Dating techniques in geology
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.Thus, we can deduce that the mudstone and shale are older than the rhyolite dike.But, since the rhyolite dike does not cut across the shale, we know the shale is younger than the rhyolite dike.In the diagram to the right,the fault cuts the limestone and the sandstone, but does not cut the basalt.
Thus, if the strata are later uplifted and then cut by a canyon, we know that the same strata would be expected to occur on both sides of the canyon.
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.
This principle is often useful for distinguishing between a lava flow and a sill.
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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In fact, sedimentary rocks are, in a sense, trash from the Earth's surface deposited in basins.