Earth's water came from different places. Condensing water vapour, and comets and asteroids hitting Earth, made the oceans. Within a billion years (that is at about 3.6 billion years ago) the first life evolved, in the Archaean era. Some bacteria developed photosynthesis, which lets plants make food from the Sun's light and water. This released a lot of oxygen, which was first taken up by iron in solution. Eventually, free oxygen got into the atmosphere or air, making Earth's surface suitable for aerobic life (see Great Oxygenation Event). This oxygen also formed the ozone layer which protects Earth's surface from bad ultraviolet radiation from the Sun. Complex life on the surface of the land did not exist before the ozone layer.
Earth's land and climate has been very different in the past. About 3 to 3.5 million years ago almost all land was in one place. This is called a supercontinent. The earliest known supercontinent was called Vaalbara. Much later, there was a time (the Cryogenian) when Earth was almost entirely covered by thick ice sheets (glaciers). This is discussed as the Snowball Earth theory.
Earth is rocky. It is the largest of the rocky planets moving around the Sun by mass and by size. It is much smaller than the gas giants such as Jupiter.
Overall, Earth is made of iron (32.1%), oxygen (30.1%), silicon (15.1%), magnesium (13.9%), sulfur (2.9%), nickel (1.8%), calcium (1.5%), and aluminium (1.4%). The 1.2% left over is made of many different kinds of other chemicals. Chemicals that are very uncommon (such as gold and platinum) can be very valuable.
The structure of Earth changes from the inside to the outside. The center of earth (Earth's core) is mostly iron (88.8%), nickel (5.8%), sulfur (4.5%), and less than 1% other things. The Earth's crust is largely oxygen (47%). Oxygen is normally a gas but it can join with other chemicals to make compounds like water and rocks. 99.22% of rocks have oxygen in them. The most common oxygen-having rocks are silica (made with silicon), alumina (made with aluminium), rust (made with iron), lime (made with calcium), magnesia (made with magnesium), potash (made with potassium), and sodium oxide, and there are others as well.
Earth's shape is a spheroid: not quite a sphere because it is slightly squashed on the top and bottom. The shape is called an oblate spheroid. As Earth spins around itself, centrifugal force forces the equator out a little and pulls the poles in a little. The equator, around the middle of Earth's surface, is about long.
The highest mountain above sea level—the well-known Mount Everest (which is above sea level)—is "not" actually the one that is the farthest away from the center of the Earth. Instead, the sleeping volcano Mount Chimborazo in Ecuador is; it is only above sea level but it is almost at the equator. Because of this, Mount Chimborazo is from the center of the Earth, while Mount Everest is closer to it. Similarly, the lowest point below sea level that we are conscious of is the Challenger Deep in the Mariana Trench in the Pacific Ocean. It is about below sea level, but, again, there are probably places at the bottom of the Arctic Ocean that are nearer to the center of the Earth.
The deepest hole ever dug is only about . We know something about the inside of the Earth, though, because we can learn things from earthquakes and the times when volcanoes erupt. We are able to see how quickly the shock waves move through Earth in different places.
The inside of Earth is very different from the outside. Almost all of Earth's liquid water is in the seas or close to the surface. The surface also has a lot of oxygen, which comes from plants. Small and simple kinds of life can live far under the surface, but animals and plants only live on the surface or in the seas. The rocks on the surface of Earth (Earth's crust) are well known. They are thicker where there is land, between thick. Under the seas they are sometimes only thick. There are three groups of rocks that make up most of the Earth's crust. Some rock is made when the hot liquid rock comes from inside the earth (igneous rocks); another type of rock is made when sediment is laid down, usually under the sea (sedimentary rocks); and a third kind of rock is made when the other two are changed by very high temperature or pressure (metamorphic rocks). A very few rocks also fall out of the sky (meteorites).
Below the crust is warm and almost-liquid rock that is always moving around (the Earth's mantle). Then, there is a thin liquid layer of heated rock (the outer core). This is very hot: . The middle of the inside of the Earth would be liquid as well but all the weight of the rock above it pushes it back into being solid. This solid middle part (the inner core) is almost all iron. This is what makes the Earth magnetic.
The Earth's crust is solid but made of parts which move very slowly. The thin level of hard rock on the outside of the Earth rests on hot liquid material below it in the deeper mantle. This liquid material moves because it gets heat from the hot center of the earth. The slow movement of the plates is what causes earthquakes, volcanoes and large groups of mountains on the Earth.
There are three ways plates can come together. Two plates can move towards each other ("convergent" plate edges). This can form islands (such as Japan), volcanoes, and high mountain ranges (such as the Andes and Himalayas). Two plates can move away from each other ("divergent" plate edges). This gives the warm liquid rock inside the earth a place to come out. This makes special mountain ranges below the sea or large low lands like Africa's Great Rift Valley. Plates are able to move beside each other as well ("transform" plate edges, such as the San Andreas Fault). This makes their edges crush against each other and makes many shocks as they move.
The outside of the Earth is not even. There are high places called mountains, and high flat places called plateaus. There are low places called valleys and canyons. For the most part, moving air and water from the sky and seas damages rocks in high places and breaks them into small pieces. The air and water then move these pieces to lower places. Because of this, the Earth would have been very flat a long time before now. The fundamental cause of the differences in the Earth's surface is plate tectonics. The shape of the entire planet itself is not even a ball. Because of its velocity, Earth has a slight bulge at the Equator. Other than that, Earth is shaped more like a pear than an actual sphere.
All places on Earth are made of, or are on top of, rocks. The outside of the Earth is usually not uncovered rock. Over 70% of the Earth is covered by seas full of salty water. This salty water makes up about 97% of all Earth's water. The fresh water people can drink is mostly ice. Only a very small amount is in rivers and under the Earth for people to drink and use. The air above the Earth stops the water from going away into outer space. Also, much of the land on Earth is covered with plants, or with what is left from earlier living things. Places with very little rain are dry wastes called deserts. Deserts usually have few living things, but life is able to grow very quickly when these wastes have rainfall. Places with large amounts of rain may be large woods. Lately, people have changed the environment of the Earth a great deal.
All around the Earth is a large amount of air (the atmosphere). The mass of the Earth pulls the gasses in the air down and does not let them go into outer space. The air is mostly made of nitrogen (about 78%) and oxygen (about 21%) but there are a few other gasses as well. Most living things need the air (or parts of the air gripped in the water) to breathe and live. They use the gasses—especially oxygen and carbon dioxide—to make and use sugar and to give themselves power.