A Trip Through Geologic Time Review and Assessment

How many years is a "long time"? We often express fourth dimension in hours or days, and ten or xx years certainly feels like a long time. Imagine if you needed to think well-nigh one 1000000, 100 million, or fifty-fifty several billion years. These exceptional lengths of time seem unbelievable, simply they are exactly the spans of times that scientists use to draw the Earth.

The Earth is 4 ^one⁄₂ billion years old. That's 4,500,000,000 years! Have places similar the G Canyon and the Mississippi River been around for all of those years, or were they formed more recently? When did the behemothic Rocky Mountains course and when did dinosaurs walk the Earth? To answer these questions, you accept to think most times that were millions or billions of years ago.

Historical geologists are scientists who study the Earth's by. They study clues left on the World to learn two main things: theorder in which events happened on Earth, andhow long it took for those events to happen. For example, they have learned that the Mississippi River formed many millions of years subsequently the K Canyon began forming. They take also concluded that dinosaurs lived on the Globe for about 200 million years.

Scientists have put together thegeologic fourth dimension scale to describe the gild and duration of major events on Earth for the last four ⁱ⁄₂ billion years. Some examples of events listed on the geologic time scale include the commencement advent of plant life on Earth, the first appearance of animals on Earth, the formation of Earth's mountains, and theextinction of the dinosaurs.

You volition learn about some of the scientific principles that historical geologists use to describe Globe's past. You will also acquire some of the clues that scientists use to learn about the past and shows you what the geologic time scale looks like.

Evaluating Prior Noesis

Before you work through this lesson, think almost the following questions. Exist sure that you can answer each 1. They will help you better understand this lesson.

• What is a fossil and how does a fossil form?
• How does a sedimentary rock form?
• In what types of locations practise sedimentary rocks class?
• How practice you decide the relative and absolute ages of rock layers?

Geologic Time

The first principle you need to understand about geologic time is that the laws of nature are always the same. This means that the laws describing how things piece of work are the same today as they were billions of years ago. For example, h2o freezes at 0°C. This police force has always been truthful and ever will be true. Knowing the natural laws helps you lot think nearly Globe's past, because it gives you clues about how things happened very long ago. It ways that we can use present-day processes to interpret the past. Imagine you findfossils of body of water animals in a rock. The laws of nature say that sea animals must live in the body of water. That law has never changed, and then the rock must take formed near the ocean. The stone may be millions of years old, merely the fossils in information technology are a clue for united states today about how it formed.

Now imagine that you find that same rock with fossils of a bounding main animal in a place that is very dry and nowhere near the sea. How could that be? Remember that the laws of nature never modify. Therefore, the fossil ways that the rock definitely formed by the sea. This tells you that even though the surface area is now dry, it must have in one case been underwater. Clues like this have helped scientists learn that Earth's surface features have changed many times. Spots that were once covered by warm seas may at present be cool and dry. Places that now have tall mountains may accept one time been low, apartment ground. These kinds of changes take place over many millions of years, but they are notwithstanding slowly going on today. The place where yous live right now may look very dissimilar in the far futurity.

Relative and Absolute Historic period Dating of Rocks

The clues in rocks assist scientists put together a flick of how places on Earth accept changed. Scientists noticed in the 1700s and 1800s that like layers of sedimentary rocks all over the world contain similar fossils. They usedrelative dating to lodge the stone layers from oldest to youngest. In the process of relative dating, scientists do not determine the verbal historic period of a fossil just do learn which ones are older or younger than others. They saw that the fossils in older rocks are dissimilar from the fossils in younger rocks. For example, older rock layers comprise simply reptile fossils, simply younger rock layers may too comprise mammal fossils.

Scientists divided Earth'southward history into several chunks of fourth dimension when the fossils showed similar things living on the Earth. They gave each chunk of fourth dimension a name to help them keep track of how Globe has changed. For example, one chunk of time when many dinosaurs lived is called the Jurassic. We discover fossils of World'south starting time green plants from the chunk of time named the Ordovician. Many of the scientists who first assigned names to times in Earth's history were from Europe. As a result, many of the names they used came from towns or other local places where they studied in Europe.

Ordering rock layers from oldest to youngest was a start step in creating the geologic time calibration. Information technology showed the society in which life on Earth changed. Information technology also showed u.s. how sure areas changed over time in regard to climate or type of environs. All the same, the early geologic time scale only showed the order of events. It did not show the bodily years that events happened. With the discovery of radioactive decay in the late 1800s, scientists were able to measure the exact historic period in years of different rocks. Measuring the amounts of radioactive elements in rocks let scientists utiliseaccented dating to give ages to each chunk of time on the geologic fourth dimension scale. For example, they are now able to state that the Jurassic began nearly 200 one thousand thousand years ago and that it lasted for about 55 million years.

Geologic Fourth dimension Scale

Today, the geologic time calibration is divided into major chunks of fourth dimension calledeons. Eons may be further divided into smaller chunks called eras, and each era is divided intoperiods. Figure 12.ane shows y'all what the geologic time scale looks like. We at present live in the Phanerozoic eon, the Cenozoic era, and the Quarternary menstruation. Sometimes, periods are further divided into epochs, but they are ordinarily simply named "early" or "tardily", for example, "late Jurassic", or "early on Cretaceous". Note that chunks of geologic time are not divided into equal numbers of years. Instead, they are divided into blocks of time when the fossil record shows that there were like organisms on Earth.

Effigy 12.i: The geologic time scale.

I of the first scientists to empathize geologic fourth dimension was James Hutton. In the belatedly 1700s, he traveled around Great Great britain and studied sedimentary rocks and their fossils. He believed that the same processes that work on Earth today formed the rocks and fossils from the by. He knew that these processes take a very long time, so the rocks must have formed over millions of years. Before Hutton, most people believed the World was only several thousand years old. His work helped u.s.a. understand that the laws of nature never change and that the Earth is very former. He is sometimes chosen the "father of geology".

The geologic time scale is oftentimes shown with illustrations of how life on Earth has changed. It sometimes includes major events on Earth, likewise, such every bit the formation of the major mountains or the extinction of the dinosaurs. Figure 12.two shows you a dissimilar style of looking at the geologic fourth dimension calibration. It shows how Earth'south environment and life forms have inverse.

Figure 12.2: A different style of looking at the geologic fourth dimension scale.

Lesson Summary

• The Earth is very old, and the report of Earth's past requires us to recall well-nigh times that were millions or even billions of years ago. Scientists utilise the geologic time calibration to illustrate the order in which events on Globe have happened.
• The geologic time scale was developed after scientists observed changes in the fossils going from oldest to youngest sedimentary rocks. They used relative dating to dissever Earth's past in several chunks of fourth dimension when similar organisms were on Earth.
• After, scientists used absolute dating to determine the actual number of years ago that events happened. The geologic time scale is divided into eons, eras, periods, and epochs.

Review Questions

1. How old is the World?
2. Why did early geologic time scales not include the number of years ago that events happened?
3. Dinosaurs went extinct about 66 million years ago. Which period of geologic fourth dimension was the last in which dinosaurs lived?
4. Can scientists use the same principles they employ to study World's history to as well report the history of other planets?
5. Suppose you are hiking in the mountains of Utah and find a fossil of an beast that lived on the ocean floor. Yous learn that the stone that holds the fossil is from the Mississippian period. What was the environment like during the Mississippian in Utah?
6. Why are sedimentary rocks more than useful than metamorphic or igneous rocks in establishing the relative ages of rock?
7. Which is likely to exist more often found in rocks: fossils of very old sea creatures or very old country creatures?

Vocabulary

accented dating

Methods used to decide how long ago something happened.

extinction

When an organism completely dies out.

fossils

The remains of past life, such as bones, shells, or other hard parts; may also include testify of past life such as footprints or leaf impressions.

geologic time scale

A timeline that illustrates World's past.

relative dating methods

Used to determine the club of geologic events in Earth'due south history.

Points to Consider

• How did life on World change from 1 menses of geologic fourth dimension to the next?
• When did life first appear on Earth?
• What atmospheric condition were necessary on Globe for living things to survive?

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Source: https://courses.lumenlearning.com/earthscience/chapter/geologic-time-scale/

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