Geology For Dummies. Alecia M. Spooner

Modern Geologic Understanding

      In this section, I introduce three of the most prominent geologists of the seventeenth and eighteenth centuries and describe the theories they proposed: theories that stand up to current scientific scrutiny and still form the basis for modern geologic understanding.

      Reading the rock layers: Steno’s stratigraphy

      In the mid-seventeenth century, Nicholas Steno, a Danish physician, made great contributions to geology and especially paleontology: the study of fossil life. When Steno began his observations, only a few other scientists had proposed, tested, and attempted to prove that fossils found in rocks were the remains of once-living organisms. Steno advanced these ideas through observations and the study of rocks. His work led him to other questions, such as how could any solid object (a rock, mineral, or fossil) become trapped within another solid object, such as a rock?

      

Steno is considered the father of modern stratigraphy, which is the study of layers of rock. He described four principles of stratigraphy that still hold true today:

       Principle of superposition: States that in an uninterrupted sequence of sedimentary rocks (those composed of pieces of other rocks; see Chapter 7), the rock layers below are older than the rock layers above (as long as they have not been deformed, which I describe in Chapter 9). In Figure 3-1, the principle of superposition indicates that layer A is older than layer B, C, or E.

       Principle of original horizontality: States that sediments forming sedimentary rocks are usually laid down in a horizontal position (due to gravity). Therefore, rock layers that appear vertical have been moved from their original, horizontal position by some natural force (such as an earthquake).

       Principle of lateral continuity: States that when sediments are laid down, creating sedimentary rocks, they spread out until they reach some other object that confines them. This principle is illustrated when you fill your bathtub with water. The water spreads to fill all the space, confined only by the edges of the tub. Pour that same amount of water on the bathroom floor, and it spreads out until it hits the bathroom walls. Sedimentary rocks, like water, continue laterally until they are stopped by some other object.

       Principle of cross-cutting relationships: States that where one type of rock cuts across or through another type of rock, the rock being cut is older and the rock cutting through is younger. After all, a rock must already exist in order to be cut through by another rock. This principle is illustrated in Figure 3-1 where rock unit D is younger than the rocks A, B, and C that it cuts through.

      FIGURE 3-1: In this sketch of rock layers, the oldest is A, and the youngest is E.

      These things take time! Hutton’s hypothesis

      While Plutonists and Neptunists were arguing about the origin of Earth’s rocks, Scottish physician James Hutton was observing the rocks in his native Scotland and thinking about the different rock types and rock layers. He proposed theories about their relationship to one another and how current rock formations came to be.

      FINDING SHARKS’ TEETH ON MOUNTAINTOPS

      Nicholas Steno scientifically examined the head of a shark and reported that the teeth in the shark’s head were very similar to stones found in the mountains. These stones were called glossopetrae, or “tongue stones,” and at the time were believed to result from lightning strikes. Indeed, what Steno had discovered was that sharks’ teeth were actually buried in the rocks on the mountaintop!

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.

      

The basic principle that Hutton proposed, called uniformitarianism, is still the foundation of geologic science. Simply put, it states that past geologic phenomena can be explained by drawing on observable processes occurring today.

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,