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Investigation of Rocks and Fossils – The Significance

Class 7
Jun 5, 2023

Investigation of Rocks and Fossils


Scientists have predicted that the Earth formed along with the solar system around 4.6 billion years ago. Several scientists have put forth various hypotheses regarding the origin of Earth. “Nebular Hypothesis” is the most popular theory of the origin of Earth. It was proposed by Immanuel Kant, a German philosopher.

This theory states that the solar system is formed from a big cloud of dust and gas known as a nebula. Scientists have predicted that around 3 billion years ago, the first life on Earth appeared. It was a tiny unicellular prokaryote. Scientists have identified stone imprints of these microbes. The remains or traces of ancient life that have been preserved by natural processes are known as “fossils.”

Fossils are important evidence of evolution. They act as an important tool for understanding the geological history. Scientists make use of these fossils to calculate the geological time scale. The geological time scale provides the calendar for the events of Earth’s history. It divides Earth’s history into geological time periods. Based on fossil records, scientists have classified geological time scales into four subdivisions, namely – Eons, eras, periods, and epochs.

Geological time scale

Plate Tectonics and Rock Cycle:

Plate tectonics is a widely accepted scientific theory. According to this theory, the lithosphere of the Earth is composed of a large number of tectonic plates. These tectonic plates move slowly, thereby resulting in the formation of various landforms on Earth. Plate tectonics is directly associated with the rock cycle.


Plate tectonics results in the formation of igneous rocks. These igneous rocks, in turn, form the basis of the rock cycle. The rate of movement of tectonic plates determines the production and destruction of three rock types.

Types of rocks

Types of rocks

Plate tectonics and rock cycle


The remains or traces of ancient life that have been preserved by natural processes are known as “fossils” (Latin: fossils – something dug up from the ground). Fossils are important evidence of evolution. Fossils are known as our window to the past. They also help us to learn about the future.


Examples of fossils: Bones, shells, impressions of animals/bacteria on stones, exoskeletons, petrified wood, hair, coal, etc.

The oldest fossils identified so far include hematite tubes, stromatolites, seaweed-like fossils, pikaia fossils, redlichiida, metaspriggina, Tortotubus and Rhyniognatha hirsti.

Significance of Fossils:

The study of fossils and the evolution of life on Earth is known as “Paleontology.”  Fossils provide evidence about ancient life on Earth. Scientists use fossils to understand the process of evolution. They also provide evidence about the changes in Earth’s surface over time.

Fossils provide information about ancient environments and climates. Fossils act as important evidence to calculate the age of rocks. Fossils are used as important evidence to study geological history. Geologists make use of fossil records to create geological maps and discover new mineral deposits.

How are Fossils Formed?

Fossils represent the preserved remains of plants and animals. During ancient times, when plant/animal bodies used to get buried in sediments, seas, lakes, or rivers, their soft body parts decayed fast while the hard parts like bones, shells, and teeth get replaced by minerals and hardened into rocks.

During ancient times, when the remnants of animals or plants became hardened and fixed within a sedimentary rock or volcanic ash, it led to the formation of fossils. When the remnants of living organisms get saturated and hardened with minerals like calcite or silica, they get transformed into stone. Most of the fossils are found in sedimentary rocks.

The Following are Four Main Ways of Fossil Formation:

  • Petrification
  • Compression
  • Molds and casts
  • Preserved remains

Types of Fossils:

The Five Types of Fossils are:

  • Body fossils
  • Molecular fossils
  • Trace fossils
  • Carbon fossils
  • Pseudo fossils

Fossil formation

Fossil formation

Fossils and geological time scale

Dating Rocks and Fossils:

Scientists use fossils as direct evidence to study the biological events of Earth’s history.  Scientists use fossils to calculate the age of rocks. Fossils that are used to calculate geological periods are known as index fossils. They are also known as guide fossils or indicator fossils.

Index fossils are found in layers of sedimentary rocks. These fossils act as an index to a particular geological period. Scientists make use of these fossils to calculate the age of rocks. Scientists use two main approaches to calculate the age of rocks and fossils.

  • Relative dating
  • Absolute dating

Relative dating involves analyzing the relative position of rock layers. It is based on the principle that younger rocks are positioned on top of older rocks. Using relative dating, scientists are able to determine which rocks are younger and which are older. But they are not able to determine the exact age of rocks.

Principles of Relative Dating:

Relative dating is based on five major principles.

  1. Younger rocks are formed on top of older rocks. Therefore, deeper layers of rocks are older than the layers that lie above them.
  2. Layers of rocks that contain the same types of fossils are considered to be of the same age.
  3. When rocks are found to be enclosed within another, then the enclosed rock is considered to be older, and the surrounding rock is considered to be younger.
  4. Anything that cuts through a rock (crack/dike) is considered to be younger than the rock.
  5. A strong break between rock layers is known as ‘unconformity.’ It represents a break (missing rock records) in the geological record. Rocks below the unconformity are predicted to be formed millions of years before the rocks that are present above them.

Absolute dating is used to find the exact age of rocks and fossils. Scientists predict the exact number of years since they formed. This technique is also known as numerical dating or radiometric dating. Scientists determine the age of rocks by analyzing the amount of radioactive material contained in them.

Relative dating of rocks and fossils

Use of index fossils to date rocks

Relative dating vs absolute dating of rocks and fossils

Dating rocks and fossils

Investigation of rocks and fossils


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