Dating geology

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A more modern way of stating the same principle is that the laws of nature (as outlined by the laws of chemistry and physics) have operated in the same way since the beginning of time, and thus if we understand the physical and chemical principles by which nature operates, we can assume that nature operated the same way in the past.

Principle of Superposition Because of Earth's gravity, deposition of sediment will occur depositing older layers first followed by successively younger layers.

Look at the many photographs of the Grand Canyon in your textbook.

Note that you can follow the layers all along the walls of the canyon, and you can find the same layers on both sides of the canyon.

Once these age relations were worked out, another principle fell into place - the principle of fossil succession.

We discuss the 7 principles of stratigraphy first and then see how these apply to fossils.

In fact, sedimentary rocks are, in a sense, trash from the Earth's surface deposited in basins.

Principle of Original Horizontality Sedimentary strata are deposited in layers that are horizontal or nearly horizontal, parallel to or nearly parallel to the Earth's surface.

But, since the rhyolite dike does not cut across the shale, we know the shale is younger than the rhyolite dike.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?Principles of Stratigraphy Stratigraphy is the study of strata (sedimentary layers) in the Earth's crust.Geologist in the 1800s worked out 7 basic principles of stratigraphy that allowed them, and now us, to work out the relative ages of rocks.In either case, the inclusions had to be present before they could be included in the younger rock, therefore, the inclusions represent fragments of an older rock.

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