Geological Evidence of Earth's Formation

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

How did the Earth form? When did it form? And more importantly, how do we know? Learn about the evidence we have for the formation of the Earth. See how many clues you picked up by taking a quiz.

The Formation of Earth

Determining how Earth formed isn't easy. You might think you need Sherlock Holmes to do it because it happened so long ago and there were no eyewitnesses. There's no newspaper article to read, YouTube video to watch, or people to interview. But scientists certainly can look for clues to lead them in the right direction.

Modern science has tools that help determine the ages of rocks, but there's a few problems. First, erupting volcanoes gradually cover Earth and replace the land with fresh new rock that hides the older rock below. This is like a criminal who takes a cloth and cleans away his fingerprints. And then there's the fact that we can only dig so far underground, so we can't tell how old the rocks are that we might find really deep down.

But like Sherlock Holmes, scientists are clever and have figured out a lot. Before the sun formed, it was surrounded by a disk of dust, rock, and metals. We think the sun formed before any planets, from a nebula, which is a cloud made up of bits and pieces left over from old stars that have exploded. The force of gravity pulls everything in the universe together, and it's thought that gravity pulled those lumps of rock together to form the planets in our solar system that we know and love today.

It wasn't an easy process though. At first, Earth contained seas of hot magma, which cooled very slowly over several hundred million years. There were also lots of random rocks constantly hitting Earth. But how do we know all this for sure? What clues led us to these ideas?

An artistic impression of the forming solar system
Artist Impression of Forming Solar System

Evidence for the Formation of Earth

We have good evidence for the formation of Earth, but we're still collecting more today. One piece of the puzzle is aging rocks, a process called radiocarbon dating. This process utilizes what scientists know about radioactive materials, which are materials that are unstable but gradually turn into a stable material over time. We know the rate of that process, so by looking at the amount of radioactive elements left in a rock we can do some math to figure out how old those rocks are.

Using this, scientists were able to age rocks that were 3.5 billion years old! So they knew Earth must be at least that old. But later, they found even older rocks made of zirconium silicate crystals in Australia, and determined their age to be 4.3 billion years.

The Grand Canyon, with its layers upon layers of rock, is a great place to find really old rocks for radiocarbon dating
The Grand Canyon is a great place to find really old rocks to date

But how do we know there aren't even older rocks to find? We can only dig so deep on Earth, so to go further we have to look to space. By looking at rocks on Mars, asteroids, the moon, and meteorites from other planets, we can see how old other parts of our solar system are. So far, the oldest thing found is an asteroid that hit Earth 5000 years ago in Arizona, and was dated to 4.53 to 4.58 billion years. We know the whole solar system formed around the same time, and even contains the same mix of materials. So this tells us that Earth is also at least 4.53 billion years old.

Meteorites can tell us the age of other parts of the solar system
Meteorites can tell us the age of other parts of the solar system

But how do we know that the solar system formed the way it did? The laws of gravity given to us by Newton explain why the planets orbit the sun the way they do, and why our solar system orbits the center of the Milky Way galaxy. They explain why balls fall to the ground, and how NASA rockets make it safely into space. Everything we see fits with these laws, and help us figure out how the solar system formed.

A page from the Principia, where Newton published his laws of gravity
A page from the Principia, where Newton published his laws of gravity

There are other pieces of evidence: young solar systems that are still forming look how we would expect them to, most of the solar system orbits in the same direction, and planets close to the sun contain less hydrogen than ones further out.

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