Page 4--Geologic Time

Figure 5. Xiphactinus was a type of fish that lived in the sea that covered western Kansas in the Cretaceous Period of the Mesozoic Era. This one, which is about 13 feet long, was found in the late 1800's and is on display at the University of Kansas Museum of Natural History (photo courtesy of the University of Kansas Museum of Natural History).

After years of studying the rocks, minerals, and features of the Earth, geologists have learned much about its geologic history. They have determined that the Earth is approximately 4 1/2 billion years old, give or take a few million years. The Earth is so old, no one knows its exact age.

Because 100 years seems like a long time, something billions, even millions, of years old is almost impossible to imagine. To make it easier to understand geologic history, scientists have divided all the time since the Earth was formed into four eras.

Figure 6. Scientists have divided time into units called eras. Eras are divided into periods, and periods into epochs. Dinosaurs lived during the Triassic, Jurassic, and Cretaceous Periods of the Mesozoic Era. Wel live in the Holocene Epoch of the Quaternary Period of the Cenozoic Era.

The first era, called the Precambrian, was much longer than any of the others. It lasted almost 4 billion years, or 88 percent of the Earth's history. Although the Precambrian Era includes much of that time, we know very little about it. Rocks formed during the Precambrian have gone through many changes; in Kansas and many other places, these rocks can only be found hundreds and even thousands of feet underground, making them difficult to study.

After the Precambrian Era, more plant and animal species began to develop. During the Paleozoic Era, which followed the Precambrian and lasted 245 million years, plants and reptiles began moving from the sea to the land. The era has been divided into seven smaller units of time called periods. Several times during the Paleozoic Era, seas appeared and disappeared in Kansas. Rocks from the last three periods in the era, called the Mississippian, the Pennsylvanian, and the Permian periods, can be found at the surface in central and eastern Kansas.

Dinosaurs appeared and disappeared during the Mesozoic Era, which lasted 160 million years. Not many dinosaur fossils have been found in Kansas. During the first part of the Mesozoic, much of the surface in Kansas was being eroded. If dinosaurs lived in Kansas then, they weren't preserved. Later in the era, in the Cretaceous Period, much of the state was covered by seas. Because the land was under water, not many dinosaur fossils have been found, but bones from giant swimming and flying reptiles have been recovered in western Kansas chalk.

We're now in the Cenozoic Era, which began about 63 million years ago. Because it is the most recent era, we know the most about it. The glaciers that moved into Kansas from the north came during this era. Later, giant dust storms carried the soil away from some areas and deposited it in other places. As mountains were formed by volcanic activity in the western United States, layers of volcanic ash were deposited in Kansas.

Figure 7. This spiral shows the length of each era and period. Start at the bottom. If the spiral were 10 miles long, you would travel nearly nine miles before reaching the end of the Precambrian Era. The Paleozoic Era would be a little more than half a mile long or about the length of 11 football fields. The Mesozoic Era would be less than half a mile, or about eight football fields long. The Cenozoic Era would only be 200 yards long, or the length of a couple of football fields. People in Kansas would appear about two inches from the edge. As time passes, the spiral will slowly continue to grow, about one inch every 7,500 years (after Geologic Time, U.S. Geological Survey publication).

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Kansas Geological Survey
Updated March 13, 1996
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