may be only 100 metres (about 300 feet) across, but the winds are so strong it destroys nearly everything in its...
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Weather
Weather is the conditions and changes that take place in the lower atmosphere, up to about 20 kilometres (about 12 miles) high. It includes temperatures by day and night, wind speed and direction, cloud type and cover, rain, hail, snow, frost, ice, droughts and storms. Weather can change by the minute or from day to day. The study of the weather is called meteorology.
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What causes winds to blow?
Wind is air moving from one area to another. When warm air rises, cooler air flows in to take its place. Winds are the result of the Sun’s uneven heating of the Earth’s surface. As a result, some places warm up more quickly than others. In the tropics, the surface is hot. The air rises and cooler air from both to the north and the south blow in to replace it...
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Great volcanic eruptions
Volcanic eruptions, the most powerful explosions on Earth, are incredibly destructive. During the eruption of Krakatoa (today called Krakatau), a volcano situated between the islands of Sumatra and Java in Indonesia, in 1883, about 20 cubic kilometres (5 cubic miles) of rock—nearly the entire exploded mountain—was blasted high into the sky. The noise of the...
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Eruption of Vesuvius
In AD 79, the volcano Mount Vesuvius in southern Italy erupted violently. It was the days of the Roman Empire, then at its height. During the eruption, nearby Roman towns and cities, notably Pompeii and Herculaneum, were completely destroyed and buried by fast-moving pyroclastic flows, avalanches of red-hot gas and rock particles. The towns were rediscovered centuries...
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Earthquake survival
Thousands of earthquakes occur every year, some of them major ones. But only where these quakes affect densely populated regions is there a great loss of life, usually resulting from the collapse of large buildings or from flooding caused by powerful tsunamis. New buildings in earthquake-prone areas can be designed to withstand tremors and absorb shocks—and thus...
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Tsunamis
When the epicentre of an earthquake is on the sea bed, a large submarine landslide can result. The sudden movement of the thousands of tonnes of water above it produces a series of fast moving waves, travelling at around 800 km/h (500 mph), called tsunamis (also sometimes known as tidal waves). The wave travels out in all directions at hundreds of kilometres an hour...
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Seismology
Seismology is the study of earthquakes and how their shock waves travel through the Earth. Seismic waves produced by deliberate explosions are one of the main methods of underground exploration, used by seismologists to look for oil wells and geological faults. They can even help with finding long-buried meteorite craters. For example, the Chicxulub Crater, which was...
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Grand Canyon
The Grand Canyon is a great gorge that twists across the dry, rocky region of Arizona, USA for nearly 450 kilometres (280 miles). It is one of the world’s most spectacular examples of river erosion. The Colorado River cut a deeper and deeper channel as the land over which it flowed slowly began to push upwards. The soft rock layers wore away more easily than the...
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Coral atolls
Layers of lava from eruptions eventually build underwater volcanoes high enough for them to rise above sea level as islands. In tropical waters, coral reefs sometimes form in the warm shallows around these volcanic islands. After they stop erupting, the volcanoes will, over thousands of years, sink back into the Earth's crust. But the reef keeps growing upwards so...
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Oceans
The oceans cover more than 360 million square kilometres (139 million square miles) of the Earth’s surface, approximately 71% of its total area. More than 1350 million cubic kilometres (324 million cubic miles) of water is contained within it, representing nearly 97% of the Earth’s entire supply. The oceans hold enough salt in them to cover Europe to a depth of 5...
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Ocean currents
The ocean waters move about the globe in great “rivers” called ocean currents. Surface currents have a big effect on the world’s climates. They make polar regions milder and the tropics cooler. There are two kinds of ocean currents. Surface currents are driven by the winds and move in roughly circular patterns, called gyres. These go round in a clockwise...
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Earth's interior
On the outside, the Earth seems hard and solid. But if you could make a journey down a deep hole almost 6400 kilometres (4000 miles) to the centre of the planet, you would notice several changes as you descend. It becomes warm, then hot. This comes from what is called geothermal energy. The average increase in temperature is about 3°C for every 100 metres (8°F for...
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Plate tectonics
The Atlantic Ocean gets wider by about the width of your thumb every year, pushing North and South America away from Europe and Africa. The Himalayan mountains, already the highest in the world, grow taller by about the length of your thumb every year. Many other parts of the Earth are moving and changing shape, too. This is because the Earth’s outer layer is...
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Continental drift
The vast landmasses we call the continents have not always been where they are today. Earth’s outer shell is divided up into large slabs, called tectonic plates. These plates, which include both the continents and the floors of the oceans, shift very slowly about. This is called continental drift, part of the process known as plate tectonics. Over hundreds of...
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Fossils
Fossils are remains of once-living things preserved in rock. Most living things are eaten or die and their soft parts rot away leaving no trace. But sometimes hard body parts remain, like the shells, bones, teeth, horns and claws of animals and the bark, cones and seeds of plants. These are the parts most likely to form fossils. Trace fossils are not actual body parts...
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Story of the Earth
Planet Earth is one of eight planets orbiting the Sun, one of billions of stars in the Milky Way Galaxy, itself one of billions of galaxies in the Universe. The Universe came into existence 13.7 billion years ago in an incredibly hot, dense explosion called the Big Bang. A few billion years later our Galaxy began to form, but it was not until about 4.6 billion years...
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Seasons
In tropical regions it is hot all year round, but farther north or south the temperature varies through the year. These different periods, spring, summer, autumn and winter, are called the seasons. They occur because of the way the Earth orbits the Sun. The Earth’s orbit is not an exact circle around the Sun, but an oval-like ellipse. Also, the Earth spins each day...
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Amphibians
Amphibians are a small group of cold-blooded, vertebrate animals. They include two main groups: the salamanders and newts, and the frogs and toads. The word “amphibian” means “double life”. Most amphibians spend the first part of their lives underwater, breathing through gills. Their adult lives are generally spent on land, breathing through lungs. They...
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Frogs
Frogs are short, tail-less amphibians with long hind legs and smooth skin. They spend most of their time in water. Their very long legs and streamlined bodies mean that they can make huge leaps and swim quickly. Adult frogs are carnivorous, feeding on insects, snails or worms. Larger frogs will also eat fish, other frogs and even small mammals. There are about 5800...
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Salamanders and newts
Salamanders and newts are long-bodied amphibians with long tails. They are often nocturnal, and are all carnivorous. They feed on insects, worms, slugs and snails, and some also prey on frog tadpoles. Some salamanders live in water all the time, and several kinds even keep their gills into adulthood. Others live on land but return to the water to lay their eggs. Many...
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Toads
Toads are amphibians with short legs, fat bodies and lumpy skin. There are more than 500 species of true toads, called bufonids. They live mostly on land, in dark, damp places to keep their skins moist and cool. Toads crawl or hop rather than leap like frogs. Adults are carnivorous, feeding on insects, snails or worms. Larger toads will also eat fish, frogs and even...
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Adélie penguin
The Adélie penguin is the commonest Antarctic penguin. It is medium-sized with a distinctive white ring around each eye. Every spring, Adélie penguins undertake a long migration to their rookeries (breeding grounds). Here they raise their young over the Antarctic summer, while food is most plentiful. Like all penguins, the Adélies are fast and graceful swimmers...
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Albatrosses
The albatrosses are a family of large sea birds. Both "great albatrosses", the wandering albatross and royal albatross, have wingspans of up to 3.4 metres (11 feet)—the largest of any living bird. Their wings are narrow and can lock in one position, enabling the birds to soar on ocean winds for days at a time, using little energy to cover great distances...
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Birds
Birds are warm-blooded, vertebrate animals with four limbs, the front two of which are adapted into wings. They are the only animals that have feathers. Birds have light, hollow bones and a toothless beak. Most can fly, but a few cannot. Birds lay hard-shelled eggs, and most species protect their young until they leave the nest. Some species migrate to warmer climates...
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Birds of prey
Birds of prey include eagles, falcons, kites, buzzards, harriers, hawks, vultures and the osprey. Often called raptors from the Latin word rapere, to seize, they are all meat-eating birds, with sharp, hooked beaks for tearing flesh and strong talons for gripping or killing prey. They swoop down through the air, often at very high speeds, on to their prey: small...
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Bustards
Bustards are ground-nesting birds that live on the plains of Africa, Europe and Asia. They have heavy, brown bodies and long legs. They feed mostly on leaves, seeds and invertebrates, especially grasshoppers, beetles and caterpillars. The kori bustard and great bustard are the world's heaviest birds capable of flight. Although they are strong flyers, they rarely take...
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Game birds
Chickens, turkeys, pheasants, guineafowl, peafowl, grouse, quail and partridges are among the birds known as game birds. The scientific name for the order, which includes around 250 species, is Galliformes. Strong runners, they have short wings and are ungainly flyers, most being unable to fly for long periods. They feed on seeds, fruits and invertebrates. Many kinds...
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