Built by a geologist
Geologic Time
Geologic Time One of geology's most profound contributions to human knowledge is the concept of deep time — the realization that Earth is incomprehensibly old, and that most of its history occurred long before humans existed. How Old Is Earth? Earth formed approximately 4.54 billion years ago from the accretion of dust and gas in the solar nebula. We know this from radiometric dating of meteorites and the oldest Earth rocks. The Geologic Time Scale Geologists divide Earth's history into a hierarchical time scale: | Division | Duration | Example | |----------|----------|---------| | Eon | Hundreds of millions to billions of years | Phanerozoic 541 Ma – present | | Era | Tens to hundreds of millions of years | Mesozoic 252–66 Ma | | Period | Tens of millions of years | Jurassic 201–145 Ma | | Epoch | Millions of years | Holocene 11,700 years ago – present | The Four Eons 1. Hadean 4.54–4.0 Ga : Earth forms, molten surface, no life, heavy bombardment 2. Archean 4.0–2.5 Ga : First oceans, first life single-celled organisms , no oxygen in atmosphere 3. Proterozoic 2.5 Ga–541 Ma : Oxygen rises, first complex cells, first multicellular life, Snowball Earth events 4. Phanerozoic 541 Ma–present : Explosion of complex life, the eon we live in The Phanerozoic is divided into three eras: - Paleozoic "ancient life" : Fish, amphibians, reptiles, first forests - Mesozoic "middle life" : Age of dinosaurs, first mammals, first flowering plants - Cenozoic "recent life" : Age of mammals, rise of humans Relative vs. Absolute Dating - Relative dating : Determining the order of events using principles like superposition older rocks on bottom and cross-cutting relationships - Absolute dating : Determining the actual age in years using radiometric methods measuring radioactive decay of isotopes like uranium-lead, potassium-argon, and carbon-14 Putting It in Perspective If Earth's entire history were compressed into a single 24-hour day: - Life appears around 4:00 AM - Fish appear at 9:30 PM - Dinosaurs appear at 10:40 PM and go extinct at 11:40 PM - Humans appear at 11:59:59 PM — the last second of the day