The Geologic Time Scale

Learning Objectives

Explain the purpose of the geologic time scale.

List the four major divisions of geologic time (eons) in order.

Identify the three eras of the Phanerozoic Eon (Paleozoic, Mesozoic, Cenozoic) and a key feature of each.

Understand that the geologic time scale is based on major changes in Earth's geology and life forms.

Earth's Diary

The Earth is incredibly old—about 4.6 billion years old! To comprehend this vast history, geologists have created a timeline called the geologic time scale. This scale organizes Earth's history into different units based on major events, such as the formation of continents, mass extinctions, and the evolution of new life forms.

The largest divisions of the time scale are Eons. Earth's history is divided into four eons.

1. Hadean Eon (4.6 to 4.0 billion years ago)

This is the very beginning of Earth's history. The name 'Hadean' comes from Hades, the Greek god of the underworld, because during this time the Earth was a hot, hellish world. It was a molten ball of rock with constant volcanic activity and frequent impacts from asteroids. No life existed yet.

2. Archean Eon (4.0 to 2.5 billion years ago)

During the Archean (meaning 'ancient'), the Earth began to cool down. The first oceans formed, and the first continents began to appear. Most importantly, the first life on Earth evolved during this eon. These were very simple, single-celled prokaryotic organisms, similar to bacteria.

3. Proterozoic Eon (2.5 billion to 541 million years ago)

'Proterozoic' means 'earlier life'. This was a time of great change.

Oxygenation: Photosynthetic bacteria evolved and began to pump huge amounts of oxygen into the atmosphere. This 'Great Oxygenation Event' was toxic to much of the early life but paved the way for more complex life to evolve.

First Eukaryotes: The first eukaryotic cells (cells with a nucleus) appeared.

First Multicellular Life: Near the end of the eon, the first simple multicellular organisms evolved.

These first three eons, which cover almost 90% of Earth's history, are collectively known as the Precambrian.

4. Phanerozoic Eon (541 million years ago to Today)

'Phanerozoic' means 'visible life'. This is the eon we are currently in. It is marked by the appearance of abundant and complex animal life. Because life was so diverse, this eon is subdivided into three smaller units called Eras.

Paleozoic Era ('Ancient Life'): Started with the Cambrian Explosion, a relatively rapid burst of evolutionary diversity where most major animal groups first appeared. This era saw the rise of fish, insects, amphibians, and reptiles. The continents were joined together as the supercontinent Pangaea. The era ended with the largest mass extinction event in Earth's history.

Mesozoic Era ('Middle Life'): This is the Age of Reptiles, dominated by the dinosaurs. The first mammals and birds also appeared during this era. The supercontinent Pangaea began to break apart. The era ended with the mass extinction that wiped out the dinosaurs.

Cenozoic Era ('Recent Life'): This is the Age of Mammals. After the dinosaurs went extinct, mammals diversified and became the dominant large animals on land. This is the era in which humans evolved.

Key Terms

**Geologic Time Scale: A timeline that organizes the events in Earth's long history.
**Eon: The largest unit of geologic time. The four eons are the Hadean, Archean, Proterozoic, and Phanerozoic.
**Era: A subdivision of an eon. The Phanerozoic Eon is divided into the Paleozoic, Mesozoic, and Cenozoic eras.
**Precambrian: The informal name for the first three eons of Earth's history (Hadean, Archean, and Proterozoic), which covers the time from Earth's formation up to the appearance of complex animal life.
**Mass Extinction: An event in which a large number of species on Earth die out over a relatively short period of time.