Geology

I

Introduction

Geology (Greek, gē, “Earth”; -logia, “knowledge of”), field of science concerned with the origin of the planet Earth, its history, its shape, the materials forming it, and the processes that are acting and have acted on it. It is one of several related subjects commonly grouped as the Earth sciences, or geoscience, and geologists are Earth scientists concerned primarily with rocks and derivative materials that make up the outer part of the Earth. To understand these materials, geologists make use of knowledge from other fields, such as physics, chemistry, and biology; thus, geological fields such as geochemistry, geophysics, geochronology (which uses dating methods), and palaeontology, now important disciplines in their own right, incorporate other sciences, enabling geologists to understand better the working of Earth processes through time.

Although each Earth science has a particular focus, they all frequently overlap with geology. Thus, the study of the Earth's waters in relation to geological processes involves knowledge of hydrology and oceanography, and the measurement and mapping of the Earth's surface forms involve knowledge of cartography (maps) and geodetics (surveying). Clues to the origin of the Earth are also sought by the study of extraterrestrial bodies, especially the Moon, Mars, and Venus. Originally limited to Earth-based telescopic observation, such studies were given a powerful impetus by the space exploration that began in the 1960s.

As a major science, geology not only involves the study of landforms and other surface features of the Earth but also is concerned with the structure and inner parts of the planet. Such knowledge is of basic scientific interest, but it is also placed at the service of humanity. Thus, the focus of applied geology is on the search for useful minerals within the Earth, the identification of geologically stable environments for human constructions, and the foreknowledge of natural hazards associated with the geodynamic forces described below.

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.