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25 Aug

From the beginning of this course, we have stated that the Earth is about 4.6 billion years old.

How do we know this and how do we know the ages of other events in Earth history?

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They did not know how many thousands, millions, or billions of years ago the Cambrian period began, but they knew that it came after the Proterozoic Eon and before the Ordovician Period, and that the fossils unique to Cambrian rocks were younger than Proterozoic fossils and older than Ordovician ones.

In a family like this it’s hard to tell how people are related simply by age. Early geologists had no way to determine the absolute age of a geological material.

With rock units we use certain principles to tell their ages relative to each other. If they didn’t see it form, they couldn’t know if a rock was one hundred years or 100 million years old.

Remember Nicholas Steno, who determined that fossils represented parts of once-living organisms? Superposition refers to the position of rock layers and their relative ages (Figure below).

Steno also noticed that fossil seashells could be found in rocks and mountains far from any ocean. Steno proposed that if a rock contained the fossils of marine animals, the rock formed from sediments that were deposited on the seafloor. Relative age means age in comparison with other rocks, either younger or older.