The geologic time scale, major eras and periods, and key events in Earth's history.
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- What is a geological eon?
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The largest division of geologic time, lasting hundreds of millions to billions of years.- What is a geological era?
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A major subdivision of an eon, typically lasting tens to hundreds of millions of years.- What is a geological period?
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A subdivision of an era, typically lasting tens of millions of years.- What is a geological epoch?
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The smallest major subdivision of geologic time, lasting millions of years.- Name the four major eons of Earth history in order from oldest to newest.
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Hadean, Archean, Proterozoic, Phanerozoic.- When did the Hadean Eon end?
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Approximately 3.8 to 4.0 billion years ago.- What major event defines the Hadean Eon?
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The Late Heavy Bombardment and the formation of Earth's earliest solid crust.- When did the Archean Eon begin and end?
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From approximately 4.0 billion years ago to 2.5 billion years ago.- When did the Proterozoic Eon occur?
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From approximately 2.5 billion years ago to 541 million years ago.- Name the three major eras of the Phanerozoic Eon.
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Paleozoic, Mesozoic, Cenozoic.- When did the Paleozoic Era occur?
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From approximately 541 million years ago to 252 million years ago.- What event marks the end of the Paleozoic Era?
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The Permian-Triassic extinction event (the Great Dying), killing about 96 percent of marine species.- When did the Mesozoic Era occur?
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From approximately 252 million years ago to 66 million years ago.- What event marks the end of the Mesozoic Era?
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The Cretaceous-Paleogene extinction event, caused by an asteroid impact.- When did the Cenozoic Era begin?
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Approximately 66 million years ago and continues to the present.
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