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Geologic Time

ImageDoes it sometimes seem as if the school day will never end? If you think it takes a long time for school to let out, imagine how long it must take for a mountain to form or an ocean to dry up. It takes millions or even billions of years. The long periods of time that it takes for major changes to happen on Earth are called geologic time. Geologic time stretches from when the Earth first formed until today.


Geologists (scientists who study Earth) have special names for periods of time on Earth. They name stages in the history of Earth just as you name stages in the life of a person.

You say a person was a baby and then a child and then a teenager and then a grown-up. Geologists use the words eon, era, period, and epoch when they talk about the stages of Earth.

An eon is the longest of the time periods. The Archean Eon started about 3.8 billion years ago, after Earth formed from gas and dust in space. It ended about 2.5 billion years ago. The Proterozoic Eon started 2.5 billion years ago and lasted until about 570 million years ago. Then the Phanerozoic began. We are living in the Phanerozoic Eon.

Geologists divide eons into eras just as you divide years into months. They divide eras into periods. They divide periods into epochs.


Geologists group all the time periods before the Phanerozoic into a time they call the Precambrian. The atmosphere and oceans formed during Precambrian time. Simple kinds of life formed during this time. These tiny creatures changed Earth’s air. By the end of Precambrian time, the air was similar to the air we breathe today.

Things really started happening in the Phanerozoic Eon. Lots of invertebrates (animals without backbones) appeared at the beginning of the eon. Geologists say the Phanerozoic Eon has three eras called the Paleozoic, Mesozoic, and Cenozoic.

The Paleozoic Era lasted from about 570 million to 240 million years ago. There were many different kinds of life on Earth during this time. At the end of the Paleozoic Era almost all life on Earth was wiped out. Scientists are trying to find out what happened.


Dinosaurs lived during the Mesozoic Era. This era began about 240 million years ago and lasted until about 65 million years ago. It had three periods called the Triassic, Jurassic, and Cretaceous. The movie Jurassic Park is named for the Jurassic Period.

All the dinosaurs went extinct (died off) at the end of the Mesozoic Era. Scientists think that a space rock called an asteroid may have crashed into Earth at that time. The crash may have changed Earth so that dinosaurs could no longer live here.

Mammals, animals with fur or hair that feed their young on milk, survived the extinction. Lots of mammals appeared during the Cenozoic Era. Mammals started getting bigger. Geologists say the Cenozoic Era has two periods called the Tertiary and the Quaternary. Horses, cats, dogs, and huge mammoths—hairy, elephant-like creatures—appeared during the Tertiary Period.


The earliest ancestors of human beings appeared about 6 million years ago during the Tertiary Period. These ancestors were shorter, hairier, and less smart than people are today.

Geologists divide the next period, the Quaternary, into the Pleistocene and Holocene epochs. Modern people did not appear until about 200,000 years ago in the Pleistocene Epoch.

During the Pleistocene, huge sheets of ice covered most of Earth. These times were called ice ages. The last Ice Age ended about 10,000 years ago at the beginning of the Holocene Epoch. We are living in the Holocene—it hasn’t ended yet.


Geologists use layers of rock to tell geologic time. Rock on Earth formed in layers. The deepest layers are usually the oldest layers. The layers on top are the youngest layers.

Layers of rock have fossils in them. A fossil is the remains or print of an animal or plant that lived long ago, at the time the rock layer formed. Scientists have found fossils in rocks all over the world. They have found fossils of different plants and animals in each layer of rock.

Fossils in the deepest layers show scientists what kind of plants and animals first lived on Earth. Fossils in other rock layers show what kinds of plants and animals lived during the different eras, periods, and epochs.

Scientists have tests for telling how old a rock is. They use the tests and look at fossils in rock layers to put together the history of Earth.

Source: Microsoft ® Encarta