Geologic Time Sca

Geologic Time Scale

Overview of Earth's History

• The Geologic Time Scale (GTS) provides a chronological record of life forms and geological events in Earth's history over approximately 4.6 billion years.
• Time Frame: From 650 million years ago to the present.
  • Major divisions include Eons, Eras, Periods, and Epochs.
  • Events in the GTS:
  • 650 million years ago: Evolution of humans (Quaternary)
  • Dinosaurs extinct (Cretaceous)
  • Diversification of mammals (Tertiary)
  • First primates and flowering plants (Jurassic)
  • Diversification of reptiles and first mammals (Triassic)
• The GTS is illustrated visually, showing major events and periods.

Measuring Time

• The Geological time scale was developed by:
  • Studying rock layers and fossils worldwide.
  • Utilizing radioactive dating to determine absolute ages of time divisions.

Naming Time Divisions

• The largest divisions of the GTS are called Eons.
• Eons are subdivided into Eras.
• Eras are further divided into Periods.
• Periods can be divided into Epochs.

Eons

• Definition: The longest, most general divisions of geological time.
• Types of Eons:
  • Cryptozoic (Precambrian) Eon
  • Phanerozoic Eon
• Each eon is subdivided into three eras.

Characteristics of Eons

1. Cryptozoic Eon (Precambrian)

   • Duration: 4600 million years ago to 540 million years ago.
   • Represents approximately 90% of Earth's history.
   • Characterized by the emergence of simple life forms, such as bacteria, algae, and protozoa.
   • Oldest known rocks form during this eon, dating back about 3.5 billion years.
   • Divided into:
     • Proterozoic Era
     • Archean Era

2. Phanerozoic Eon

   • Commenced 540 million years ago and continues to present.
   • Divided into three major eras:
     • Paleozoic Era
     • Mesozoic Era
     • Cenozoic Era

Eras

• Each Eon consists of several Eras.
• Major Eras in Earth's History:
  • Archean Era (4600 mya - 2500 mya)
  • Proterozoic Era (2500 mya - 540 mya)
  • Paleozoic Era (540 mya - 250 mya)
  • Mesozoic Era (250 mya - 65.5 mya)
  • Cenozoic Era (65.5 mya - present)
  • Notes:
  • m.a. indicates million years ago.

Periods

• Each Era is subdivided into Periods.
• Each Period is notable for specific geological events or life forms that define it.
• Periods are further classified into different Epochs.

Time Divisions and Major Events

• Major changes in Earth's history delineate boundaries between time sections.
• Most divisions correlate with significant evolutionary events or extinctions.

Cryptozoic Eon (Precambrian Time)

• Duration: 4600 million years ago to 540 million years ago.
• Oldest and longest eon in Earth's history.
• Characterized by:
  • The emergence of simple organisms like bacteria and algae.
  • Formation of the oldest known rocks dating to about 3.5 billion years.
• Divided into two eras:
  • Archean
  • Proterozoic

Archean Era

• Duration: 4600 million years ago – 2500 million years ago.
• Significant developments:
  • First marine algae appeared.
  • Existence of bacteria recorded around 3800 million years ago.
• Oldest rocks formed approximately 3500 million years ago.

Proterozoic Era

• Duration: 2500 million years ago to 540 million years ago.
• Notable characteristics:
  • Marine invertebrates emerged, with some developing shells.
  • Evidence of glaciations, possibly worldwide during this era.

Phanerozoic Eon

• Commenced 540 million years ago and continues to present.
• Divided into three significant eras:
  • Paleozoic Era (Past Life)
  • Mesozoic Era (Middle Life)
  • Cenozoic Era (Present Life)

Paleozoic Era

• Duration: 570 million years ago to 250 million years ago.
• Contains six periods:
  • Cambrian
  • Ordovician
  • Silurian
  • Devonian
  • Carboniferous
  • Permian
• Features:
  • First organisms with hard parts (such as shells and exoskeletons).
  • Evolution and diversity of various life forms including pteridophytes, amphibians, reptiles, fishes, and trilobites.

Mesozoic Era

• Duration: 250 million years ago to 65 million years ago.
• Known as the Age of Reptiles.
• Divided into three periods:
  • Triassic
  • Jurassic
  • Cretaceous
• During this era:
  • Reptiles and gymnosperms thrived.
  • Dinosaurs evolved, eventually reaching dominance.

Cenozoic Era

• Duration: 65 million years ago to present.
• Divided into two periods:
  • Tertiary
  • Quaternary
• Features:
  • Unsurpassed development of mammals, flowering plants, and the human species.

Phanerozoic Eon Summary

Important Periods in Earth's History

1. Carboniferous Period

   • Duration: 360 million years to 300 million years ago.
   • Significant developments:
     • First reptiles and pteridophytes appeared.
     • Emergence and spread of coal plants due to organic matter burial.

2. Permian Period

   • Duration: 300 million years to 250 million years ago.
   • Notable events:
     • Largest mass extinction occurred, with around 90% of ocean life and 78% of land life perishing, possibly due to climate change and volcanic activity.

3. Jurassic Period

   • Duration: 250 million years ago to 200 million years ago.
   • Characterized by:
     • Emergence of first birds and mammals, and dominance of gymnosperms.
     • Dinosaurs reached their peak during this period.

4. Cretaceous Period

   • Duration: 150 million years ago to 65.5 million years ago.
   • Marks transition from Mesozoic to Cenozoic Era.
   • Witnessed mass extinction event, likely caused by an asteroid impact blocking sunlight, leading to the death of plants, herbivores, and subsequently carnivores.

5. Quaternary Period

   • Duration: 1.8 million years ago to present.
   • Characterized by:
     • Evolution of humans as the most intelligent species.
   • Divided into two distinct Epochs:
     • Pleistocene Epoch
     • Holocene Epoch