Interpreting Earth History
Historical geology is the study of the origin and evolution of Earth. Geologists are interested not only in placing events in a chronological sequence but also, and more important, in explaining how and why past events took place. Recently, historical geology has taken on even greater importance because scientists in many disciplines are looking to the past to help explain current events (such as short- and long-term climatic changes) and using this information to try and predict future trends.
We look at Earth as a system consisting of a collection of various subsystems or related parts inter- acting with each other in complex ways. By using this systems approach, we can see that the evolution of Earth, far from being a series of isolated events, is a continuum in which the different components both affect and are affected by one another. An example is the early history of Earth in which the evolution of the atmosphere, hydrosphere, lithosphere, and biosphere are intimately related. Today, scientists are examining the effect humans have on short-term climate changes and the environment, as well as what a decrease in global biodiversity means for both humans and the planet.
Geologists seek to know not only what happened in the past but also why something happened and what the implications are for Earth today and in the future. Thus it is important to understand present-day processes and to have an accurate means of measuring geologic time so as to appreciate the duration of past events and how these events might affect Earth and its inhabitants today.
An important component of historical geology is understanding how we know what we know. How do we know that dinosaurs became extinct 65 million years ago, or that glacial conditions prevailed over what is now the Sahara Desert during the Carboniferous Period? How can we be so sure that the early atmosphere was devoid of oxygen and evolved over millions of years to one that today has oxygen? Historical geology addresses these questions by seeking answers in rocks and fossils. As more information becomes available from new observations or scientiﬁc techniques, geologists become more conﬁdent in their interpretations of past events.
One of the many exciting aspects of geology, and of science in general, is that there are still so many unanswered questions. For example, there is still heated debate on what caused the Permian mass extinction. Another exciting area of research is the determination of past environments. New studies indicate that changes in the chemistry of the oceans may have signiﬁcantly affected the carbon cycle and have important implications in present- day reef ecology and evolution.
What is important to remember is that rocks and fossils provide the clues to Earth’s evolution. By applying the various principles of geology, we can interpret Earth’s history. It is also equally important to remember that geology is not a static science but one that, like the dynamic Earth it seeks to understand, is constantly evolving as new information becomes available.
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