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In this lesson, the theory of how the planet Earth formed will be discussed.

This includes looking at how it differentiated into three layers, how it formed its atmosphere, water, and land features, and how it continues to evolve today.

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Formation of the Planet

Earth was formed 4.6 billion years ago from the same nebula cloud of gas and dust that formed the Sun and other planets. Earth back then was very different from Earth now, and it would have been impossible for life to exist on it.

The Earth is still changing even today. It has a molten layer, which causes volcanoes to occasionally erupt, and the crust of the planet is constantly moving, sliding over, under, and sideways against itself. Let’s look at how the Earth may have become like the planet it is now.

With erupting volcanoes and a constantly moving crust, the Earth is changing even today.

Earth Changing Today

You live here. This is our solar system, in one of the arms of the Milky Way galaxy. When the universe began, around 10 billion years ago, Earth wasn’t around. Neither was our solar system.

The Milky Way, was formed in a perfectly ordinary place in the universe in the normal way. Solar systems and the planets within them form from the spinning disks of matter. Slowly, the grains of matter come together to form clumps, then boulders, and eventually balls big enough to have their own gravity coalesce. At this point, these clumped matter are called planetesimals, which just means a small, irregular-shaped body formed by colliding matter.Eventually, the planetesimals grew larger by colliding and combining with other bodies of matter. As the planetesimals grew larger, their gravity was greater, and they collected even more matter. Some of the planetesimals began to orbit the main star, our Sun.

When they do this, they are considered to be a planet, an astronomical object that orbits a star and does not shine with its own light. Earth formed this way about 4.6 billion years ago and was mostly done in about 10-20 million years, although it still continues to change to this day.

Formation of Earth’s Layers

Earth is the third planet, counting outward from the Sun, and the beginning stages of its life were violent.

During the first eons of Earth’s life, it was under continuous bombardment by meteorites and comets. These bombardments helped shape the planet and brought water in the form of ice. They also enriched the Earth with carbon dioxide, methane, nitrogen, and ammonia.At first, the Earth was extremely hot and much larger than it is now. It was made of rock, different compounds, and dense elements, like solid and liquid iron.

As Earth cooled and contracted, the heavier material moved to the center of the Earth to form the core. The liquid material settled over the core to form the mantle. As the Earth cooled more, a solid crust formed over the liquid middle, much like the crust forms on a pan of brownies while the middle is still molten. This is how Earth differentiated into three layers.

Earth is made up of three layers: a core, a mantle, and a crust.
Earth Three Layers

Formation of Water and Atmosphere

As the Earth cooled even more over time, it formed a primitive atmosphere.

The solid crust was covered with active volcanoes that spewed out gases like water vapor, carbon dioxide, and ammonia that add to the helium and hydrogen from the original solar nebula. Light from the Sun broke down the ammonia, which released nitrogen into the atmosphere. It wasn’t until the evolution of bacteria a few billion years later that the atmosphere contained oxygen.The water vapor that was in the atmosphere condensed and formed clouds. As Earth cooled more, the water vapor formed droplets in the clouds, and it started to rain.

This water, along with the ice from the comets, formed the oceans and lakes. The water was all fresh at first but eventually became salty as chemicals from the Earth’s crust were mixed in.

Formation of Land

At one point, most of the surface of the Earth was covered in water, but underneath, the crust was broken into pieces.

Pressure and heat from the molten interior of the planet pushed solid portions above the water to form land. These pieces, called plates, move around on Earth even today, forming and reforming continents, mountain ranges, and valleys.

Pressure and heat pushed material from the molten interior of the Earth above water to form land.
plate tectonics, greenhouse gas emissions, and other natural and human-made conditions.

Learning Outcomes

After watching this lesson, you should be able to:

  • Define planet and planetesimal as well as describe how they form
  • Name Earth’s three layers and understand how they formed
  • Summarize how water, the atmosphere, and land formed on Earth

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