Chapter 9: A View into Earth’s Past Geologic Time Chapter 9: A View into Earth’s Past

Geologic Column A geologic column is an ordered arrangement of rock layers that is based on relative age older rocks on bottom, newer rocks on top

Fossils in Rock Layers Rock layers can be distinguished by the types of fossils the layers contain Fossils in the upper layers of Earth contain remains of modern plants and animals Fossils in lower, older layers are different than those living today; some may even be extinct

Index Fossils Index fossils are used to help geologists match different rock layers that may be located far apart To be an index fossil it must be… Abundant Widespread around the world Remains of organisms that lived for a short period of time trilobite

Geologic Time Scale A summary of the major events in Earth’s history Divided into units of time (eon, era, period, epoch) Changes in these time periods are typically marked by major changes in Earth’s surface, climate, and types of organisms Mass extinction (dinosaurs) Ice ages

Geologic Time Scale EON – largest segment of geologic time ERA PERIOD EPOCH – smallest segment of geologic time

Geologic Time Scale see page 213

Geologic Time compared to 1 Day

Precambrian Time The time period from the formation of Earth until about 542 million years ago Accounts for 88% of Earth’s history! We know very little about what happened during that time Rock record from this time difficult to interpret due deformed rock from disturbing forces within Earth Fossils are rare from this time, most of Earth was covered by ocean

Precambrian Time

Paleozoic Era 542 million years ago (mya) to 251 mya Earth’s atmosphere becomes oxygen rich at this point At the end of this era Pangaea existed Large fossil record Trilobites was the common marine invertebrate used as an index fossil.

Mesozoic Era 251 mya to 65.5 mya Age of the reptiles There is a rich fossil record for this era Divided into 3 periods: Triassic Jurassic Cretaceous Ended with a mass extinction

Cenozoic Era Current era Age of the mammals 2 periods: Tertiary – before the last ice age Quaternary – after the last ice age

Geologic Time Video https://www.youtube.com/watch?v=H2_6cqa2cP4