Dating the Earth Study Guide (page 3)

Updated on Sep 25, 2011

The Geological Time Scale

Planet Earth condensed from a gas nebula and was brought together by gravity about 4.6 billion years ago. The entire time from that fiery origin to today has been carefully divided by geologists into various named intervals of time. This is something like the way our calendar divides the year into months, weeks, days, hours, and so forth.

To create the geologic time scale, geologists have relied to a large extent on the presence of different kinds of organisms, preserved as fossils, who have inhabited the earth at different times. For example, thanks to the movie Jurassic Park, most people know that one of the times that dinosaurs lived is called the Jurassic. As we go back in time, the fossil record becomes more and more incomplete. Indeed, for much of Earth's history, the only fossils are tiny, single-celled, mineralized organisms that don't look very distinct because they are only round, microscopic spheres or elongated rods in rock. So the divisions of geologic time are not as finely resolved back in the early Earth as the divisions are within, say, the last 100 million years.

The biggest scale of the divisions (like dividing the year into months) are called eons. The earliest eon was the Hadean (time of "hell"). There were still many bombardments from space coming to Earth. It lasted to about 3.9 billion years ago.

The next eon was the Archean, from 3.9 to 2.5 billion years ago. Life originated early in the Archean, but it was all singled-celled.

After the Archean came the Proterozoic, about 2,500 to 680 million years ago. (We've switched to millions of years ago, because the dating becomes better.) This eon saw a great rise in atmospheric oxygen about 2,000 million years ago. Near the end of the Proterozoic, multicelled life evolved (organisms with many cells to their bodies, such as simple worms).

Skipping the Vendian, we come to the eon called Paleozoic (545–250 million years ago). It started with an explosion of new kinds of life forms. By the end of the Paleozoic, plants had evolved to tall trees on land, and the dominant life forms on land were giant amphibians and early reptiles.

Next came the eon we are currently in, called the Phanerozoic. It is divided into eras, which are subdivided into periods. For example, the Mesozoic era (248–65 million years ago) is itself subdivided into three main periods called Triassic, Jurassic, and Cretaceous. A mass extinction at 65 million years ago ended the dinosaurs and the Mesozoic era.

The final era (the subdivision of the Phanerozoic called the Cenozoic) is the age of mammals, beginning 65 million years ago and lasting until today. The most recent—two million years or so—is the epoch (a subdivision of a period) called the Pleistocene, a time of the growth and then retreat of giant ice sheets, in cycles of about 100,000 years each. At the final deglaciation, about 10,000 years ago, geologists end the Pleistocene and start a new epoch, called the Holocene.

During these billions of years, plate tectonics have shaped Earth's surface. Continents have combined and split repeatedly, oceans have widened and disappeared, mountain ranges have risen and then eroded to hills, and rocks formed under water in sed iments have been lifted into plateaus and mountains.

To see the geologic time scale in more detail, study Figure 8.2. Then move right on to the practice questions. You'll need to use the figure to help you with some of the answers.

Figure 8.2 The Geologic Time Scale

Practice problems of this concept can be found at: Dating the Earth Practice Questions

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