restricted access What Is Geology?
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What Is Geology? William B. Dade Geology is about asking, and attempting to answer, questions concerning our planet, our home, the Earth. How did the Earth come to be? It sounds grand, but perhaps you’ve already asked these kinds of questions while hiking in the woods or picking up a shiny rock in a local park, or maybe even sitting in the movies and seeing a view of the Earth from outer space.We can readily see that the Earth is made up of solid parts (rocks) and liquid parts (deeply buried molten material that sometimes erupts onto the Earth’s surface; surface waters in oceans, lakes, and rivers; and the overlying atmosphere). What are its different parts made of, exactly, and how are they arranged? How has the Earth’s makeup and arrangement changed and,at the same time,how has life developed over millions, even billions, of years of Earth history and possibly influenced that process? The lucky people who tackle such questions in a scientific way, with some training in mathematics, physics, chemistry, and biology, are called geologists. That is, geologists base their answers to these and other questions on observations of the natural world that can be interpreted in a rational framework and can be used to yield predictive insight. Their scientific perspective requires no explanations arising from special belief systems or worldviews, and the answers geologists seek are rigorously tested, confirmed, and ultimately accepted by a worldwide community of investigators with similar technical training and dedication to rational thought.This approach makes geology different from the nonscientific ways throughout most of human history,such as mythology and the interpretation of religious scripture, that people have used to address questions about the form of our Earth and the processes that underlie it. Geology is a natural science,and one full of opportunities for wonder,so some geologists jokingly describe themselves as “wonder-full” (okay, and also “won- 184 William B. Dade derful”!) people. We typically share an appreciation for exploring the outdoors, our greatest classroom, and like to get together with other geologists at the end of the day to debate and laugh about our ideas. All are full of amazement and share a humble curiosity about the natural world we live in. It’s about Time We now know that the Earth is a little over 4.5 billion years old. It is difficult to grasp the magnitude of such a number, so think about it this way: Imagine that all Earth history, just over 4.5 billion years, could be condensed to a walk the length of a 100-yard football field. With this visualization each yard represents about 45 million years. If you begin walking steadily from the home-team goal line,you see the first signs of single-cell life somewhere around the near,hometeam 22-yard line; the rise of photosynthesis and the resulting introduction of oxygen into the ancient atmosphere occurs somewhere around the 25-yard line; the earliest multicellular life appears just past midfield,and sexual reproduction kicks in around the far 27-yard line of the away team.The first known footprints (of any kind!) on land appear around the far 11-yard line,and the first land plants appear around the 9-yard line.The age of dinosaurs runs from the 7-yard line to about 1.5 yards from the far goal line.The first grasses and songbirds show up at around the 1-yard line.Our earliest humanlike ancestors emerge in Africa at less than two inches from the far goal line. And Europeans arrive in eastern North America at less than one-thousandth of an inch from the goal line. How did we arrive at this humbling understanding? It’s not like there was a stack of calendars to look at—or chalk marks on a wall marking time, as a castaway on a desert island or a prisoner in a cell might have. Or is there? That is, are there some clues in the rocks around us? Indeed there are! You just need to know how to look! Using chemistry we can detect in rocks small amounts of certain naturally occurring elements that decay at known rates compared to other,readily detectable elements.Such elements are naturally radioactive,meaning that they spontaneously emit energy from, and thus alter, their own atomic structure to become a different, “daughter” element. Radioactive elements, and especially their unique rates of decay,are well studied.So,by...