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