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Age of Formation of Earth – Fossils, Rocks and Carbon Dating

Class 7
Jun 5, 2023

Age of Formation of Earth


It is difficult to determine how the Earth formed because it occurred so many years ago, and there were no eyewitnesses, you might assume you need Sherlock Holmes to do it. There are no articles to read, videos to watch on YouTube, or people to interview. Scientists, on the other hand, can look for evidence to point them in the right direction.

Tyrannosaurus rex

Do you recognize the skeleton of this animal? You’re correct if you guessed Tyrannosaurus Rex. T. rex, like other dinosaurs, became extinct 65 million years ago. How do we understand what this long-extinct animal looked like? The solution is right in front of you: it is from fossils left behind.

This Tyrannosaurus rex isn’t a real fossil. It’s only a replica on exhibit in a museum. However, many T. rex fossils have been discovered. Fossils not only reveal the appearance of extinct animals. They also provide information about previous environments and geological processes. In this section, you will learn how scientists use fossils to learn about Earth’s history.


People have been discovering fossils for thousands of years. They have been wondering about the creatures who abandoned them. Myths were inspired by fossils in ancient times. Monsters and other incredible creatures were the subjects of stories. For example, two thousand years ago, dinosaur fossils discovered in China were thought to be dragon bones.


Do you know what fossils are and how they form? And what do you think they can tell us regarding the past?


What are Fossils?

Fossils are the preserved remains or traces of organisms that once lived. The majority of the preserved remains are hard parts like teeth, bones, or shells. The figure below shows examples of these types of fossils. Footprints, burrows, and even wastes can be preserved traces. The figure below shows some examples of trace fossils.

Trace fossils

Trace fossils


Trace fossils

Fossil Formation:

Fossils form in a variety of ways, but the majority form when a living organism (like a plant or animal) dies and is immediately buried by sediment (like mud, sand, or volcanic ash). Soft tissues frequently decompose, leaving only hard bones or shells (but in specific conditions, the soft tissues of organisms can be preserved).

After the organism is buried, more sediment, volcanic ash, or lava can accumulate on top of it, and eventually, all the layers harden into rock (they become lithified). Only when the process of erosion occurs, when the rocks are worn down and washed away, do we see these once-living organisms revealed from within the stones.

Fossil formation (Fossilization)

Fossilization is the process by which the remains or traces of living things become fossils. The majority of fossils are found in sedimentary rocks.

Sedimentary Rocks:

The majority of fossils are formed when a dead organism is buried in sediment. Layers of sediment accumulated over time. The sediment is buried and eventually becomes sedimentary rock. The remains within the rock are also transformed into rock. Minerals replace what is left (remains). The remains are literally transformed into stone. The figure depicts the process of fossilization.

Fossilization and sedimentary rocks

Types of Fossils

Fossils can form in a variety of ways. The organism does not change much when completely preserved. As shown below, tree sap can cover an organism before turning it into amber. The original organism has been preserved so that scientists can study its DNA. Organisms can also be maintained completely in tar or ice.

Molds and casts are other methods for fossilizing organisms. A mold is an imprint of an organism that has been left in the rock. The organism remains entirely degraded. The rock that fills in the mold cavity resembles the original. A cast is a fossil that forms in the mold. Molds and casts are most commonly formed in sedimentary rock.

Compression places an organism’s remains under extreme pressure within rock layers.

Types of fossils: amber, mold and cast, compression

Fossils are Rare

It is extremely unlikely that any organism will become a fossil. Many organisms remain are consumed. Other living things or elements can also break down remains. Bones, for example, are much more likely to become fossils. Even so, they rarely survive long enough to become fossils. Organisms with no hard parts have the lowest chance of fossilization. Soft organism fossils, ranging from bacteria to jellyfish, are extremely rare.

Fossils are our best source of information about the evolution of life on Earth.

Fossil Traces [clues]

Fossils provide information about main geological events. Fossils can also reveal information about past climates.

Fossils of ocean animals are discovered at the summit of Mt. Everest. Mt. Everest is the highest point on the planet. The presence of these fossils indicates that the area was once located at the bottom of a sea. The Himalayan mountain range was formed by uplifting the seabed. The figure depicts an example.

Plant Fossils have been discovered in Antarctica. Antarctica is currently almost entirely covered in ice. The fossil plants demonstrate that Antarctica used to have a much warmer climate.

Fish fossil, plant fossil

Fish fossil, plant fossil

Index Fossils

The ages of rock layers are determined using fossils. Index fossils are the best for this. Index fossils are organisms that lived over a large geographic area. They were only alive for a short time. A scientist can use an index fossil to determine the age of the rock it is found in.

Trilobite fossils are widely known index fossils, as shown in Figure. Trilobites were common marine animals. They lived 500 to 600 million years ago. Trilobite fossil-bearing rock layers must be that old. Various trilobite fossil species can be used to narrow the age even further.

Trilobite fossil

Relative Age of Rocks

Scientists can use fossils to measure the ages of rocks. Some evidence only shows that one rock is older or younger than another. Other types of proof indicate the actual age of a rock in years.

Things are happening in the same way they have in the past. The processes of the Earth have not changed over time. Mountains grow and gradually erode, just as they did billions of years ago. Living creatures adjust to changes in their environment. They evolve over time. Some organisms may be incapable of adapting. They become extinct, which means they are completely extinct.

Historical geologists investigate the past of the Earth. They use evidence from rocks and fossils to determine the sequence of events. They consider how long it took for those incidents to occur.

Laws of Stratigraphy

Stratigraphy is the study of rock strata. Stratigraphic laws can help scientists understand Earth’s past. The laws of stratigraphy are commonly attributed to a Danish geologist named Nicolas Steno. He was born in the 1600s. The laws are depicted in Figure.

Laws of stratigraphy

A: Law of superposition

B:  Law of lateral continuity

C:  Law of original horizontality

D:  Law of cross-cutting relationship

Law of Superposition:

The position of rock layers and their ages are referred to as superposition.

Relative age refers to the age of a rock in comparison to other rocks, either younger or older. The ages of rocks are essential in understanding Earth’s history. Existing rock layers are always deposited on top of new rock layers. As a result, deeper layers must be older than those closer to the surface. This is well-known as the law of superposition.

Law of superposition

Law of Lateral Continuity

Laterally, or out to the sides, rock layers extend. They have the potential to cover very large areas, especially if they are created at the bottom of ancient seas. Although erosion has removed some of the rock, layers on either side of eroded areas will still “match up.”

The figure shows the Grand Canyon. It’s an excellent example of lateral continuity. The same rock layers can be seen on both sides of the canyon. Because the matching rock layers were formed at the same time, they are of the same age.

The Grand canyon

Law of Original Horizontality

In ancient seas, sediments were deposited in horizontal or flat layers. If sedimentary rock layers are slanted (tilted), this indicates that they move after being deposited.

Law of Cross-Cutting Relationships

Another rock, such as the igneous rock shown in Figure, may cut through rock layers. Which rock is the oldest? We use the law of cross-cutting relationships to determine this. The rock layers that have been cut are older than the rock that has been cut across them. Rock D is called a dike that cuts through all of the other rocks.

Cross-cutting relationships


Sedimentary rock layers can tell geologists a lot about Earth’s history. However, there are some places where no rock layers can be found. An unconformity is a gap in the structure of rock layers.

James Hutton discovered the unconformity in the 1700s. Hutton discovered that the lower rock layers are extremely old. The upper layers are significantly younger. Between both the ancient and latest layers, there are no layers. Hutton hypothesized that the intermediate rock layers eroded before the latest rock layers were deposited.

Hutton’s discovery was a watershed moment in geology! Hutton discovered that the rocks were deposited gradually over time. Some have been eroded away. Hutton was well aware that deposition and erosion are both slow processes. He realized that both would take an inordinate amount of time. This made him realize that the Earth is much older than previously thought.


Geological Timescale

The Earth was created 4.5 billion years ago. This time span is divided into smaller periods by geologists. Many of the divisions commemorate significant events in one’s life.

The geologic time scale records divisions in Earth’s history. Geologists correlated rock layers to create the geologic time scale. Steno’s laws were used to calculate the ages of rocks. The older rocks are at the bottom, and the younger rocks are on top.

The order of events could only be shown by the early geologic time scale. That altered with the invention of radioactivity in the late 1800s. Scientists were able to determine the precise age of some rocks in years. The time scale divisions were given dates. For example, the Jurassic period began approximately 200 million years ago. It lasted approximately 55 million years.

The Absolute Age of Rocks

The absolute age of a rock is expressed in years. Absolute ages differ greatly from relative ages. The method for determining them is also distinct. Radiometric methods, such as carbon-14 dating, are used to determine absolute ages. These techniques rely on radioactive decay.

Radiometric Dating:

The decay rate of unstable isotopes can be used to calculate the absolute ages of fossils and rocks. This method of dating is well-known as radiometric dating.

Carbon-14 Dating

Carbon-14 dating is the most well-known method of radiometric dating. Carbon-14 is absorbed by living things (along with stable carbon-12). Carbon-14 is replaced with more carbon-14 as it decays. When an organism dies, it stops absorbing carbon. Carbon-14 is one of them.

The carbon-14 in its body continues to degrade. As a result, as time passes, the organism contains less and less carbon-14. By measuring the ratio of carbon-14 to carbon-12, we can estimate the amount of carbon-14 that has decayed. We know how quickly carbon-14 degrades. We can determine how long ago the organism died using this information.

Age of formation of Earth


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