Geological Time Scale, Dating Methods, and Earth History

Posted on Dec 24, 2025 in Geology

Geological Time Periods and Key Events

Paleozoic Era

  • Cambrian: Marine invertebrates with external skeletons.
  • Ordovician: First vertebrates appear.
  • Silurian: First jawed fish; first land plants.
  • Devonian: First amphibians.
  • Carboniferous: Forests of giant ferns; first reptiles.
  • Permian: Pangaea forms. The climate became very dry toward the end, leading to the greatest mass extinction event.

Mesozoic Era

  • Triassic: First dinosaurs; first mammals.
  • Jurassic: Gymnosperm plants dominate; first birds appear.
  • Cretaceous: First flowering plants. Ashes and dust from volcanic activity led to a massive extinction event.

Precambrian History

Geological Events in Precambrian

  • Formation of the supercontinent Rodinia.
  • Global glaciation (Snowball Earth).
  • Formation of the ozone layer, leading to increased biodiversity.

Biological Events in Precambrian

  • Photosynthesizing prokaryotic cells.
  • The Great Oxygenation Event.
  • Unicellular eukaryotic cells.
  • Multicellular organisms (Ediacaran biota).

Mesozoic and Cenozoic Tectonic Events

Mesozoic Geological and Biological Events

By the end of the Jurassic period, Gondwana split into two parts. The Tethys Sea separated Gondwana and Laurussia.

Cenozoic Geological and Biological Events

Continents collide, initiating the Alpine Orogeny. The Alps, Caucasus, Pyrenees, and Himalayas rose up. The Isthmus of Panama formed as North and South America joined. Ice sheets spread globally.

Formation of Pangaea

  • Cambrian–Ordovician: The largest fragment of Rodinia, Gondwana, moved toward the South Pole and underwent glaciation. Laurentia, Siberia, and Baltica were separated from Gondwana.
  • Silurian–Devonian: Baltica and Laurentia collided in the Caledonian Orogeny, forming Laurussia.
  • Carboniferous Period: Laurussia collided with Gondwana in the Hercynian Orogeny. The middle section suffered deformations. Gondwana became very cold, and glaciers formed.
  • Permian Period: The Hercynian Orogeny continued; Siberia collided with Laurussia. PANGAEA WAS COMPLETED.

Fundamental Geological Theories

Catastrophism

Based on the belief that there had been catastrophes that had changed the Earth quickly.

Uniformitarianism

Slow processes could cause changes in the Earth over vast periods of time.

Neocatastrophism

An intermediate view: both slow and quick events could change the Earth.

Stratigraphy and Geological Structures

Stratigraphy

The study of rock strata (layers).

Stratum

A layer of rock deposited horizontally.

Deposition

Occurs in episodes, not as a continuous process.

Geological Structures

  • Dyke: Veins that run perpendicular to the strata.
  • Sill: Sheets that run parallel to the strata.
  • Batholith: A huge, bulbous mass intruding surrounding strata deep underground.

Geological Dating Methods

Relative Dating

Determining the sequence of events without specifying an exact numerical age.

  • Principle of Superposition: A stratum is newer than those below it, but older than those above it. This is true if the layers have never been overturned.
  • Principle of Cross-Cutting Relationships: Any dyke or fault is newer than the rocks it affects, but older than the rocks it does not.
  • Erosion of the Strata: Erosion is greater in continents where relief is exposed to weathering.
  • Erosion: Depends on the weather above sea level; greater on exposed rocks and land.
  • Sediments in Marine Environment: An erosion surface indicates a sea regression has occurred in between depositional events.

Absolute Dating

Specifying a numerical age.

  • Radioactive Dating: Atoms go through a process of disintegration, making them more stable. This process occurs at a constant speed.
  • Ice Core Dating: Dust and bubbles are trapped in the layers of glaciers. Studying them allows us to determine past climate changes or volcanic eruptions.

Fossils and Fossilization

Fossil

The remains of living things or the molds/cavities formed by them in sedimentary rocks.

Fossil Formation Process

  1. A crocodile dies in a river.
  2. Sediments cover the body.
  3. Sediments become rock, and the remains become a fossil.
  4. The rock erodes, exposing the fossil.