Geological Dating and the History of Earth

Dating consists of estimating the age of an event or object or placing it in a specific time or period.

Geological Dating Methods

• Absolute dating: Dates events or materials by determining how many millions of years old they are.
• Relative dating: This puts events, rocks, and fossils in chronological order without specifying exact dates.
• Radiometric dating: A method used to date an object by comparing the number of specific radioactive isotopes it contains.

The Science of Fossils

Fossils: Paleontology is the branch of science that studies fossils. Fossils are the remains of living beings or their activity preserved in rocks; the majority of fossils come from the hardest and most resistant parts of the organism.

Geological Importance of Fossils

Fossils can provide two types of information:

• Temporal information: As species evolve, scientists can learn about specific species through fossils; this means that species and rocks can be dated.
• Paleoecological information: As living beings adapt to the environment, paleontologists can learn about the environmental conditions of a particular age.

The Earth's History

The Pre-Cambrian Eon

• Hadean Eon (4800 Ma – 3800 Ma): First atmosphere; the Earth was separated into crust, core, and mantle; meteorites continually bombarded the Earth; the Moon was formed; first life forms probably existed at the end of the Hadean Eon.
• Archean Eon (3800 Ma – 2500 Ma): First primitive prokaryote life forms; Cyanobacteria (the first autotrophic organisms that obtained their energy through photosynthesis) reduced the concentration of CO₂ and increased the concentration of O₂.
• Proterozoic Eon (2500 Ma – 542 Ma): First organisms that used aerobic respiration; unicellular eukaryotic cells evolved. At the end of this eon, Ediacaran fauna appeared—the first multicellular organisms. They were soft-bodied organisms, and their name comes from Australia.

The Phanerozoic Eon

The Phanerozoic Eon is composed of the Paleozoic Era (542–251 Ma), Mesozoic Era (251–65 Ma), and Cenozoic Era (65 Ma–present day).

The Paleozoic Era (542–251 Ma)

Started in the Cambrian period and ended in the Permian period. A lot of living beings appeared and became extinct very fast. Plants lived on dry land; arthropods, reptiles, and amphibians also lived on dry land.

The Mesozoic Era (251–65 Ma)

Began and ended with major extinction events (the P-T and K-T extinctions).

• Cretaceous Period: Appearance of plants with flowers; extinction of dinosaurs and ammonites.
• Jurassic Period: Gymnosperm plants dominate the Earth’s surface; dinosaur diversity; the appearance of flying and aquatic reptiles; first birds.
• Triassic Period: First dinosaurs and first mammals.

The Cenozoic Era (65 Ma–Present)

Mammals and flowering plants replaced reptiles and gymnosperms. Mammals dominated land and water, and birds dominated the sea. It is divided into the Tertiary and Quaternary periods.