By Robert Hazen, Ph.D., George Mason University
Rocks reveal the complex history of the Earth’s landforms and its life. How do they do that? How can we look at a rock and say what happened millions, or even billions, of years ago? The answer lies in geology, the science of rocks. Geologists go into the field and make observations. They use subtle clues to deduce the Earth’s history. Isn’t that fascinating?
Geologists and Rocks
When geologists go into the field, they make observations; they make measurements on the rocks themselves. They measure, for example, the relative positions and the thicknesses of different rock formations. They identify the rock types. Once they identify the rock type, it tells a great deal right there about the history of that outcrop.
The geologists also map folds and faults in the rock; they look for places where younger sediments lie on top of older sediments. Moreover, they collect fossils and lots of rock samples.
These rock samples are analyzed in the laboratories. The geologists can measure the ages of the rocks using radiometric techniques. By making thin sections of the rock and looking at the minerals and the relationship of the minerals, one can learn much about their history.
History is the key here. Every outcrop, every roadcut has an ancient history. It’s sad, in a way, that because the Earth keeps recycling its rocks through plate tectonics, most of the history is absolutely lost forever. The oldest oceanic rocks, the oldest oceanic crust, is only about 200 million years old; all the rest of the oceanic crust has been subducted.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
The Oldest Known Rocks
Continental material can be much, much older, because the continents are permanently floating around on top of the plates. Nevertheless, there are very few rocks that are older than about two billion years, and that’s less than half of the Earth’s total age. In fact, there are only a handful of outcrops older than about three billion years.
The oldest known rocks are from Australia, and they are about 3.8 billion years old.
Thus, we know almost nothing about the geology for the first half of the Earth’s existence. On the other hand, thanks to the abundance of accessible rock formations, and especially fossils, we know a great deal about the last billion years of Earth history.
Learn more about the plate tectonics theory.
The Fossil-rich Rocks of Cincinnati
Cincinnati is a vast storehouse of fossils from the Ordovician period. Virtually any roadcut, any place one stops in the Cincinnati area, they’ll find abundant, beautiful fossils.
These fossils are beautiful not only because they’re varied, because they’re large, and so forth, but also because they’re a very particular kind of fossilization, called silicification.
The Cincinnati fossils, the original calcium-carbonate shells, were replaced by silica, which is tough and hard; and so these fossils are preserved in exquisite detail and beauty. There are hundreds and hundreds of different species in the Cincinnati area, so it’s a rich treasure ground for fossil hunters and professional paleontologists.
The Coral Reef System
Imagine the Cincinnati area about 500 million years ago, when Cincinnati was under a shallow sea. It was an inland sea that covered, in fact, much of North America, and there was a great coral reef system.
That coral reef system teemed with life and deposited thick layers of limestone; it probably was very similar to the Great Barrier Reef today, depositing layer after layer of limestone, and numerous other shells and organisms that died and fell down, and helped build up these layers of fossiliferous rock.
Gradually, for another 150 million years, the reefs were buried; the inland sea of North America was filled with many thousands of feet of sediments.
By 350 million years ago, the basin was much, much shallower than it is right now, and land sediment started to fill in that basin. Apart from other things, it also included coal layers.
Learn more about the geochemical cycles.
The Great Tectonic Forces
Coal layers represent swampland far above the Cincinnati Ordovician layers. By 200 million years ago, there were great tectonic forces that gradually arched North America and started bending it upward. Imagine how this happened: we have a thick layer of sediments, which has been deeply buried, but then other forces come into play and start uplifting those layers of sediment.
Gradually, as the layers of sediment are lifted up, streams and rivers cut through that, cut more deeply; erosion takes place, and layers and layers are stripped off.
At the highest point of that arching, we have the greatest erosion, and that was right near Cincinnati. This is why Cincinnati is the place where the oldest and the most deeply buried rocks come closest to the Earth’s surface.
Of course, the Ohio River helps as it cuts down deeply through these rocks and exposes more and more of them each year. It exposes these ancient rocks, the ancient coral reef, which was buried 500 million years ago.
Common Questions about How Rocks Tell the Earth’s History
When geologists go into the field, they measure the relative positions and the thicknesses of different rock formations. They identify the rock types, and also map folds and faults in the rock. They collect fossils and rock samples. These rock samples are analyzed in the laboratories, and the geologists can measure the ages of the rocks using radiometric techniques.
The oldest oceanic rock, the oldest oceanic crust, is only about 200 million years old; all the rest of the oceanic crust has been subducted. This is because the Earth keeps recycling its rocks through plate tectonics, and so most of the history is lost forever.
Silicification ocurred when, in the fossils, the original calcium-carbonate shells were replaced by silica, which is tough and hard.
