Geology

C

Medieval and Renaissance Periods

The nature and origin of minerals and rocks interested many ancient writers, and mineralogy may have been the first systematic study to arise in the earth sciences. The Saxon chemist Georgius Agricola wrote De Re Metallica (On the Subject of Metals) following early work by both the Islam natural philosopher Avicenna and the German naturalist Albertus Magnus. De Re Metallica was published in 1556, a year after Agricola’s death. Many consider this book to be the foundation of mineralogy, mining, and metallurgy.

Medieval thought was strongly influenced by Aristotle, but science began to move in a new direction during the Renaissance Period. In the early 1600s, English natural philosopher Francis Bacon reasoned that detailed observations were required to make conclusions. Around this time French philosopher René Descartes argued for a new, rational system of thought. Most natural philosophers, or scientists, in this era studied many aspects of philosophy and science, not focusing on geology alone.

Studies of the earth during this time can be placed in three categories. The first, cosmology, proposed a structure of the earth and its place in the universe. As an example of a cosmology, in the early 1500s Polish astronomer Nicolaus Copernicus proposed that the earth was a satellite in a sun-centered system. The second category, cosmogony, concerned the origin of the earth and the solar system. The Saxon mathematician and natural philosopher Gottfried Wilhelm, Baron von Leibniz, in a cosmogony, described an initially molten earth, with a crust that cooled and broke up, forming mountains and valleys. The third category of study was in the tradition of Francis Bacon, and it involved detailed observations of rocks and related features. English scientist Robert Hooke and Danish anatomist and geologist Nicolaus Steno (Niels Stenson) both made observations in the 17th century of fossils and studied other geologic topics as well. In the 17th century, mineralogy also continued as an important field, both in theory and in practical matters, for example, with the work of German chemist J. J. Becher and Irish natural philosopher Robert Boyle.

D

Geology in the 18th and 19th Centuries

By the 18th century, geological study began to emerge as a separate field. Italian mining geologist Giovanni Arduino, Prussian chemist and mineralogist Johan Gottlob Lehmann, and Swedish chemist Torbern Bergman all developed ways to categorize the layers of rocks on the earth's surface. The German physician Georg Fuchsel defined the concept of a geologic formation—a distinctly mappable body of rocks. The German scientist Abraham Gottlob Werner called himself a geognost (a knower of the earth). He used these categorizations to develop a theory that the earth's layers had precipitated from a universal ocean. Werner's system was very influential, and his followers were known as Neptunists. This system suggested that even basalt and granite were precipitated from water. Others, such as English naturalists James Hutton and John Playfair, argued that basalt and granite were igneous rocks, solidified from molten materials, such as lava and magma. The group that held this belief became known as Volcanists or Plutonists.

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By the early 19th century, many people were studying geologic topics, although the term geologist was not yet in general use. Scientists, such as Scottish geologist Charles Lyell, and French geologist Louis Constant Prevost, wanted to establish geology as a rational scientific field, like chemistry or physics. They found this goal to be a challenge in two important ways. First, some people wanted to reconcile geology with the account of creation in Genesis (a book of the Old Testament) or wanted to use supernatural explanations for geologic features. Second, others, such as French anatomist Georges Cuvier, used catastrophes to explain much of earth’s history. In response to these two challenges, Lyell proposed a strict form of uniformitarianism, which assumed not only uniformity of laws but also uniformity of rates and conditions. However, assuming the uniformity of rates and conditions was incorrect, because not all processes have had constant rates throughout time. Also, the earth has had different conditions throughout geologic time—that is, the earth as a rocky planet has evolved. Although Lyell was incorrect to assume uniformity of rates and conditions, his well reasoned and very influential three-volume book, Principles of Geology, was published and revised 11 times between 1830 and 1872. Many geologists consider this book to mark the beginning of geology as a professional field.

Although parts of their theories were rejected, Abraham Gottlob Werner and Georges Cuvier made important contributions to stratigraphy and historical geology. Werner's students and followers went about attempting to correlate rocks according to his system, developing the field of physical stratigraphy. Cuvier and his co-worker Alexandre Brongniart, along with English surveyor William Smith, established the principles of biostratigraphy, using fossils to establish the age of rocks and to correlate them from place to place. Later, with these established stratigraphies, geologists used fossils to reconstruct the history of life's evolution on earth.

E

Age of Geologic Exploration

In the late 18th and the 19th centuries, naturalists on voyages of exploration began to make important contributions to geology. Reports by German natural historian Alexander von Humboldt about his travels influenced the worlds of science and art. The English naturalist Charles Darwin, well known for his theory of evolution, began his scientific career on the voyage of the HMS Beagle, where he made many geological observations. American geologist James Dwight Dana sailed with the Wilkes Expedition throughout the Pacific and made observations of volcanic islands and coral reefs. In the 1870s, the HMS Challenger was launched as the first expedition specifically to study the oceans.

Expeditions on land also led to new geologic observations. Countries and states established geological surveys in order to collect information and map geologic resources. For example, in the 1860s and 1870s Clarence King, Ferdinand V. Hayden, John Wesley Powell, and George Wheeler conducted four surveys of the American West. These surveys led to several new concepts in geology. American geologist Grove Karl Gilbert described the Basin and Range Province and first recognized laccoliths (round igneous rock intrusions). Reports also came back of spectacular sites such as Yellowstone, Yosemite, and the Grand Canyon, which would later become national parks. Competition between these survey parties finally led the Congress of the United States to establish the U.S. Geological Survey in 1879.

F

Geologic Time

Determining the age of the earth became a renewed scholarly effort in the 19th century. Unlike the Greeks and most eastern philosophers, who considered the earth to be eternal, western philosophers believed that the planet had a definite beginning and must have a measurable age. One way to measure this age was to count generations in the Bible, as the Anglican Archbishop James Ussher did in the 1600s, coming up with a total of about 6000 years. In the 1700s, French natural scientist George Louis Leclerc (Comte de Buffon) tried to measure the age of the earth. He calculated the time it would take the planet to cool based on the cooling rates of iron balls and came up with 75,000 years. During the 18th century, James Hutton argued that processes such as erosion, occurring at observed rates, indicated an earth that was immeasurably old. By the early 19th century, geologists commonly spoke in terms of 'millions of years.' Even religious professors, such as English clergyman and geologist William Buckland, referred to this length of time.

Other means for calculating the age of the earth used in the 19th century included determining how long it would take the sea to become salty and calculating how long it would take for thick piles of sediment to accumulate. Irish physicist William Thomson (Lord Kelvin) returned to Buffon's method and calculated that the earth was no more than 100 million years old. Meanwhile, Charles Darwin and others argued that evolution proceeded slowly enough that it required at least hundreds of millions of years.

With the discovery of radioactivity in 1896 by French physicist Henri Becquerel, scientists, such as British physicist Ernest Rutherford and American radiochemist Bertram Boltwood, recognized that the ages of minerals and rocks could be determined by radiometric dating. By the early 20th century, Boltwood had dated some rocks to be more than 2 billion years old. During this time, English geologist Arthur Holmes began a long career of refining the dates on the geologic time scale, a practice that continues to this day.

G

Theory of Continental Drift

In 1910 American geologist Frank B. Taylor proposed that lateral (sideways) motion of continents caused mountain belts to form on their front edges. Building on this idea in 1912, German meteorologist Alfred Wegener proposed a theory that came to be known as Continental Drift: He proposed that the continents had moved and were once part of one, large supercontinent called Pangaea. Wegener was attempting to explain the origin of continents and oceans when he expanded upon Taylor’s idea. His evidence included the shapes of continents, the physics of ocean crust, the distribution of fossils, and paleoclimatology data.

Continental drift helped to explain a major geologic issue of the 19th century: the origin of mountains. Theories commonly called on the cooling and contracting of the earth to form mountain chains. The mountain-building theories of German geologist Leopold von Buch and French geologist Leonce Elie de Beaumont were catastrophic in nature. American geologists James Hall and James Dwight Dana proposed the geosynclinal theory of mountain building—a theory based on the downward bending of the earth’s crust (a geosyncline). Austrian geologist Eduard Suess developed a related theory. Hall, Dana, and Suess believed that continents and ocean basins were ancient, permanent features on earth and that mountain belts formed at their edges.

Most geologists did not accept the theory of continental drift in the 1920s and 1930s. British geologist Arthur Holmes supported continental drift and proposed that convection (a type of heat movement) inside the earth drove continental drift. Others who favored the idea included South African geologist Alex du Toit, who studied geologic evidence for the southern continents of Gondwanaland, part of the hypothetical supercontinent Pangaea. Other scientists, such as British geophysicist Harold Jeffreys, argued that continental drift was physically impossible. Paleontologists, such as American George Gaylord Simpson, said that the distribution of fossils could be explained by other means.

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