Age of Earth

A timeline shows when events happen. For example, you could show your life on a timeline, beginning when you were born and ending with today.

The Story of Earth Timeline

In your science journal, answer these questions based on your timeline.

1. Birth year

2. A memory from childhood before starting school.

3. Age you started school.

4. A memory from elementary school.

5. A memory from this school year.

During its history, our planet has changed. Early Earth looked much different than today.

Like a science-fiction movie set, Early Earth was a place of erupting volcanoes, pouring lava, and a red sky. Poisonous gases were in the atmosphere. Back in those days, people could not have lived on Earth.

Earth Before and After

Earth Before and After

Earth scientists use radiometric dating, index fossils, and other clues to figure out the age of Earth. Using these methods, they know that Earth is 4.6 billion years old. It did not form until 9.1 billion years after the Big Bang.

One way scientists tell the timeline of Earth is with the geochronological scale. The geochronological scale shows layers of Earth stacked on each other, with newer layers on top. Older layers have the remains of animals and plants and different rocks.

Catastrophism v. Uniformitarianism

Changes to Earth can happen because of a big event, like the planet being hit by a meteor, or smaller events, such as when soil erodes and changes the landscape over the years.

Some people think big events mostly shaped our planet. This is called catastrophism—like the word catastrophes. Others believe our planet changed mostly because of smaller, slower processes. This is called uniformitarianism.

Scientists tend to prefer uniformitarianism because many changes to Earth have happened slowly. But there are times when big events suddenly change the Earth.

Let’s Practice

Relative Dating

Relative dating is a process used to place rocks and events in the order in which they occur.

An index fossil helps scientists figure out how old a rock is. Index fossils were created by organisms living only a short time, so scientists know the rock must be from that time. Because index fossils are spread across the Earth, rocks in different parts of the world can be from the same time if they both have an index fossil of the same plant or animal. Other fossils within that rock also are the same age as the index fossil.

After a fossil has been cleaned, it can be studied in a laboratory. Using microscopes, scanners, and different kinds of analysis, scientists can figure out a fossil's age and explore its features. Scientists can learn about an animal's diet and the environment by looking at fossil teeth and bones. DNA analysis from fossils might show how different animals related to each other. Fossils help solve the mystery of what life was like on Earth long ago.

Giant Fish Fossil

Giant Fish Fossil

In your science journal, answer these questions based on the "Giant Fish Fossil."

1. How old do you think the fish fossil is—100 years old, 12,000 years old, or 5 billion years old?

2. What kinds of living things could become a fossil?

3. What does a paleontologist do?

4. In the lab, fossils show that some bones of early humans were the remains of food eaten by big cats. What might you conclude?

As you have seen, rocks on the surface of Earth are younger than rocks towards the center.

Scientists use Steno's laws to describe patterns in Earth's rock layers.

Law of Superposition

Superposition refers to the position of rock layers and their relative ages. Younger rocks stack on top of older rocks. Rocks closer to the surface are younger than rocks at the bottom of the stack farther down.

Law of Superposition

Law of Cross-Cutting Relationships

One rock may cut over or across another rock, causing this second rock's layers to be cut off. The rock that gets cut is older.

Cross-Cutting

Law of Original Horizontality

Rock layers are flat or horizontal because rock sediments were deposited in ancient seas horizontally. If a rock layer is not flat, it must have moved after it formed, showing that something changed at that point.

Horizontality

Law of Lateral Continuity

Horizontal rock layers go from side to side, extending laterally, as shown in the Grand Canyon image. For instance, the same rock layers can be seen on both sides of a canyon. Knowing the age of a layer on one side can be applied to the other.

Grand Canyon

Relative dating is based on the Law of Superposition, in which rocks in the top layers formed more recently than those in the lower layers.

Absolute Dating

A "tree ring" is an example of absolute dating. A tree grows one tree ring for every year that it is alive. For pine trees, one tree ring includes two bands – one light and one dark.

See the source image

Tree Ring

In your science journal, answer these questions.

1. How many trees rings equal one year?

2. Have you ever noticed a tree has rings?

3. Is tree rings a method of absolute dating?

Absolute dating is used to determine the more precise age of a rock. It is a method of measuring the age of an object such as a rock or fossil. Scientists use both absolute and relative dating to develop geologic time scales. Absolute dating requires the use of a natural clock. The clock is the radioactive decay of certain natural elements like uranium and carbon.

Radioactive decay refers to how unstable atoms lose energy and matter over time. Unstable atoms are called radioactive isotopes. As a result of radioactive decay, an isotope turns into another element over a period of time. An element is a substance composed of only one kind of atom.