MEPX
Chapter 9 of 10All chapters
  1. 1Inside the Earth
  2. 2Plate tectonics
  3. 3Rocks and minerals
  4. 4Shaping the surface
  5. 5Water on Earth
  6. 6Earthquakes
  7. 7Volcanoes
  8. 8The atmosphere
  9. 9Geological time
  10. 10Resources and hazards

Chapter 9 of 10

Geological time

Reading the record.

Dating

Layers are deposited in order, so lower is older. Radioactive decay gives absolute ages, and several independent methods agree on about 4.5 billion years.

  • The geological timescale is divided by major changes in fossils.
  • Humans occupy the last fraction of one percent of it.

Reading a cliff face

Each band is a period of deposition, and breaks in the sequence record erosion or missing time. Sequences like the Grand Canyon are a legible history if you know the conventions.