Geologic Time Unit 8.1

Geologic Time In the 18th century, James Hutton recognized the immensity of Earth history and the importance of time as a component in all geologic processes. In the 19th century, others effectively demonstrated that Earth had experienced many episodes of mountain building and erosion, which must have required great spans of geologic time.

Geologic Time Although these pioneering scientists understood that Earth was very old, they had no way of knowing its true age. Was it tens of millions, hundreds of millions, or even billions of years ago?

Geologic Time Because scientists didn’t know exactly how old Earth was, they developed a geologic time scale that showed the sequence of events based on relative dating principles. What are these principles? What part did fossils play?

Geologic Time With the discovery of radioactivity and the development of radiometric dating techniques, geologists can now assign fairly accurate dates to many of the events in Earth history. What is radioactivity? Why is it a good “clock” for dating the geologic past? This unit answers those questions.

Geologic Time The nature of our Earth has been a focus of study for centuries. However, in the late 1700’s James Hutton published his important work The Theory of the Earth, marking beginning of modern geology. Prior to this time, many explanations of Earth’s history was based on supernatural events.

Catastrophism In the mid 1600’s, James Ussher, a respected biblical scholar, published work that had immediate and profound influence. He constructed a chronology of human and Earth history in which he determined that Earth was only a few thousand years old, having been created in 4004 BC. His work was widely accepted and even printed in the margins of the Bible itself.

Catastrophism During the 1600’s and 1700’s, the doctrine of catastrophism strongly influenced people’s thinking about Earth. Briefly stated, catastrophists believed that Earth’s varied landscapes had been fashioned primarily by great catastrophes. These events occurred suddenly, sometimes worldwide, and where the result of unknowable causes that no longer operate.

Birth of Modern Geology Modern geology begin in the late 1700’s, when James Hutton, published his Theory of the Earth. In this work, Hutton put forth a fundamental principle that is a pillar of geology today: uniformitarianism. Uniformitarianism states that the physical, chemical, and biological laws that operate today have also operated in the geologic past.

Birth of Modern Geology This means that the forces and processes that we observe presently shaping our planet have been at work for a very long time. Thus, to understand ancient rocks, we must first understand present-day processes and their results. This idea is commonly expressed by saying “The present is the key to the past.”

Birth of Modern Geology Prior to Hutton, no one had effectively demonstrated that geologic processes occur over extremely long periods of time. However, Hutton persuasively argued that processes that appear to be weak and slow acting, can, over long spans of time, produce effects that are just as great as catastrophic events.

Birth of Modern Geology Unlike his predecessors, Hutton cited verifiable observations to support his ideas. Hutton argued that mountains are sculpted and ultimately destroyed by weathering and the work of running water and that their wastes are carried to the oceans by processes that can be observed.

Geology Today The basic tenets of uniformitarianism are just as viable today as in Hutton’s day. We realize more strongly than ever before that the present gives us insight into the past and that the physical, chemical, and biological laws that govern geologic processes remain unchanging through time.

Geology Today The acceptance of the concept of uniformitarianism, however, meant the acceptance of a very long history for Earth. Although Earth’s processes vary in their intensity, they still take a long time to create or destroy major landscape features.

Geology Today It is important to remember that although many features of our physical landscape may seem to be unchanging over the decades we observe them, they are nevertheless changing – but on time scales of hundreds, thousands, or even many millions of years.

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