Geologic time is a measure of the Earth's life span. In the past, people thought that the world was only a few thousand years old. Today we know that the actual age of the Earth is much greater, approximately 4.56 billion (4,560,000,000) years. To put things in perspective, if all of geologic time were reduced to one year, the first human-like beings would arrive at about 11:30 pm on December 31, and the first modern humans would arrive about 11:59 pm - just in time to watch the New Years' Eve countdown!
A big part of what geologists do is to analyze rocks for clues about the Earth in order to understand how the planet has developed over time. Rocks tell us that Earth is constantly changing. For example, they show us that all of the continents were joined together into one huge land mass at least twice in the last billion or so years and that the Atlantic Ocean is only about 200 million years old.
Before it was known that the Earth is immensely old, people who thought the Earth was only a few thousand years old thought that the landscape was formed by catastrophic events, such as enormous floods. This explanation is known as Catastrophism. However, even though the actual age of the Earth was not known until the mid-1900's, some scientists in late 1800's described evidence for an Earth that was much older than a few thousand years. If so, then even very slow rates of change could result in huge changes, given a wide expanse of time. This concept is known as Uniformitarianism. Most recently, with the age of the earth firmly established, scientists see geologic change as being largely the product of slow processes over long periods of time punctuated by occasional catastrophes, such as meteorite impacts, of local to global importance.
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