1. The earth takes shape

2. The Earth formed as planetesimals in the solar system collided and combined.

3. When Earth was still a young planet, it had an irregular shape
When Earth was still a young planet, it had an irregular shape. But as Earth gained more matter, gravity became greater than the strength of the rock. Therefore, the rock at the center of Earth was crushed by gravity and Earth started to become round.

4. As the Earth was changing shape, it was also heating up.
Platentesimals continued to collide with the Earth and the energy of their motion heated the planet.
Radioactive material, which was present in the Earth as it formed, also heated the young planet.

5. After Earth reached a certain size, the temperature rose faster than the interior could cool, and the rocky material inside began to melt. Today, the Earth is still cooling from the energy that was generated when it formed.

6. Volcanoes, earthquakes, and hot springs are effects of this energy trapped inside the Earth.
The effects of heat and gravity also helped form the Earth’s layers when the Earth was very young.

7. As rocks melted, denser elements, such as nickel and iron, sank to the center of the Earth and formed the core. Less dense elements floated to the surface and became the crust.

8. The three layers of Earth are:
1. crust
2. mantle
3. core

9. The crust is the thin, outermost layer of the Earth.
It is 5 to 100 km thick.
Crustal rock is made of elements that have low densities, such as oxygen, silicon, and aluminum.

10. The mantle is the layer of Earth beneath the crust.
It extends 2,900 km below the surface.
Mantle rock is made of elements such as magnesium and iron and is denser than crustal rock.

11. The core is the central part of the Earth below the mantle.
It contains the densest elements (nickel and iron) and extends to the center of Earth – almost 6,400 km below the surface.

12. Today, Earth’s atmosphere is 78% nitrogen, 21% oxygen, and about 1% argon.
Scientists think that the Earth’s atmosphere has changed several times since Earth formed.

13. Scientists think that Earth’s first atmosphere was a steamy mixture of carbon dioxide and water vapor.

14. Scientists think that Earth’s first atmosphere contained carbon dioxide and water vapor. Later, volcanoes added carbon dioxide, water vapor, chlorine, nitrogen, and sulfur. Comets brought water, carbon, hydrogen, nitrogen, and oxygen. Solar energy created new chemicals that led to the formation of living organisms. These organisms greatly changed the composition of the atmosphere by adding oxygen.

15. Scientists also think that ultraviolet (UV) radiation, the same radiation that causes sunburns, helped produce the conditions necessary for life.

16. Sometime before 3.4 billion years ago, organisms that produced food by photosynthesis appeared.
During the process of making food, these organisms released oxygen – a gas that was not abundant in the atmosphere at that time.

17. As oxygen levels increased, some of the oxygen formed a layer of ozone in the upper atmosphere. This ozone blocked most UV radiation and made it possible for life, in the form of simple plants, to move onto land about 2.2 billion years ago.

18. Scientists think that the oceans formed when Earth was cool enough for rain to fall and remain on the surface. After millions of years of rainfall, water began to cover Earth and eventually formed the global ocean.

19. Earth’s continents formed as heavy elements sank close to the core of Earth and light elements rose to Earth’s surface. The light elements were light enough to pile upon the surface and began to form the earliest continents. These continents gradually thickened and slowly rose above the surface of the ocean.