Geology

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4

Paleontology and Paleobiology

Paleontology is the study of ancient or fossil life. Paleobiology is the application of biological principles to the study of ancient life on earth. These fields are fundamental to stratigraphy and are used to reconstruct the history of organisms' evolution and extinction throughout earth history. The oldest fossils are older than 3 billion years, although fossils do not become abundant and diverse until about 500 million years ago. Different fossil organisms are characteristic of different times, and at certain times in earth history, there have been mass extinctions (times when a large proportion of life disappears). Other organisms then replace the extinct forms. The study of fossils is one of the most useful tools for reconstructing geologic history because plants and animals are sensitive to environmental changes, such as changes in the climate, temperature, food sources, or sunlight. Their fossil record reflects the world that existed while they were alive. Paleontology is commonly divided into vertebrate paleontology (the study of organisms with backbones), invertebrate paleontology (the study of organisms without backbones), and micropaleontology (the study of microscopic fossil organisms). Many other subfields of paleontology exist as well. Paleobotanists study fossil plants, and palynologists study fossil pollen. Ichnology is the study of trace fossils—tracks, trails, and burrows left by organisms. Paleoecology attempts to reconstruct the behavior and relationships of ancient organisms.

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5

Paleoceanography and Paleoclimatology

Paleoceanography (the study of ancient oceans) and paleoclimatology (the study of ancient climates) are two subfields that use fossils to help reconstruct ancient conditions. Scientists also study stable isotopes, or different forms, of oxygen to reconstruct ancient temperatures. They use carbon and other chemicals to reconstruct aspects of ancient oceanographic and climatic conditions. Detailed paleoclimatic studies have used cores from ice sheets in Antarctica and Greenland to reconstruct the last 200,000 years. Ocean cores, tree rings, and lake sediments are also useful in paleoclimatology. Geologists hope that by understanding past oceanographic and climatic changes, they can help predict future change.

VI

History of Geology

Geology originated as a modern scientific discipline in the 18th century, but humans have been collecting systematic knowledge of the earth since at least the Stone Age. In the Stone Age, people made stone tools and pottery, and had to know which materials were useful for these tasks. Between the 4th century and 1st century bc, ancient Greek and Roman philosophers began the task of keeping written records relating to geology. Throughout the medieval and Renaissance periods, people began to study mineralogy and made detailed geologic observations. The 18th and 19th centuries brought widespread study of geology, including the publication of Charles Lyell’s book Principles of Geology, and the National Surveys (expeditions that focused on the collection of geologic and other scientific data). The concept of geologic time was further developed during the 19th century as well. At the end of the 19th century and into the 20th century, the field of geology expanded even more. During this time, geologists developed the theories of continental drift, plate tectonics, and seafloor spreading.

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Ancient Greek and Roman Philosophers

In western science, the first written records of geological thought come from the Greeks and Romans. In the 1st century bc, for example, Roman architect Vitruvius wrote about building materials such as pozzolana, a volcanic ash that Romans used to make hydraulic cement, which hardened under water. Historian Pliny the Elder, in his encyclopedia, Naturalis Historia (Natural History), summarized Greek and Roman ideas about nature.

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Science as an organized system of thought can trace its roots back to the Greek philosopher Aristotle. In the 4th century bc Aristotle developed a philosophical system that explained nature in a methodical way. His system proposed that the world is made of four elements (earth, air, fire, and water), with four qualities (cold, hot, dry, and wet), and four causes (material, efficient, formal, and final). According to Aristotle, elements could change into one another, and the earth was filled with water and air, which could rush about and cause earthquakes. Other philosophers of this era who wrote about earth materials and processes include Aristotle's student Theophrastus, the author of an essay on stones.

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Chinese Civilizations

Chinese civilizations developed ideas about the earth and technologies for studying the earth. For example, in 132 AD the Chinese philosopher Chang Heng invented the earliest known seismoscope. This instrument had a circle of dragons holding balls in their mouths, surrounded by frogs at the base. The balls would drop into the mouths of frogs when an earthquake occurred. Depending on which ball was dropped, the direction of the earthquake could be determined.

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