Life on Earth - the planet Earth
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Life on Earth

Molten lava
Molten lava

Most of the molecules that smashed together to make the Earth were iron - the smashing together released a lot of heat, so the whole center of the Earth is made of hot melted iron. Iron is heavier than most other metals, so it sank to the center of the Earth, while lighter molecules like silicon and carbon rose to the top. At the outermost part of the Earth were the lightest molecules, hydrogen, helium, and nitrogen, making our atmosphere. Most of the hydrogen and helium was so light that it floated back out into space, so more than three-quarters of our air is made of nitrogen. Perhaps about the same time, because Jupiter's gravity was messing with their orbits, a lot of comets made of ice smashed into the Earth. When they reached Earth, the ice melted into water, and the water boiled to make a steamy atmosphere. Ultraviolet light from the sun (the rays that give you sunburns now) hit the water molecules and broke them apart into hydrogen and oxygen. A layer of oxygen (the ozone layer) formed and blocked most of the ultraviolet rays. Along with the comets, in the ice, came amino acids that had formed in space out of carbon and hydrogen atoms.

Granite cliffs
The Earth might have looked
like these granite cliffs (New Zealand)

But space is cold, so by a little less than four billion years ago (about 3,800,000,000 years ago), the surface of the Earth had cooled down enough so that the silica began to cool down into solid rock - these were the first igneous rocks. That was the first land. On this cooler earth, the steam began to cool back into water, and the water ran down wherever the rocks were lower, and that made the first oceans. This cycle where water condenses and makes oceans and then evaporates again caused the beginnings of weather - rain, wind, snow, thunderstorms, and so on.

This water, and the wind in the air, gradually eroded the igneous rocks and made sand and dirt and clay, which then fell into the water. The water pressed the sand and clay together and made the particles into new sedimentary rocks like shale and sandstone. Now the atmosphere was mainly made of carbon dioxide molecules.

By about three and a half billion years ago, long strings of amino acids began cooperating to make the earliest living cells. Soon these cells began cooperating with other cells, uniting to form more complicated cells that could use photosynthesis to make energy out of sunlight and carbon dioxide. These cells broke apart the carbon dioxide and used the energy to eat and the carbon to repair and reproduce themselves, but they didn't want the oxygen. So the atmosphere gradually got more and more oxygen molecules in it. By around 2,500,000,000 years ago, there was so much oxygen that some cells began to use oxygen for energy instead of carbon dioxide, and by about 1,500,000,000 years ago, plants and animals with more than one cell began to develop.

Meanwhile, the center of the Earth was still mainly melted iron, and the silica crust was floating on that melted iron. So the land drifted around, sometimes forming one big continent, and sometimes breaking up into a lot of smaller continents. This is known as plate tectonics. About a billion years ago, all of the land was lumped together into one continent, and then it broke up and the continents floated apart. Around half a billion years ago, the land all smashed together again, and then floated apart again. About 250,000,000 years ago, in the time of the dinosaurs, the land all came together again, and now it is all floating apart again. Some of the pieces smash into each other as they float around, and that's what makes big mountain ranges like the Himalayas, the Alps, and the Rocky Mountains.

Learn by doing: the Earth
More about the Earth

Bibliography and further reading about planets:

More about Earth
Planets
Physics
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Professor Carr

Karen Eva Carr, PhD.
Assoc. Professor Emerita, History
Portland State University

Professor Carr holds a B.A. with high honors from Cornell University in classics and archaeology, and her M.A. and PhD. from the University of Michigan in Classical Art and Archaeology. She has excavated in Scotland, Cyprus, Greece, Israel, and Tunisia, and she has been teaching history to university students for a very long time.

Professor Carr's PSU page

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