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The History of Planet Earth – Geological Timescale

Grade 6
Jun 5, 2023

Geological Timescale


Geologic time is the number of years since the planet Earth first started to form. Geologists are researchers that focus on the structure and past of Earth. Their field of study is termed geology.


How Rocks and Fossils Depict Earth’s History

Geologists examine rocks and fossils, or the traces of extinct organisms that have survived underground.

Rocks and fossils depict Earth’s history from the time of the formation of its crust billions of years ago to the present.

A time scale that serves as a “calendar” of Earth’s geologic history has been mapped out by geologists.


The formation of the Earth’s crust about 4 billion years ago marks the beginning of geologic time. The planet Earth is slightly older than this, but at the time of its formation, the planet was a hot, thick liquid. The planet’s surface solidified as it cooled. The oldest rocks discovered so far, according to geologists, date back roughly 4 billion years. They assume that the crust of the Earth must be roughly the same age.

The oldest rocks

Geologic Record

Layers of rock made of pebbles, sand, and mud that were left behind by glaciers, wind, or water make up the Earth’s surface.

The most recent layers are piled on top of the oldest ones, which are located further below.

Geologists who studied these rocks discovered that each layer contained distinct and distinguishable fossils.


Additionally, they discovered that the layers were similar everywhere. The fossils recovered in each layer of rock were then dated and categorized by geologists.

Geologic Record

Earth’s History

Geology is the scientific study of how rocks are formed. The study of solid substances, such as rock or rock layers, is the main focus of this science. Additionally, the rock layers convey the Earth’s history and life, particularly that which is recorded in them.

The stratum that is generated from deposits or piles of layers can be studied for many years in order to better understand the meaning of the term “rock strata.”

When a single rock is made up of several (multiple parallel layers) layers, strata is the term used. To indicate a massive collection of deposited sediments, the word “strata” is used, and stratum is a single layer of rock.

Stratigraphy is the study of the features of stratified rocks, including their relationship to time.

By examining the record of earlier events that are recorded in the rocks, we can learn more about Earth’s history.

rock strata


The layering of such rocks or sediment is known as stratification. Sedimentary rocks are formed by the accumulation or deposition of mineral or organic particles at the Earth’s surface, followed by the cementation of the deposits naturally over time that can be visually distinguished from adjacent beds or layers.

The majority of the rocks that are visible at the Earth’s surface are sedimentary, which means that they were created from fragments of older rocks that were blown apart by wind or water.


Relative Age

In rivers, lakes, and seas, the gravel, sand, and mud sink to the bottom. On the lake or ocean floor, these sedimentary particles may bury both alive and dead creatures and plants.

The sediments at the bottom of the pile turn into rock with the passage of time, the addition of new particles, and frequent chemical changes. Sand turns into sandstone. Mud turns into mudstone or shale, gravel turns into a conglomerate, and plant and animal fragments can turn into fossils.

The formation of rock layers and the order in which they occurred are both depicted by strata.

Scientists can establish the relative age of each rock layer if they are aware of the sequence in which the rock layers originated.

Relative age– It indicates that one layer is older or younger than another layer but does not indicate the rock’s age in years.

Relative age

Relative Positions of Sedimentary Rocks

The relative locations of sedimentary strata were explored by the Danish scientist Nicholas Steno as early as the mid-1600s. In agreement with their relative weight or size, he discovered that solid particles separate from a fluid.

The biggest or heaviest things land first, and the smallest things land last. Layers, sometimes known as beds, are created in the rock as a result of small variations in particle size or composition. The most noticeable characteristic of sedimentary rocks is layering or bedding.

Relative positions of sedimentary rocks

The Principle of Superposition

Layers of sedimentary rocks are built up, one on top of the other, particle by particle and bed by bed. As a result, in any layered rock sequence, a given bed must be older than any bed on top of it.

Because it identifies the relative ages of rock layers and the fossils they contain at any given site, the Law of Superposition is crucial to understanding Earth’s history.

The layer at the bottom was deposited first and is, therefore, the oldest, according to the superposition principle.

Layers of sedimentary rocks

The Principle of Horizontality

Particles from the air or water settle and cause layered rocks to build. As per Steno’s Law of Original Horizontality, the majority of sediments were first deposited horizontally.

Many of the stratified rocks, however, are no longer horizontal. We know that sedimentary rocks that are not horizontal either were created in unique ways or, more frequently, were shifted from their horizontal position by subsequent processes, such as tilting during episodes of mountain formation. This is because of the law of Original Horizontality.

Original Horizontality

The Principle of Original Lateral Continuity

The idea of original lateral continuity states that rock strata that were initially deposited extends laterally until they thin out or run into another rock layer that cuts them off.

original lateral continuity states

Stratigraphy in the Copper Basin, Idaho.

These important principles serve as the foundation for the geologic science of stratigraphy, which is the study of stratified rock (strata).

A geologist studying stratigraphy in the Copper Basin, Idaho. These rock layers were deposited horizontally and uplifted later, so they are now tilted at an angle (along the red arrow).

stratified rock

Grand Canyon Rock Layers

Rock layers provide clues about conditions on Earth when the layers formed.

Grand Canyon rock layers

Geological timescale of Earth's History


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