The geologic time scale is an essential tool for understanding the history of Earth and the evolution of life. In this lesson, explore the principal eons, eras, periods, and epochs that help us track major events in geologic history.
The Geologic Time Scale
How do we know when the dinosaurs died out? How do we know when birds first appeared on Earth or when humans evolved? What about the beginning of life itself? How was our planet formed and populated by living things over time?
To answer these questions, geologists use a special timeline called the Geologic Time Scale. It's a record of the earth's geologic history as scientists have come to understand it by studying the layers in rock. The geologic time scale is broken up into larger and smaller subdivisions, which help us get a better sense of how historical events fit together. So, in this lesson, we're going to learn how the time scale was created and how its major subdivisions fit together to tell the story of Earth's history.
Study of Strata
People have been studying earth and rock formations for a very long time. In the 19th century, geologists took a closer look at the layers that they saw in sedimentary rocks. They noticed that the rock tended to lie in horizontal sections that had different colors, textures, and fossils inside. The top rock layer might have been limestone containing lots of snail fossils. The next layer may have been chunky conglomerate rock, while the next was a layer of shale and fish fossils. Geologists called these layers of different rock types strata. They studied rock strata all around the world in order to figure out major events in geologic history. Over time, geologists and other scientists put all that information together to make the geologic time scale.
Before we learn the parts of the geologic time scale, let's first talk about how we measure time in our own daily lives. For instance, how do you measure the time it takes to get ready for work or school? Do you measure it in minutes? Hours? Seconds? Days? Chances are you probably measure in minutes. Minutes are fine for measuring daily chores, like driving to an appointment, fixing dinner, or doing the laundry. But what if you're talking about a bigger chore, like training for a marathon? You probably plan out your preparation on the scale of days, hours, weeks, and months. We measure our age in terms of years. We measure generations in terms of decades. And when we look at human history, we talk about it in terms of hundreds and thousands of years.
Obviously, it doesn't make sense to talk about everything on the same time scale. That's why we've broken up our time. Years are made up of months, months are made up of weeks, weeks are made up of days, and so on. Geologists use the very same strategy to talk about the history of the earth. They break up geologic time into larger and smaller chunks, so that major events are easier to talk about. Okay, now let's go ahead and check out the major divisions of the geologic time scale.
Eons and Eras
The first principal subdivision is called the eon. An eon, the largest division of the geologic time scale, spans hundreds to thousands of millions of years. Geologists generally agree that there are two major eons: the Precambrian eon and the Phanerozoic eon. The Precambrian goes from the formation of the earth to the time when multicellular organisms first appeared - that's a really long time - from 4,500 million years ago to just about 543 million years ago. Then begins the Phanerozoic eon, which continues up to today.
Eons are made up of eras, divisions that span time periods of tens to hundreds of millions of years. The three major eras are the Paleozoic, the Mesozoic, and the Cenozoic. The Cenozoic era is the one we are in today. It began 65 million years ago, right about the time that the dinosaurs went extinct.
Keep in mind that these three eras are all grouped within the Phanerozoic eon. Remember that other eon, the Precambrian eon? Well, that one doesn't get to have any eras inside it. We don't have a lot of information about it, so we leave it as one big chunk in geologic history.
Periods and Epochs
The next subdivision down from the era is the period, a division of geologic history that spans no more than one hundred million years. You're probably most familiar with the periods of the Mesozoic era: the Triassic, the Jurassic, and the Cretaceous periods. These are the periods that included our most favorite dinosaur species, like the Triceratops and Tyrannosaurus Rex. Today, we live in the Quaternary period of the Cenozoic era.
Finally, we have the smallest division of the geologic time scale, called the epoch. Epochs are the chunks of time that describe the evolutionary ups and downs of mammals and birds. You've probably seen pictures of giant prehistoric creatures, called 'megafauna', like the wooly mammoth, the giant ground sloth, and a Saber-tooth Cat. These all appeared sometime in the Tertiary period, which includes the Eocene, Miocene, and Pliocene epochs. The Pleistocene epoch marks the bulk of human evolution, beginning around 1.8 million years ago. Our current epoch is the Holocene epoch; it only started about 12,000 years ago.
So let's recap about the geologic time scale and how its divisions fit together. Epochs are the smallest divisions. Many epochs make up a period, many periods make up an era, and many eras make up an eon. In defining the boundaries between major divisions, we often use markers, like the dinosaur extinction or the appearance of certain organisms.
But fossil evidence isn't the only information we rely on. As you'll see in other lessons, the geologic time scale arises from an in-depth study of the trends in rock strata, like how the layers are arranged and how they are composed of certain chemicals. The more information we gather about the rock strata, the more we can tweak the Geologic Time Scale to fit the evidence. But, now that we've mapped out a pretty good timeline, we can use it as a universal tool for talking about our planet and studying its history. After all, the whole point of having the geologic time scale is to give scientists a logical framework on which to reference major events in geologic time.
This lesson should teach you to:
- Define strata and the Geologic Time Scale
- Explain how geologists developed the Geologic Time Scale
- Describe eons, eras, periods, and epochs