Fix the mistake: Geologic History and Geologic Time

Fundamentals of Geologic Time Divisions (Eons, Eras, and Periods)

The history of Earth spans approximately 4.6 billion years. To organize this vast amount of time, geologists have constructed a system of geologic time based on fossil records and radiometric dating. Understanding the hierarchy of eons, eras, and periods is the first step in reading Earth’s history.

Basic Knowledge

The largest unit of geologic time is the eon, and Earth’s history is divided into four eons: the Hadean (4.6–4.0 billion years ago), Archean (4.0–2.5 billion years ago), Proterozoic (2.5–0.54 billion years ago), and Phanerozoic (0.54 billion years ago to the present). The Hadean, Archean, and Proterozoic together are called the Precambrian and account for approximately 88% of Earth’s history.

Eons are divided into eras. The Phanerozoic consists of three eras: the Paleozoic (0.54–0.25 billion years ago), Mesozoic (0.25–0.066 billion years ago), and Cenozoic (0.066 billion years ago to the present). Eras are further subdivided into periods. The Paleozoic has six periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.

Boundaries in geologic time are often established using evidence of mass extinctions or major environmental changes. For example, the boundary between the Mesozoic and Cenozoic (the K–Pg boundary) corresponds to the mass extinction at the end of the Cretaceous. Numerical ages are determined mainly by radiometric dating, such as the uranium–lead method.

Example Problem
State the three eras of the Phanerozoic in order from oldest to youngest.
Answer: Paleozoic (0.54–0.25 billion years ago) → Mesozoic (0.25–0.066 billion years ago) → Cenozoic (0.066 billion years ago to the present). Each is separated by mass-extinction events.
Key Points
  • Earth’s history is approximately 4.6 billion years; the Precambrian accounts for 88%
  • Eons: Hadean → Archean → Proterozoic → Phanerozoic (from oldest)
  • Three Phanerozoic eras: Paleozoic → Mesozoic → Cenozoic
  • Six Paleozoic periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian
  • Time boundaries = records of mass extinctions and major environmental changes
  • Absolute ages are determined by radiometric dating (such as the U–Pb method)
  • K–Pg boundary = mass extinction at the end of the Cretaceous (66 million years ago)

Points to Note

① The Precambrian is not the name of a single geologic period; it is a common term for the Hadean, Archean, and Proterozoic together. ② The order of the periods, especially the six Paleozoic periods, is frequently tested on entrance examinations, so memorize the order. ③ The Permian and Dyassic are alternative names for the same period (the European term and the former Japanese term).

Practice

  1. Approximately what percentage of Earth’s 4.6-billion-year history is occupied by the Precambrian?
  2. State the six Paleozoic periods in order from oldest to youngest.
  3. How many million years ago was the boundary between the Mesozoic and Cenozoic (the K–Pg boundary)?

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