Geologic Time Scale

• Geologic Time Scale:

  • Earth's age is believed to be 4.6 billion years
  • It is broken down into different units and sub-units based upon the rocks and fossils within those rock layers
  • Life on Earth arose around 3.5 billion years ago
  • Over Earth’s vast history, both gradual and catastrophic processes have produced enormous changes
• Terms:

  • Eon: largest division of the geologic time scale; spans hundreds to thousands of million of years ago
  • Era: division in an Era that spans time periods of tens to hundreds of million of years
  • Period: a division of geologic history that spans no more than one hundred million years
  • Epoch: the smallest division of the geologic time scale characterized by distinctive organisms
• Precambrian Eons:

  • More ancient Eon covering approximately 88% of Earth’s History
  • Divided into Hadean Era, Archean Era, and Proterozoic Era
  • Erosion has destroyed most rocks and fossils, making surviving fossils rare and simple in structure
• Hadean Era:

  • 4.6 billion years ago to roughly 3.8 billion years ago
  • Chaotic time due to meteorite bombardment
  • Atmosphere reduction with methane, ammonia, carbon dioxide
• Archean Era:

  • 3.8 - 2.5 billion years ago
  • Lots of volcanic activity and eruptions
  • Atmosphere made up of methane gas, carbon dioxide, sulfur dioxide but no free oxygen gas
  • Presence of stromatolites created by photosynthetic cyanobacteria
• Proterozoic Era:

  • 2.5 billion years ago to 542 million years ago
  • Massive and rapid continental accretion
  • Atmosphere enriches with oxygen due to bacteria
  • Rise of multicellular organisms represented by the Vendian fauna
  • Formation of the protective ozone layer
• Phanerozoic Eons:

  • The most recent eon
  • Greatest diversity of land and ocean organisms
  • Fossil record indicates complex organisms thrive
  • Currently living in the Phanerozoic
• Paleozoic Era:

  • Longest and diverse era among the three eras of the Phanerozoic eon
  • Started 300 million years ago
  • Divided into 6 major periods: Cambrian, Devonian, Ordovician, Carboniferous, Silurian, Permian
• Cambrian Period:

  • 550 - 500 million years ago
  • Explosion of new organisms
  • Abundance of trilobites
  • Age of invertebrates
• Ordovician Period:

  • 500 - 450 million years ago
  • First appearance of jawless fish known as AGNATHA
  • Plants dominate the land while animals remain in the water
• Silurian Period:

  • 450 - 400 million years ago
  • Corals appear in the ocean
  • Vascular plants evolve on land
  • Emergence of terrestrial life like air-breathing scorpions and millipedes
• Devonian Period:

  • 400 - 350 million years ago
  • Known as the "age of fishes"
  • First seed plants and lobe-finned fish evolved
  • Cartilaginous fish like sharks and rays were common
• Carboniferous Period:

  • 350 - 300 million years ago
  • Widespread forests led to coal and crude oil deposits
  • First amphibians and reptiles evolved
  • Tropical and humid climate
• Permian Period:

  • 300 - 250 million years ago
  • All major land masses collided to form Pangea
  • Greatest mass extinction of all time
  • Extreme temperatures and dry climate
• Mesozoic Era:

  • Rise of dinosaurs
  • Spans from about 252 million years ago to about 66 million years ago
  • Known as the age of reptiles or dinosaurs
  • Rapid spread of true pines, redwoods, and flowering plants
• Jurassic Period:

  • 200 - 150 million years ago
  • Golden age of dinosaurs
  • Earliest birds evolved from reptile ancestors
  • Major group of mammals evolved
• Triassic Period:

  • 250 - 200 million years ago
  • First dinosaurs branched off from reptiles
  • Huge seed ferns and conifers dominated the forest
  • Modern corals, fish, and insects evolved
• Cretaceous Period:

  • 150 - 65 million years ago
  • Dinosaurs reach their peak and distributions
  • Extinction of dinosaurs
  • Warm climate with poles lacking ice caps
• Cenozoic Era:

  • Rise of mammals
  • Represents more than 2% of Earth’s history
  • Currently in the Halocene epoch
  • Fossils of organisms that survived the Cretaceous-Tertiary extinction
• Quarternary Period:

  • 2 million years ago to present
  • Climate cooled leading to ice ages
  • Homo sapiens evolved
  • Woolly mammoths adapted to the cold
• Tertiary Period:

  • 65 - 0.01 million years ago
  • Climate was warm and humid
  • Mammals evolved to fill niches vacated by dinosaurs
  • Modern rain forests and grasslands appeared