Geologic time encompasses all the time that has passed since the formation of Earth. When looking at Earth's history -- from its formation to its current state — geologic time is a way to describe the continuous order of events and environmental interactions that have shaped the planet. For purposes of study, analysis, and discussion, the geological timeline can be broken down into different periods based on major geological and astronomical events as reconstructed through scientific investigation.

An eon is the longest division of geologic time and is further divided into eras, periods, and epochs. From the perspective of the geological time scale, we are living during the Phanerozoic eon, Cenozoic era, Neogene period, and Holocene epoch — a unit of time described by flourishing animal life, especially mammals and birds.

Using theoretical models, as well as geological evidence and dating techniques, we are able to assemble a history of the planet going back to its formation about 4.6 billion years ago. However, our understanding of Earth's history is not evenly distributed — we know less and less about it as we explore further back in time. In fact, most of our knowledge extends back only about 600 million years. The events that happened earlier in Earth's history — in the Precambrian eons — are still vastly unknown.

The different layers of rock on Earth's surface are like chapters in the story of Earth's development. As each layer forms at a particular time in history, it keeps a biological, geological, and chemical record of the conditions on Earth at that time. In addition to reflecting the relative sequence of events, each of the layers can be analyzed to determine their absolute age. Scientists rely heavily on radiometric dating to determine the absolute age and duration of the geological ages.

The extinction of the dinosaurs is one event that shows how Earth's layers can tell us a story. The event that caused the death of the dinosaurs is clearly captured in a layer of clay that is rich in iridium. This layer is found in rock formations all around the world, and shows a dramatic difference between the fossils found in the layers above and below it. At the time that the iridium-rich layer was deposited — now determined to be about 65 million years ago — Earth underwent a mass extinction in which the majority of all plants and animals were wiped out. Because iridium is rare on Earth but much more abundant in space, the layer could be explained by a massive asteroid impact on Earth that spewed material worldwide. This theory gained convincing evidence when a large impact crater found in Mexico was dated to about 65 million years ago.

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