Earth’s Materials and Processes-Part 5 The Geologic Time Scale & Earth’s History

The Geologic Time Scale A record of the geologic events and the evolution of life forms as shown in the rock and fossil record. Developed by scientists by studying rock layers and index fossils worldwide. Used relative and radioactive dating to determine the relative and absolute ages of the divisions of the geologic time scale.

Dividing Geologic Time As geologists studied rock and fossil records, they found major changes in life forms at certain times. Fossils provide evidence of change on Earth. These changes are used to identify major events in Earth’s history and mark where one unit of geologic time ends and the next begins.

Precambrian Time Geologic time begins with a span called the Precambrian time. This period covers about 88% of Earth’s history. Few fossils survive from this period. Estimated 4.6 billion years ago to 541 million years ago.

Eras Geologists divide the time between Precambrian and present into three units of time called eras. Paleozoic era: Life increased in complexity, Pangaea formed. Mesozoic era: Dominance of dinosaurs, Pangaea breaking apart. Cenozoic era: Mammals are the dominant land animals, continents move to present locations.

Periods Eras are subdivided into smaller units called periods. Mesozoic era includes three periods: Triassic, Jurassic, and Cretaceous Period.

Major Events of Paleozoic Era The Cambrian Period: many new life forms appeared in a short period of time. Pieced together by scientists using the law of superposition and radioactive dating. Many invertebrates present.

Major Events of Paleozoic Era Vertebrates and land plants appeared. The supercontinent, ‘Pangaea’ formed. Mass extinction occurred, but Scientists are still unsure of the cause. Some scientists think that volcanoes erupted, blocking the sun. Others hypothesize that climate change was to blame.

Major Events of Mesozoic Era Reptiles, Mammals, Birds, and Flowering Plants present. Diverse life forms, including dinosaurs. Scientists hypothesize that another mass extinction occurred.

Major Events of Cenozoic Era Mammals thrive. Ice ages occur. Humans are the dominant land organism.