When the Earth Shakes by Simon Winchester
Author:Simon Winchester
Language: eng
Format: epub
Published: 2019-01-29T00:00:00+00:00
Left: Weathered limestone near Puget Sound, Washington; Right: Fossil fish and leaf, western Wyoming .
Another clue that hints at whether a rock is sedimentary is the presence of fossils, remains of once-living animals or plant life. Geologists use evidence of fossils as one way of telling how old a rock is. They know when in the millions of years Earth has existed each kind of animal and plant lived. So if a rock is embedded with a plant fossil from 1.2 million years ago, that fossil helps scientists date the rock as having been laid down at around that time.
Where there are no fossils, no layers, and the rock is made up of an accumulation of recognizable crystals, then it is more likely to be an igneous rock. To figure out the details of an igneous rock is more difficult—though to me, much more satisfying and interesting, because I am finding out about the history of the whole earth, and not just the things that lived on it. To uncover the composition of the rock is important: I use a magnifying glass to determine what minerals it is made of. I know what they look like, but you can easily use a rock guidebook to look up specific material, such as shiny mica or iron oxide or the kind of crystals that make up granite. When someone comes up to me with a piece of igneous rock and says, “What is it?” I take great pains to examine the rock. When I am able to say, without a doubt, that it is this or that, and is made of this and that, and then perhaps, by using specialized instruments, to measure the rock’s chemistry to help show how old it is—to me, that is a moment of greatest pleasure. It is detective work of the highest order—not uncovering the commission of a crime, but the mechanics of creation.
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