Geologic Time Scale

Uniformitarianism Earth is an always changing place. Plate tectonics, weathering, erosion etc. The same forces of change that are working today were at work in the past. The Earth has changed gradually over time.

Geologic Time Scale Shows Earth’s history divided into time units based on life forms that existed and major events. Geologist piece together rock layers and fossils to get this time line.

Earth is estimated to be 4.6 billion years old. Oldest living things were single celled organisms (bacteria) that first showed up in the fossil record around 3.5 billion years ago. Humans appeared in the fossil record 100,000 years ago.

Divisions of Geologic Time Eons: Largest unit Eras: eons are divided into eras Paleozoic, Mesozoic, Cenozoic Periods: eras are then divided into periods. Epochs: periods are divided into epochs.

Precambrian Time Longest part of Earth’s history About 4.5 billion years long First igneous rocks formed 3.9 billion years ago. First living things appeared in fossil record (bacteria)

Precambrian Time Invertebrates (no backbone) animals appear at the end of the Precambrian. This time ended with a global ice age.

Paleozoic Era 544 mya – 248 mya Means “ancient life” and referred to as the age of fish. At the beginning of this era there is an explosion of life in the warm, shallow seas. Organisms with shells and vertebrates form (trilobites, mollusks, fish and amphibians) 1st land plants. Increase oxygen in atmosphere

Paleozoic Era Life moves unto land at the end of this era due to increased oxygen. First reptiles (able to reproduce on land) and large insects form. Pangaea forms

Paleozoic Era Largest mass extinction marks the end of this era. 95% of all life disappears in the fossil record. One theory is global-climate change due to massive volcanic activity.

Mesozoic 248 mya – 65 mya Means “middle life” and often know as the age of the reptiles. Dinosaurs ruled the Earth. First flowering plants. First birds and mammals. Pangaea separates.

Mesozoic Another mass extinction at the end of this era. Most believe that an asteroid hit the Earth causing global-climate change.

Cenozoic Era 65 mya - present Means “recent life” and is considered the age of mammals. Several ice ages occur during this time. Fossil of 1st human shows up (100,000 years old).