Introduction to Geologic Time
Geologic time refers to the vast expanse of time that has passed since the formation of the Earth, and spans a period of over 4.6 billion years. Understanding geologic time is critical to the study of geology, as it provides the context for the formation of rocks, fossils, and the evolution of life on Earth.
Key Concepts:
- Geologic time is divided into several eons, eras, periods, and epochs, based on significant changes in the Earth’s environment, climate and life during those times.
- The oldest eon is the Hadean, which lasted from the Earth’s formation to around 4 billion years ago.
- The Phanerozoic eon, which started about 542 million years ago and continues to the present day, is the era during which life diversified and evolved into the complex forms we see today.
- The boundaries between geologic time periods are often defined by mass extinction events, such as the one that wiped out the dinosaurs at the end of the Cretaceous period.
- Radiometric dating uses the decay of isotopes in rocks to establish the age of geologic materials, and is an essential tool for determining the timing of events in Earth’s history.
Equations and Formulas:
- Radiometric dating involves measuring the ratio of parent isotopes (which decay at a known rate) to daughter isotopes in a given sample. The age of the sample can be calculated from this ratio using a formula that takes into account the isotopic decay rate and the time since the sample was last heated enough to reset the isotopic clock.
Examples:
- The Great Unconformity is a boundary in the Grand Canyon between Precambrian rocks (over 1 billion years old) and overlying sedimentary rocks that are much younger in age (around 500 million years old). This unconformity represents a gap in the geologic record that spans hundreds of millions of years.
- The Permian-Triassic boundary (about 252 million years ago) is marked by a mass extinction event that wiped out over 90% of marine species and over 70% of terrestrial species. This event is thought to have been caused by severe environmental changes, such as global warming and ocean acidification.
References:
- Stratigraphy and Geologic Time, USGS
- The Rocks Don’t Lie: A Geologist Investigates Noah’s Flood, David Montgomery
- The Geologic Time Scale, Geological Society of America
