II.

History of Geological Thought

Ancient peoples considered many geological features and processes as the work of gods and goddesses, and they regarded the natural environment with fear and wonder as dangerous and mysterious. Thus, the ancient Sumerians, Babylonians, and other peoples, although they made remarkable discoveries in mathematics and astronomy, went astray in geological inquiries by simply personifying geological processes. Irish legends, for example, suggest that giants were responsible for certain natural phenomena such as a weathered formation of basaltic columns, now known as the Giant's Causeway. Such mythology was also popular among the civilizations of the New World; for example, furrows on the flanks of what came to be known as Devil's Tower in Wyoming were thought by Native Americans to be the claw marks of a giant bear.

A.

Ancient to Medieval Times

Similarly, in ancient Greece and Rome, many of the gods were identified with geological processes. For example, volcanic eruptions in Sicily were ascribed to the local Roman volcano god, Vulcan. The Greek philosopher Thales of Miletus, in the 6th century bc, has been credited with making the first clean break with this traditional mythologizing. He regarded geological processes as natural and orderly events that could be studied in the light of reason, rather than as supernatural interventions. The Greek philosopher Democritus advanced this naturalistic philosophy with the theory that all matter is composed of atoms. Building on his atomic theory, he offered rational explanations of all manner of geological processes: earthquakes, volcanic eruptions, the hydrologic cycle, erosion, and sedimentation. His teachings, as expounded by the Roman poet Lucretius in his poem On the Nature of Things, are readily available in English translation. Aristotle, the most influential natural philosopher of ancient times, recognized in the 4th century bc that fossil seashells embedded in sedimentary rock strata were similar to shells found along the beach. From this observation he surmised that the relative positions of land and sea must have fluctuated in the past, and he also realized that such changes would require great lengths of time. Theophrastus, Aristotle's pupil, contributed to geological thought by writing the first book on mineralogy. Called Concerning Stones, it formed the basis of most mineralogies throughout the Middle Ages and even later.

B.

The Renaissance

The Renaissance was truly a new beginning for the Earth sciences; people began to observe geological processes much as the ancient Greeks had done. Were Leonardo da Vinci not better known as a painter and engineer, he might still be recognized as a pioneer of natural science. He realized, for example, that landscapes are sculptured by erosive processes and that fossil shells in Apennine limestones were the remains of marine organisms that had lived on the floor of a former sea that must have extended over Italy.

Following Leonardo, the French natural philosopher Bernard Palissy wrote on the nature and scientific study of soils, groundwater, and fossils. The classic works on minerals written in this period, however, were by Georgius Agricola, a German mining expert who published De Re Metallica (1556) and De Natura Fossilium (1546). Agricola recorded the most recent developments in geology, mineralogy, mining, and metallurgy at that time, and his works were widely translated.

C.

17th Century

Niels Stensen, a Dane—better known by the Latinized version of his name, Nicolaus Steno—stands prominent among 17th-century geoscientists. In 1669 he showed that the interfacial angles of quartz crystals were constant, regardless of the shape and size of the crystals, and that by extension, the structure of other crystal species should also be constant. Thus, by drawing attention to the significance of crystal form, Steno laid the foundation for the science of crystallography. Steno's observations on the nature of rock strata led him to formulate the law of superposition, one of the basic principles of stratigraphy (see below).

D.

18th and 19th Centuries

Geological thought during the 18th century was characterized by debates between contrasting schools. Plutonists, who proposed that the Earth's rocks were all originally solidified from a molten mass and later altered by other processes, were opposed by Neptunists, whose leading exponent was the German geologist Abraham Gottlob Werner. Werner hypothesized that the Earth's crust is a series of layers derived from mechanical and chemical sedimentary deposits laid down by a vast ocean, in a regular sequence, like the layers of an onion. By contrast, the Scottish geologist James Hutton and the Plutonists, as his followers were called, distinguished sedimentary rocks from intrusive rocks of volcanic origin.

In 1785, Hutton introduced the concept of uniformitarianism, according to which the history of the Earth can be interpreted solely on the basis of everyday geologic processes familiar to modern observers. He reasoned that most such processes, operating as slowly as they do today, would have taken millions of years to produce the modern landscape. This theory set him at odds with theological opinions of the day, which held that the Earth was barely 4000 years old. Hutton's antagonists, led by the French naturalist Georges Cuvier, believed that abrupt, violent changes—natural catastrophes such as floods and earthquakes—were responsible for the Earth's geologic features. For this reason, they were known as catastrophists.

The debate that raged between these two schools began to tip in favour of the uniformitarians with the publication of Charles Lyell's Principles of Geology (1830-1833). Born in 1797, the year Hutton died, Lyell became a major influence on modern geological theory, courageously attacking theological prejudices concerning the age of the Earth and rejecting attempts to interpret geology in the light of scripture.

In the American colonies, the noted surveyor, draftsman, and mapmaker Lewis Evans had already made remarkable contributions to American geological knowledge before Lyell's influential work. For Evans, river erosion and fluvial deposition were self-evident processes that had been at work in the past. Through the work of Evans, in addition, the concept of isostasy—that the density of the Earth's crust decreases as its thickness increases—also appeared for the first time in American geological writings.

Besides Lyell's work, the primary 19th-century developments in geology were the following: new reactions to traditional geological concepts, the fostering of glacial theory, the beginnings of American geomorphology, theories of mountain building, the advent of marine exploration, and the development of the so-called structuralist school (see below). Geological explorations, mainly in the American West, were major scientific events.

D.1.

Glacial Theory

The glacial theory drew on the work of Lyell and many others. First propounded in about 1840 and later universally accepted, the theory states that glacial drift had been deposited by glaciers and ice sheets moving slowly from higher to lower latitudes during the Pleistocene epoch (see Quaternary Period). The Swiss naturalist Horace Bénédict de Saussure had been among the first to credit glaciers in the Alps with the power to move large boulders. The Swiss-American naturalist Louis Agassiz correctly interpreted the environmental impact of this erosive and transporting agent and, with his colleagues, accumulated various forms of evidence that supported concepts of glacial advance and retreat for continental and mountain glaciers.

D.2.

Stratigraphy

Advances in stratigraphy were made by the English geologist William Smith, who traced out the strata of England and represented them on a geological map that remains substantially unchanged today. Smith first traced strata over relatively short distances; he then correlated stratigraphic units of the same age but of different rock content. After the development of evolutionary theory by Charles Darwin later in the 19th century, this knowledge led to the principle of faunal succession. According to this principle the life in each period of Earth history is unique for that specific period, fossil remains provide a basis for recognizing contemporaneous deposits around the world, and fossils can be used to assemble scattered fragments of the record into a chronological sequence known as the geological timescale (see below).

D.3.

Cycles of Geological Activity

Many 19th-century geologists came to understand the Earth as a thermally and dynamically active planet, internally as well as externally. Those known as structuralists or neocatastrophists believed that catastrophic or structural upheavals accounted for the formation of the Earth's topographic features. Thus, the English geologist William Buckland and his followers postulated frequent changes of sea level and upheaval of land masses to explain geological successions and breaks, or unconformities, in stratigraphic sequences. Hutton, by contrast, regarded Earth history in terms of overlapping, successive cycles of geological activity. He referred to long belts of folded rocks, which were taken to be the result of a variety of such cycles, as orogenic belts, and he referred to mountain formation through the processes of folding and uplift as orogenesis. Other geologists later supported these orogenic concepts, and they distinguished four major orogenic periods: the Huronian (end of the Precambrian); the Caledonian (Lower Palaeozoic era); the Hercynian, or Variscan (end of the Palaeozoic era); and the Alpine (end of the Cretaceous period).

D.4.

Surveys

Exploration of the western United States in the 19th century provided a whole new body of geological data that had an immediate effect on geomorphological theory. Early survey parties to the American West, under the auspices of the government, were headed by such figures as Clarence King, Ferdinand Vandeever Hayden, and John Wesley Powell, among others. Grove Karl Gilbert, the most outstanding of Powell's associates, recognized a form of topography caused by faults in the Earth's crust, and he deduced a system of laws governing landform development.

E.

20th Century

Technological advances made in the 20th century provided new, sophisticated tools for geologists, enabling them to measure and monitor Earth processes with a precision previously unattainable. In terms of basic theory, the field of geology underwent a major revolution with the introduction and development of the plate tectonics hypothesis, that the Earth's crust is divided into a number of plates that move about, collide, and separate over geological time. The great crustal plates of the Earth are now understood to begin at midocean and other ridges, or spreading centres, and to move towards submarine trenches, or subduction zones, where the crustal material again descends. The places on the Earth where major earthquakes occur tend to outline the boundaries between these crustal plates, suggesting that seismic activity can be interpreted as the result of the horizontal movements of the crustal plates.

This hypothesis is related to the concept of continental drift, first proposed in modern form by the German geophysicist Alfred Wegener in 1912. The hypothesis gained support later in the century as deep-sea exploration provided evidence for seafloor spreading—the outflow of new crustal material along midocean ridges. The concept of plate tectonics has since been related to the origin and growth of continents, the generation of continental as well as oceanic crust, and the nature of the Earth's underlying layers and their evolution through time. Thus, 20th-century geologists have developed a theory that unifies many of the major processes that have shaped the Earth and its landforms.