Geologic Time Scale

The Absolute Geologic Time Scale

By modern scientific calculations, the earth is many millions of years old. The Absolute Geologic Time Scale has been developed by using a process called “Isotopic Dating”, in which the decay rates of certain radioactive materials are established and measured, then used as “clocks” to calculate the ages of various rocks.

The Geologic Time Scale is illustrated here to help you see the way geologists have broken down geologic time into sections, each with an interesting and fascinating name, and each containing a package of rocks deposited during a certain time period….all over the world.

The scale shows how old certain rocks are in millions of years. For example, look at the Jurassic. You will see the Jurassic rocks were deposited about 138-205 million years Geologic Time Scaleago. The Jurassic was the “age of the dinosaurs.”

A More Detailed Time Scale

Here is a much more detailed time scale, courtesy of the GSA (Geological Society of America). Click on it for a

Detailed Geologic Time Scale (Courtesy GSA)

Detailed Geologic Time Scale (Courtesy GSA)

full-sized view (large file).

The PG is not as interested in the age of rocks in years as she is in the relative sequence of their deposition, and the time period to which they belong. That is because the PG knows that certain conditions existed on earth during each of the geologic time periods. Knowing that a certain rock was deposited in the “Pennsylvanian”, for example, may help the PG to interpret rocks in a certain area.

Also, petroleum geologists are mainly interested in rocks from the Mesozoic and Paleozoic Eras. This is because almost all of the oil and gas found so far is contained within these rocks. These rocks are 60 to 600 million years old, approximately. But of course, these oil and gas-bearing rocks represent only a small fraction of the total age of the earth, which is measured in billions of years.

A Little More Detailed Look:

The geologic time scale is like a big clock that shows the history of Earth and all the events that have happened over millions and billions of years. It helps scientists understand when things occurred, like when certain animals and plants existed or when major changes took place on our planet. Let’s break it down into simpler parts:

What is the Geologic Time Scale?

  1. Division of Time: The geologic time scale divides Earth’s history into different chunks of time. The largest chunks are called eons, which are divided into eras, and then those eras are split into periods. Here’s how it looks from biggest to smallest:
    • Eons
    • Eras
    • Periods
    • Epochs (though epochs are usually talked about later).
  2. Eons: The geologic time scale has four main eons:
    • Hadean Eon: This is the very first time period, starting when the Earth formed about 4.6 billion years ago. It’s called “Hadean” because it was super hot, like hell!
    • Archean Eon: This started about 4 billion years ago and lasted until about 2.5 billion years ago. During this time, the first simple life forms, like bacteria, appeared.
    • Proterozoic Eon: Starting about 2.5 billion years ago, this eon lasted until around 541 million years ago. In this time, more complex life began to develop, including the first jellyfish and algae.
    • Phanerozoic Eon: This eon began about 541 million years ago and goes to the present day. It’s the eon we live in, and it has three main eras.
  3. Eras: The Phanerozoic eon is divided into three eras:
    • Paleozoic Era: This era lasted from about 541 million to 252 million years ago. During this time, life started to move from water to land, and we saw the rise of fish, amphibians, and reptiles.
    • Mesozoic Era: This era lasted from 252 million to 66 million years ago. It’s often called the “Age of Dinosaurs” because dinosaurs were the most important animals during this time. Plants like flowering plants also became more common.
    • Cenozoic Era: This is the era we are in now, starting 66 million years ago and continuing to today. This era is known as the “Age of Mammals” because after the dinosaurs went extinct, mammals (including humans) became the dominant animals.
  4. Periods: Each era is split into periods that mark significant events in Earth’s history, such as the appearance of different types of plants and animals. For example:
    • The Jurassic Period is part of the Mesozoic Era and is famous for dinosaurs like the T. rex and long-necked sauropods.
    • The Quaternary Period is part of the Cenozoic Era and includes the time when humans appeared and developed.

Why is it Important?

The geologic time scale helps geologists understand how life and Earth have changed over time. By studying rock layers and fossils, they can put together a timeline of events, learn about ancient environments, and even see how species evolved and adapted.

In summary, the geologic time scale is a way to track the history of our planet, showing how life has changed and evolved throughout billions of years, just like a big timeline that helps us understand Earth’s past!