target="_blank" rel="nofollow" href="#fb3_img_img_4899a4e7-1b98-51c7-9253-0ae424141df2.png" alt="Remember"/> Through his observations of sites such as Siccar Point — a rocky area along the east coast of Scotland — Hutton began describing geologic processes that required long periods of time to create the rock formations visible today. He was the first geologist to propose the idea of geologic time, also called deep time, which extended the age of the earth much farther into the past than had previously been accepted.
According to Hutton, with a long enough period of time, even the small, commonplace processes that shape the earth’s surface today could result in the dramatic formations previously assumed to be the results of catastrophe.
What has been will be: Lyell’s principles
Following Hutton’s work, Charles Lyell, a Scottish professor of geology in the early nineteenth century, published a book called Principles of Geology. In this book Lyell outlined and expanded on Hutton’s ideas about deep time, geologic processes, and the formation of rock features on Earth’s surface.
In publishing his book, Lyell spread Hutton’s ideas and popularized them. The concept that “the present is the key to the past” was groundbreaking at the time and inspired scientific thought in fields outside of geology, such as Darwin’s ideas about evolution.
Uniformi-what? Understanding the Earth through Uniformitarianism
The idea that geologic processes we observe today have always been occurring and can be used to explain the features of the earth has stood the test of time. In fact, now more than ever, geologists recognize that the physical, chemical, and biological processes that occur today must have occurred in the past as well. Even a feature as spectacular as the Grand Canyon is created by the same simple process of erosion by water (see Chapter 12) that creates creeks and gullies in your backyard.
However, when Hutton and Lyell proposed the concept of uniformitarianism, they assumed that the rate and intensity of past processes were the same as those observed today. The current understanding of uniformitarianism in geology no longer makes this assumption. Modern uniformitarianism differs from the original idea in two very important ways:
Rates and intensity of processes may vary: While the processes scientists observe today occurred in the past, they may have occurred more quickly or more intensely than they do now. For example, massive layers of volcanic rocks across Siberia (called the Siberian Traps) suggest a period of very intense lava outpourings, unlike anything humans have ever observed.
Catastrophes do play a role: When uniformitarianism was first proposed, it ran counter to the ideas of catastrophism. But modern geologists recognize that occasional catastrophic events (such as volcanic eruptions and tsunamis) do play an important role in shaping the earth’s surface.
Pulling It All Together: The Theory of Plate Tectonics
During World War I, a German scientist named Alfred Wegener suggested that the continents had once been connected and had drifted apart. His ideas about continental drift — the movement of the continents — were based on fossil, rock, and stratigraphic evidence (which I discuss in detail in Chapter 8). However, he hadn’t worked out all the details – such as what force, or mechanism, propelled the continents. At the time, scientific understanding of the earth’s crust as a continuous, solid, rigid layer did not allow for moving continents, and without a clear explanation for how they moved, Wegener’s hypothesis was strongly rejected by other geologists.
A dramatic breakthrough occurred in the decades after World War I when the use of submarines in warfare led to mapping of the seafloor with sonar. The 1960s in particular was a time of new discovery and understanding. Geologist Marie Tharp discovered a long, rocky ridge, a rift, in the middle of the Atlantic Ocean while drawing an ocean floor map from sonar data. (Her map, created with Bruce Heezen, is still the standard map of the ocean floor used by all scientists.)
The idea of seafloor spreading — the moving apart of oceanic crust along ridges on the ocean floor — was being explored by scientists, led by the ideas of Harry Hess. But there were still many who “knew” the earth’s crust and the mantle below were solid rock and could not be moving. One skeptic, Canadian geologist J. Tuzo Wilson, eventually published his ideas about plates moving across hotspots (see Chapter 10 for details) and his ideas provided a foundation on which the unifying theory of geology was built. The theory of plate tectonics combines ideas about plate movement with evidence for seafloor spreading, as well as incorporating explanations for volcanoes, earthquakes, and other geologic features and phenomena. Because this theory is so crucial, I devote Part 3 of this book to it.
Scientists never stop exploring, of course, so even with a well-accepted, well-tested explanation of how the surface of the earth constantly transforms, they don’t stop asking questions.
Forging Ahead into New Frontiers
After geologists had the theory of plate tectonics laid out, they had a framework within which they could propose and test specific hypotheses to fill in the details. This work continues today, right now, as you read these words! The frontiers of earth science are being expanded in many directions. In this section, I describe just a few areas of current, exciting research and discovery.
Asking how, where, and why: Mountain building and plate boundaries
Plate tectonics theory explains that the movement of plates creates mountains by pushing crustal rocks together and up (see Chapters 9 and 10). But scientists have not gathered enough evidence to agree on what forces drive the uplift of mountains. Some suggest that a pushing force, exerted by the neighboring plate, forces the rocks upward. Others suggest that the removal of rocks by erosion (explained in Part 4) leads the continental rocks to “float” upward, like an iceberg melting in the ocean.
Mysteries of the past: Snowball earth, first life, and mass extinctions
Later in this book (in Chapter
