Theories About How Oceans Were Formed

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  • 0:01 Earth Is a Wet Planet
  • 1:19 Ocean Formation
  • 3:08 Keeping Water on…
  • 4:58 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Today, more than 2/3 of Earth is covered with water, mostly in our oceans. But Earth itself is much older than our oceans. In this lesson you'll explore theories on how our oceans formed, as well as why liquid water remains on Earth even after all this time.

Earth Is a Wet Planet

About 70% of Earth's surface is covered with water. And most of that, about 97%, is in the oceans. That's right - all the glaciers, ice caps, aquifers, groundwater, rivers, lakes, and streams only make up 3% of Earth's surface water!

But the surface of Earth was not always so wet. In fact, Earth was a very dry place for some time after its formation. It's believed that Earth is about 4.5 billion years old, and that our oceans are only about 3.8 billion years old.

There are two main theories describing how the oceans on Earth formed. First, it is theorized that Earth was formed with water and that this water came from the inside out. The other theory is that Earth did not contain any water when it was formed and it was instead brought here by other sources as they hit the young planet.

We don't know for sure which is true, or if one is truer than the other. Many scientists favor the first theory as the major source of Earth's ocean water, but most believe it is some combination of the two. For this lesson, we'll discuss the formation of the oceans using both theories so you can see how each method could have contributed to the process.

Ocean Formation

Our solar system originally started as a large swirling cloud of various gases, ice, and dust particles. Eventually, these components aggregated together in different ways to form the planets that make up what is now our solar system.

Early Earth was a very inhospitable place. It is believed to have been formed from meteorites, but it didn't start out with the neatly divided layers we have today: the inner core, the outer core, the mantle, and the crust. As this differentiation, or geological separation of layers by density was occurring, there was also much releasing of gases onto Earth's surface through volcanic activity, a process called outgassing.

And, while all this was going on, Earth was also being bombarded with materials from outer space, such as comets. These space materials also brought gases to Earth, and eventually these gases as well as those from outgassing formed our early atmosphere.

Earth's early atmosphere was H-O-T! It trapped heat on Earth like a greenhouse so that temperatures on the surface were possibly as high as 400 °C! While water did not exist in the atmosphere in a gaseous state, it was way too hot for any liquid water. It would have simply boiled in that extreme heat, instantly turning to gas and rising into the sky.

Throughout the next several million years, the atmosphere and surface cooled. Temperatures came down to below 100 °C, which allowed water to remain in liquid form. Much of the water vapor in the atmosphere fell to Earth and filled shallow basins in the ground that eventually became the large, deep oceans we have today.

Keeping Water on Earth's Surface

As we have seen with other planets, just because there is surface water at one time doesn't mean it will stay there. For example, it is likely that both Venus and Mars once had liquid water on their surfaces, but there is no evidence that it still exists on either planet.

Retaining water on the surface of a planet is tied to several different factors. First, Earth is in a 'sweet spot' in the solar system. This is because it's just close enough to the sun to stay warm, but far enough away to stay relatively cool.

It's also a good size. Earth is large enough that its gravity holds the atmosphere in place, protecting Earth's surface from both receiving and losing too much heat from solar radiation.

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