Geological Time Help

By — McGraw-Hill Professional
Updated on Aug 30, 2011

Introduction to Geological Time

Have you ever thought much about time? Or how long it takes you to do things? How much time it takes to brush your teeth? How long it takes to bake a cake? What is the time difference between riding a bicycle to school instead of walking? How long before your next birthday? How long before your brother finds out you ate the last slice of pizza? What about the amount of time before you get your driver’s license or graduate from high school or start college?

These measurements of time are all common within our daily activities, but what about larger amounts of time? How long will it take before you graduate from college and/or graduate school and start a career? How long before you finish a tour of duty in the Armed Services or Peace Corps? How long before you get married, have children, and grandchildren? How long before humans build a colony on the Moon or Mars or beyond? These things could take decades or even a century or two.

What about travel to distant stars? Without a new, as yet undiscovered fuel to travel faster than the speed of light or the “warp engine” of science fiction that travels through bends and folds of time, travel much beyond our solar system is not practical. It can only be done with current rocket engines if the travelers didn’t want to return. Generational ships that carried families into space on a grand adventure of colonization would also face radiation shielding, life support/environmental issues, micrometeor impacts, physiological adaptations, and many other challenges.

But does the human race have any other choice? From a scientific view, millions of years from now, the Sun will run out of energy and will eventually cease to exist along with most of the planets in our solar system, including the Earth. But that is a bit far out to plan for, so we might as well keep working on the geological problems we have now. To study and learn about our planet is much more fun!

Earth Time

What about time measured only in our imaginations? What about millions and billions of years? What kind of timescale can bridge vast stretches of time?

Time that spans millions of years is known as geological time. The entire history of the Earth is measured in geological time.

Geological time includes the history of the Earth from the first hints of its formation until today. Geological time is measured mathematically, chemically, and by observation.

Figure 2-1 shows a geological time clock with one second roughly equal to one million years.

Geological Time

Fig. 2-1. Over 99.9% of the Earth’s development happened before humans appeared.

In 1785, Scottish scientist James Hutton, called the father of modern geology , began to try to figure out the Earth’s age from rock layers. He studied and tested local rock layers in an attempt to calculate time with respect to erosion, weathering, and sedimentation.

Hutton knew that over the period of a few years, only a light dusting of sediments are deposited in an undisturbed area. He thought that sedimentary rock that has been compacted and compressed, tighter and tighter, from the weight of upper rock layers must have happened over many ages. He also thought that changes in the sedimentary rock layer, through uplifting and fracturing of weathering and erosion, could only have taken place over a very long period of time. Hutton was one of the first scientists to suggest that the Earth is extremely ancient compared to the few thousand years that earlier theories suggested. He thought that the formation of different rock layers, the building of towering mountains, and the widening of the oceans had to have taken place over millions of years.

Hutton wrote the Principle of Uniformitarianism that suggested that changes to the Earth’s surface happened slowly instead of all at once. His early work paved the way for geologists to consider that the Earth was not in its final form, but was still changing. Gradual shifting and compression changes were possible across different continental land forms.

Time Measurements

Ancient people, until about the 17th century, believed the Earth was approximately 6000 years old. This estimate, based mostly on the history of humankind handed down through stories and written accounts, seemed correct. Except for theory, there were no “scientific” ways to check its accuracy. However, in the 1800s scientists began to test rock samples for their age. It was during this time that scientists used dating methods to suggest the Earth might be millions or even billions of years old.

Practice problems of this concept can be found at: Geological Time Practice Test

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