J OURNAL #49 What is the age of the Earth? Since the Earth was created, what do you think happened on Earth? What changes occurred to make it like it is today? As scientists, how can we determine what changes have been made and what Earth was like millions of years ago?

T ODAY, WE WILL LIST THE MAJOR DIVISIONS OF GEOLOGIC TIME

G EOLOGIC T IME The Earth’s surface is constantly changing. To describe the sequence and length of this change, scientists have developed a geologic time scale. This scale outlines the development of Earth and of life on Earth. By studying fossils and applying the principle of superposition, scientists can determine the ages of rocks.

D IVISIONS OF G EOLOGIC T IME Geologic history of the Earth is marked by major changes in Earth’s surface, climate, and types of organisms. These indicators divide the geologic time scale into smaller units. Look on page 213. You will see a simplified geologic time scale. Ma = mega-annum (which means one million years)

U SING THE G EOLOGIC T IME S CALE When did Horses appear? When did Dinosaurs appear? What was the first organism on Earth? When did the first primates appear?

E ONS AND E RAS The largest unit of time is an eon. Geologic time is divided into 4 eons. Hadean eon Archean eon Proterozoic eon Phanerozoic eon. The first 3 eons are part of the Precambrian time. This 4 billion year interval contains most of Earth’s history. Most eons are divided into smaller units called eras.

E RAS There are 4 Eras 1. Precambrian (4.6 Billion years ago) 2. Paleozoic 3. Mesozoic 4. Cenozoic Mammals first appeared in the Cenozoic era Which era did bears first appear?

P ERIODS AND E POCHS Eras are divided into shorter time periods called periods. Each period is characterized by specific fossils and is usually named for the location in which the fossils were first discovered. Periods are divided into smaller units called epochs. These can be divided into ages. Ages are defined by the occurrence of distinct fossils in the fossil record. Eons  Era  Period  Epoch  Age

L ET ’ S R EVIEW What is the name of the time scale that outlines the development of Earth and life on Earth? Answer: Geologic Time Scale

L ET ’ S R EVIEW What are Earth’s 4 Geologic Era’s? Precambrian Era, Paleozoic Era, Mesozoic Era, and Cenozoic Era

L ET ’ S R EVIEW Which period was there a mass extinction that marked the end of the Paleozoic era? Permian

L ET ’ S R EVIEW Which period were dinosaurs the dominant life form on the Earth? Jurassic

L ET ’ S R EVIEW What is the age of the Earth? 4.6 Billion years

Y OUR A SSIGNMENT : Procedure: Part A 1. Figure 1 shows approximately how long age each major division of Earth’s geologic time scale began. Use the information to calculate how long each of these divisions lasted. Record that information in the last column of Figure Using that information, make a bar graph on the grid in the Data and Observations section to show how long each division lasted.