Chapter 17 Unraveling Geologic History

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384 CHAPTER 17: UNRAVELING GEOLOGIC HISTORY HOW CAN WE DETERMINE THE SEQUENCE OF EVENTS? There are two different ways to express age. Relative age is used to express the sequence of events. It tells us that some things are older or younger than others, or that certain events happened before or after other events. For example, you must travel to school before you can attend class. This does not tell us when you came to school or how long you were there before class began, but it does put these two events in sequence. Sometimes sequence is not enough. You may want to know exactly when or how long ago an event took place. This is known as numerical, or absolute, time. If you say that the Earth and solar system formed about 4.6 billion years ago, you are expressing an absolute age. Absolute age always includes a number and a unit of time, such as years, days, or seconds. ACTIVITY 17-1 RELATIVE AND ABSOLUTE TIME You teacher will set out a variety of objects. For this activity, you will work in laboratory groups. Divide your paper into four columns. List the name of each object in the first column. In the next column, estimate the absolute age of each object. The age of some objects will be easy to estimate, while the age of others will probably be uncertain. In the third column, indicate the reliability of your estimate by assigning a margin of error. (For example, if you are confident that the true age is within 10 years of your estimate, the margin of error will be or 10 years.) In the last column, record the method your group used to estimate the age of each object. After you have filled in the four columns, list the objects by their relative age, from youngest to oldest. Geologists use several guiding principles to help them interpret geologic history. With these ground rules and the record of Earth’s history as it is preserved in the rocks, they
have been able to piece together a remarkable account of Earth’s history. Uniformitarianism Geological features sometimes show the results of catastrophic events of the past such as violent volcanic eruptions and continental glaciers. The record of these dramatic events has led some people to assume incorrectly that these events were more common in the geological past than they are today. As another example, when some people see fossils of marine organisms in mountains thousands of meters above sea level, they may not understand that slow uplift over millions of years elevated the land. Geologists have responded to these misconceptions by adopting uniformitarianism as one of their guiding principles. Uniformitarianism is the concept that the geological processes that took place in the past are generally similar to those that occur now. This does not mean that all geological changes are slow and steady. For example, erosion caused by most streams is very slow except when the streams are in flood. Those limited flood events do most of the erosional work of streams and rivers. Most volcanic eruptions are sudden events that are not very frequent. However, the present is not really different from most of the geological past. Scientists can understand most of the features in the geological record by observing similar events in the present. Uniformitarianism is sometimes summerized by the expression, the present is the key to the past. This means that our best way to understand the geological events that shaped our planet is to look at the geological processes at work today. Superposition HOW CAN WE DETERMINE THE SEQUENCE OF EVENTS? 385 Layers of different rock are the results of a series of events. In most places, the lowest layers of sedimentary rocks are the oldest. After all, the lowest layers must be in place before
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Chapter 17 Unraveling Geologic History

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