Geological time

Introduction

This chapter looks at rocks and fossils. It explains how, by studying different rocks, scientists managed to find out about different events in the history of the Earth. It looks at the law of superposition and a geological column. It also looks at the division of time into eras, periods and epochs.

Rocks and fossils

Finding out about the events of ancient Earth might seem impossible, but by studying rocks we can study the history of our planet. Different natural forces form different types of rocks. Some rocks are made of tiny pieces of ancient sea creatures, some are made of cooled-down volcanic lava and others are formed by different types of rocks melted and mixed together by volcanic heat. Rocks also preserve things like traces of water ripples from rivers that existed millions of years ago, as well as pieces of ancient plants and animals.

The science that studies the history of the Earth as told by the rocks is called geology. The science that studies the traces of living things preserved in rocks is called palaeontology. The traces of ancient plants or animals preserved in rock are called fossils. After studying fossils and rocks for many years, scientists were able to find out about the age of our planet, its ancient landforms and the events that changed them. Scientists are also finding out about the history of life on Earth by studying different rocks and fossils.

The law of superposition

Because our planet was formed a very long time ago (about 4600 million years ago) scientists divided the Earth's age into shorter divisions of time such as eras, periods and epochs. A geological era lasts for hundreds of millions of years. Each era is divided into periods, and periods are dived into epochs.

The people of ancient Greece were the first to discover - and explain scientifically - fossils. That was the first important step in understanding that rocks and minerals take a very long time to form. Later, scientists discovered that the rocks found at the deepest levels were the oldest. In geology it is called the law of superposition. Superposition means the placement of one thing (a layer of rocks in this case), on top of another. By studying rocks further, scientists discovered that some of them had fossils, while others did not. Some layers of rocks were hard while others were soft and loose. In addition, all the rocks were coloured differently and had different textures.

Today, scientists divide all rocks into three main groups. This division is based on the rocks' texture and the time period during which the rocks were formed. The oldest group of rocks is called primary, which means first. These rocks lie deep down underneath the Earth's crust. Primary rocks were formed by active volcanoes of the early Earth. They do not have fossils. The next layer of rocks up is called secondary. These rocks were formed after the primary rocks. Some of the secondary rocks have fossils. That means they were formed during the time when different creatures were already living on Earth. The loose, soft rocks which are carried by rivers belong to the youngest group of rocks called surface materials.

Geological column

Geological time is the division of time into major periods of biological and geological activity.

• The Precambrian Era. Precambrian time makes up about seven eights of the Earth's age. Some sources refer to Precambrian time as a super-eon, which is the longest unit of geological time. Other sources refer to it as an era, which is a large division of geological time. In this unit we will refer to it as the Precambrian era.  Primitive life forms like algae and fungi appeared on Earth during this era. Various minerals, metals and gems that we use today were also formed during this period of time.
• The Palaeozoic Era. More complex, multi-cellular life forms evolved during this period of time. Sea creatures, like crabs and jelly-fish, make up most of the fossil record of this era.
• The Mesozoic Era. Many land life forms evolved during this era. Scientists have discovered many fossils of dinosaurs, crocodiles, birds and flowering plants that were common during this period.
• The Cenozoic Era. Most of the currently-existing life forms evolved during this period of time. At the beginning of this era, the Earth's climate was warmer than it is today and some of the continents were still joined together.

The geological history of the Earth is often drawn as a geological column which looks like an imaginary slice of the Earth's crust. The slice is made up of different layers of rocks. Each layer belongs to a different period of time in the Earth's history. The layers of rocks forming the geological column are called geological systems. Geological systems and geological periods have the same name, meaning that the rocks were formed in that period. When parts of geological systems have different textures they are called groups. If geological systems are made of the same material, they are called formations. See image 2.

All the changes of geological time periods are based on changes in the fossils and rocks that make up the Earth's crust. Rocks and fossils change during major environmental changes. Events like a volcanic eruption, a sudden climate change or an earthquake would form different types of rocks and change the population of living organisms in the area. See image 3.

It is difficult to define the borders between many eras, periods and epochs. Scientific research groups may have different opinions. That is why different books about geology may have conflicting information about the dates for the same period of time.

See animation 1.