James Hutton, Uniformitarianism, and Deep Time

James Hutton gave us the ideas of uniformitarianism and deep time. He's often referred to as the father of modern geology. Here's how he earned that honor. Hutton lived in Edinburgh in the mid-1700s during what is known as the Scottish Enlightenment. Geologists were trying to understand how rocks formed. In this picture of Edinburgh you can see the Salisbury Cracks which contained a lava flow from an extinct volcano, Arthur's Seat. Here James Hutton saw basalt on top of sedimentary rock. He deduced that the sedimentary rock was formed first and the basalt came later. Here's a close-up and you can see the sedimentary rock surrounded by the basalt. Now this interpretation was completely at odds with the theories of the time. One group of scientists called the Neptunists believed that rocks had precipitated out of the Earth's early ocean. First came the granite, then the gneisses and schists, then the basalts which they thought were sedimentary, and finally the limestones and other sedimentary rocks. You can see why Hutton's observation put Neptunism into question. Hutton proposed that the Earth's interior was hot and that it was this heat which This view became known as Plutonism. He proposed that heat caused the uplift of rock layers, the land was weathered and eroded into sediment, the sediment was transported to the sea, the sediment was deposited, and heat caused it to be lithified into rock, and then once again it was uplifted, weathered and eroded, and so on and so forth. Notice that this is a cycle and can happen many times. What Hutton needed to support his theory was evidence that sedimentary rocks were formed in a series of cycles. He looked for contacts between rock layers called unconformities. First he had to find out where two different kinds of rocks were in contact with each other. All he had to do was follow the stone walls. Each wall is composed of locally derived rocks. This one is made of a gray sandstone which we call greywacke. So the rock underneath this field contains greywacke. This wall, however, is composed of both greywacke as well as red sandstone. From this wall Hutton simply had to look down to the bottom of the sea cliff to find Siccar Point. At Siccar Point he found this red pebbly sandstone. Notice that it has a slight tilt to it. He also found the greywacke which he called schistos, and notice that the schistos is nearly vertical. But most importantly he found the contact between the two layers, and there it is. We would call this contact an angular unconformity. Even better, at the bottom of the red sandstone he found that there were pieces of the greywacke. So clearly the greywacke formed first and the red sandstone incorporated it when it formed. How did this happen? Well, long ago greywacke was deposited in the ocean. Notice that the layers of greywacke are horizontal. Most sedimentary rocks are horizontal when they are first deposited. Then the rocks became deformed into tight folds by compression. So now the rocks are nearly vertical. The rocks then have to be uplifted to the surface where they can be eroded by the elements. Notice the pebbles of greywacke can remain in the lower parts of the erosional surface. Next the rock is again below sea level because it's in the ocean that the sediment that will become the red sandstone was deposited. The sandstone is formed. It's uplifted and gently tilted. With a little more erosion you have the sicker point angular unconformity. How much time did it take to create the landscape? A whole lot of time. John Playfair, a scientist who accompanied Hutton to sick our point and later popularized his theories, put it like this. We felt necessarily carried back to a time when the schistos on which we stood was yet at the bottom of the sea and when the sandstone before us was only beginning to be deposited in the shape of sand or mud from the waters of the supercontinent ocean. The mind seemed to grow giddy by looking so far into the abyss of time. So the red line represents the unconformity which we now know separates greywacke which was 425 million years old and sandstone which is 345 million years old. That's 80 million years of time between those layers. That is what we call deep time. Deep time. Not the 6,000 years being proposed by theologians at the time but deep time for the processes of weathering and erosion take time as do the deposition the lithification and the uplift and of course another cycle of weathering erosion deposition lithification and uplift. Is there another layer below the greywacke? Most certainly. Will there be another layer someday above the sandstone? Why not? Sick our point represents only two cycles of what could be many many cycles. In Hutton's word we find no vestige of the beginning no prospect of an end. Hutton's ideas were popularized and expanded by John Clay Fair and Charles Lyell. Uniformitarianism refers to the idea that the earth was shaped entirely by gradual forces still in operation today acting over a very long period of time. This is in complete contrast to the existing idea of catastrophism that the land was formed by a series of catastrophes. So if you saw the Grand Canyon and you thought that the earth was young you would have to assume that it must have been carved by a giant flood for certainly the little Colorado River could not have done that in 6,000 years. But if you have deep time you can then say that the river eroded it over a long period of time and it did. Not only that but you have time to explain each of the layers in the Grand Canyon walls. So uniformitarianism describes changes in the earth as an ongoing process not a single catastrophe. Does that mean that catastrophic events never occurred? Well of course not. Ask a dinosaur. Well of course you can't ask a dinosaur because they're all extinct from a catastrophic meteor impact that occurred 67 million years ago. Does that mean that uniformitarianism is wrong? It doesn't. It just means that uniformitarianism must take into account that these processes could be sudden but still they're the same processes which occur today. After all we have meteor impacts today just not as big luckily. The change could be gradual but it doesn't have to be. Here are the Channel Scablands of Washington. When J Harlan Bretz proposed that the Channel Scablands were formed by a giant glacial flood he was discounted as a catastrophist. However as more about the Ice Age became understood his theory was finally accepted. It is possible for processes such as earthquakes volcanic eruptions and meteor strikes affect the landscape in a very sudden catastrophic manner. They simply differ in size and in scale. A glacially dammed outburst destroyed Eagle, Alaska in the same way as one wiped out the Channel Scablands. Here's a definition of uniformitarianism. However it's best described in this simple but powerful sentence. The present is the key to the past. So by studying natural processes that happen today we can decipher the events of the past but even better we can use the understanding of the present and the past to predict the future. That's the power of geology. If none of this would be possible we didn't understand geologic time. Why do we understand geologic time? Because James Hutton the father of modern geology. So we have him to thank. Of course I have a few other things to make and especially I want to thank you for you made it to the end of the podcast.