Explainhow geologic time is recorded in rock layers.<br />Identify important dates on the geologic time scale.<br />Explain how changes in climate resulted in the extinction of some species.<br />Objectives:<br />
Geologic Time<br />The Rock Record and Geologic Time<br />Grand Canyon National Park is one of the best places in North America to see Earth’s history recorded in rock layers.<br />These rock layers represent almost half, or nearly 2 billion years, of Earth’s history.<br />The Fossil Record and Geologic Time<br />Fossils of plants and animals are common in sedimentary rocks that belong to the Green River formation.<br />These fossils are well preserved.<br />Burial in the fine-grained lake-bed sediments preserved even the most delicate structures.<br />
The Geologic Time Scale<br />The geologic column represents the 4.6 billion years that have passed since the first rocks formed on the Earth. <br />To aid in their study, geologists have created the geologic time scale.<br />The geologic time scale<br />Standard method used to divide the Earth’s long natural history into manageable parts.<br />Divisions of Time<br />Geologists have divided the Earth’s history into sections of time.<br />An eon<br />Largest division of geologic time.<br />The four eons are the Hadean eon, theArchean eon, theProterozoic eon, and the Phanerozoic eon.<br />
Eons are divided into eras.<br />For example, the Phanerozoic Eon is divided into three eras.<br />Periods<br />Third-largest divisions of geologic time and are the units into which eras are divided.<br />Periods are divided into epochs.<br />The fourth-largest division of geologic time.<br />The Appearance and Disappearance of Species<br />At certain times during Earth’s history, the number of species has increased or decreased dramatically.<br />Result of a relatively sudden increase or decrease in competition among species.<br />
The number of species decreases dramatically over a relatively short period of time during a mass extinction event.<br />Extinctionis the death of every member of a species.<br />Events such as global climate change can cause mass extinctions.<br />
The Paleozoic Era — Old Life<br />Lasted from about 542 million to 251 million years ago.<br />First era that is well represented by fossils.<br />Marine life flourished at the beginning of the era and the oceans became home to a diversity of life.<br />However, there were few land organisms.<br />By the middle of the Paleozoic era,<br />Most modern groups of land plants had appeared.<br />By the end of the Paleozoic era,<br />Amphibians and reptiles lived on the land, and insects were abundant.<br />The era came to an end with the largest mass extinction in Earth’s history.<br />Some scientists believe that changes in seawater circulation were a likely cause of this extinction, which killed nearly 90% of all marine species.<br />
The Mesozoic Era — The Age of Reptiles <br />Began about 251 million years ago. During this era, reptiles, such as dinosaurs, dominated the land.<br />Small mammals appeared about the same time as dinosaurs, and birds evolved late in the era.<br />At the end of the Mesozoic era,<br />About 15% to 20% of all species on Earth, including the dinosaurs, became extinct.<br />Global climate change may have been the cause.<br />
The Cenozoic Era — The Age of Mammals<br />The Cenozoic era began about 65.5 million years ago and continues to the present. <br />This era is known as the “Age of Mammals.”<br />After the mass extinction at the end of the Mesozoic era, mammals flourished.<br />Mammals were able to survive the environmental changes that probably caused the extinction of the dinosaurs.<br />
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