Geologic time (SG-03)
Geologists describing rock units as being older or younger than others are using relative time. When they say that a rock or fossil is 350 million years old, they are using absolute time. The first is comparative; the other is chronological.

An important contribution of geology to science is the concept of the immensity of geologic time. Although a geologic event may have lasted from minutes to hundreds of years, its record and the evidence of similar events may stretch back millions or perhaps billions of years.

If the earth is about 4.5 billion years old, an analogy may aid us in comprehending such a vast span of time. If the earth formed at 12:01 on January 1, the first living things appeared on April 23. Primitive animals with shells became abundant on November 24. Dinosaurs appeared on December 21 and disappeared suddenly on the afternoon of December 26. Man appeared on earth at 8:06 p.m. on December 31. Recorded history began at 11:59 p.m. on December 31, and America was discovered a fraction of a second before midnight.

Our Hoosier State Beneath Us: Structural Geology

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