Geology

C

Uniformitarianism

Uniformitarianism, or actualism, helps geologists use their knowledge of modern processes and events to reconstruct the past. The principle of uniformitarianism depends on the 'uniformity of laws,' which assumes that the laws of physics and chemistry have remained constant. To test uniformity of laws, geologists can examine preserved one-billion-year-old ripples that look very much like ripples on the beach today. If gravity had changed, water and sand would have interacted differently in the past, and the ripple evidence would be different. Also, minerals in three-billion-year-old rocks are the same as minerals forming in rocks today, confirming the uniformity of chemical laws. Uniformitarianism contrasts with, for example, the idea that past events such as floods or earthquakes were caused by divine intervention or supernatural causes. Catastrophism, which calls on major catastrophes to explain earth’s history, is also sometimes contrasted with uniformitarianism. However, uniformitarianism can include past catastrophes.

III

The Geologic Time Scale

Geologists have created a geologic time scale to provide a common vocabulary for talking about past events. The practice of determining when past geologic events occurred is called geochronology. This practice began in the 1700s and has sometimes involved some personal and international disputes that led to differences in terminology. Today the geologic time scale is generally agreed upon and used by scientists around the world, dividing time into eons, eras, periods, and epochs. Every few years, the numerical time scale is refined based on new evidence, and geologists publish an update.

Geologists use several methods to determine geologic time. These methods include physical stratigraphy, or the placement of events in the order of their occurrence, and biostratigraphy, which uses fossils to determine geologic time. Another method geologists use is correlation, which allows geologists to determine whether rocks in different geographic locations are the same age. In radiometric dating, geologists use the rate of decay of certain radioactive elements in minerals to assign numerical ages to the rocks.

The process of determining geologic time includes several steps. Geologists first determine the relative age of rocks—which rocks are older and which are younger. They then may correlate rocks to determine which rocks are the same age. Next, they construct a geologic time scale. Finally, they determine the specific numerical ages of rocks by various dating methods and assign numbers to the time scale.

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A

Relative Time

Geologists create a relative time scale using rock sequences and the fossils contained within these sequences. The scale they create is based on The Law of Superposition, which states that in a regular series of sedimentary rock strata, or layers, the oldest strata will be at the bottom, and the younger strata will be on top. Danish geologist Nicolaus Steno (also called Niels Stensen) used the idea of uniformity of physical processes. Steno noted that sediment was denser than liquid or air, so it settled until it reached another solid. The newer sediment on the top layer is younger than the layer it settled upon. Since this is what happens in the world today, it should also determine how rock layers formed in the past. Crosscutting relationships are also used to determine the relative age of rocks. For instance, if a thin intrusion of granite, called a dike, cuts through a layer of limestone, the granite must be younger than the limestone.

B

Biostratigraphy

In the field of biostratigraphy geologists study the placement of fossils to determine geologic time. British surveyor William Smith and French anatomist Georges Cuvier both reasoned that in a series of fossil-bearing rocks, the oldest fossils are at the bottom, with successively younger fossils above. They thus extended Steno's Law of Superposition and recognized that fossils could be used to determine geologic time. This principle is called fossil succession. Smith and Cuvier also noted that unique fossils were characteristic of different layers. Biostratigraphy is most useful for determining geologic time during the Phanerozoic Eon (Greek phaneros, “evident”; zoic, “life”), the time of visible and abundant fossil life that has lasted for about the past 570 million years. Although fossils exist that are as old as three billion years or more, they are not common. Few fossils exist that are useful for determining geologic age from time before about 1 billion years ago, so biostratigraphy is of limited use in older sedimentary rocks.

C

Correlation

Using correlation to determine which rocks are of equal age is important for reconstructing snapshots in geologic history. Correlation may use the physical characteristics of rocks or fossils to determine equivalent age. For example, the limestone at the top of one side of the Grand Canyon can be correlated to the opposite side of the canyon. Also, ash from a volcanic eruption can be correlated over long distances and wide areas. Fossils are the most useful tools for correlation. Since the work of Smith and Cuvier, biostratigraphers have noted that 'like fossils are of like age.” This is the principle of fossil correlation.

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