4-Geologic-Time-Scale

GEOLOGIC TIMESCCALE

• A record of life forms and geological events throughout Earth’s history.

MEASURING THE AGE OF THE EARTH

• Studying rocks and fossils is essential for understanding geological time.

• Radioactive dating is a key method that helped determine absolute divisions in the geological timescale.

TIME DIVISIONS

• EON: Largest section of time.

• ERA: Second largest section.

• PERIOD: Third largest section.

• EPOCH: Smallest section of time.

I. EON

A. Cryptozoic or Precambrian Eon (540-4600 mya)

• Oldest and longest eon, featuring the oldest known rocks.

• Birthplace of simple organisms.

• Divided into two eras: Archean and Proterozoic.

B. Phanerozoic Eon (540 mya – present)

• Notable for the abundance of animal and plant life.

• Divided into three eras: Paleozoic, Mesozoic, and Cenozoic.

II. ERA

A. Archean Era (4600 mya - 2500 mya)

• Earliest development of plants and the first appearance of bacteria.

• Formation of the oldest rocks.

B. Proterozoic Era (2500 mya - 540 mya)

• Common marine invertebrates, few with shells.

• Significant worldwide glaciations.

C. Paleozoic Era (540 mya - 250 mya)

• Emergence of amphibians, reptiles, fishes, and winged insects.

• Further divided into six periods: Permian, Carboniferous, Devonian, Silurian, Ordovician, and Cambrian.

D. Mesozoic Era (250 mya - 65.5 mya)

• Known as the Time of Reptiles.

• Notable for dinosaurs, mammals, and gymnosperms.

• Divided into three periods: Triassic, Jurassic, and Cretaceous.

E. Cenozoic Era (65.5 mya – present)

• Development of modern mammals, angiosperms, and humans.

• Divided into two periods: Tertiary and Quaternary.

III. PERIOD

A. Carboniferous Period (350-300 mya)

• Duration from 360 to 300 mya.

• Emergence of first reptiles and pteridophytes.

• Spreading of coal plants.

B. Permian Period (300-250 mya)

• Marked the largest mass extinction, causes uncertain (possibly climate change and volcanic activity).

C. Triassic Period (250-200 mya)

• Pangaea began to drift apart.

• First dinosaurs and marine reptiles appeared.

D. Jurassic Period (200-150 mya)

• First birds and mammals appeared.

• Dominance of gymnosperms and peak diversity of dinosaurs.

E. Cretaceous Period (150-65.5 mya)

• Conclusion of the Mesozoic era and initiation of the Cenozoic.

• Mass extinction of dinosaurs and many other species.

F. Quaternary Period (1.8 mya – present)

• Evolution of humans as the dominant species.

• Divided into two epochs: Pleistocene and Holocene.

IV. EPOCH

A. Paleocene (65-56 mya)

• Asteroid impact led to the extinction of dinosaurs (except birds).

• Rapid evolution of mammals.

B. Eocene (56-34 mya)

• Period of many extinctions.

• Warm climate.

• Earliest fossils of most modern mammalian orders found.

C. Oligocene (34-23 mya)

• Earth experienced a large object impact.

• Cooler and dryer climate began.

D. Miocene (23-5.3 mya)

• Expansion of grasslands; highest abundance and diversity of mammals.

E. Pliocene (5.3-1.8 mya)

• Savanna and grasslands flourished.

• Further modernization of mammals; global cooling started.

F. Pleistocene (1.8 mya - 11,000 years ago)

• Ice ages occurred, glaciers advanced, and lower sea levels.

G. Holocene (11,000 years ago - present)

• Significant developments in agriculture and human civilizations.

• Emergence of written language; interglacial period began.

CURRENT AGE

• Phanerozoic Eon

• Cenozoic Era

• Quaternary Period

• Holocene Epoch

SIGNIFICANT EVENTS IN EARTH'S HISTORY

• Formation of Earth's crust and ocean.

• Oldest geological evidence of life.

• Proliferation of cyanobacteria.

• Appearance of first eukaryotes.

• Influx of multicellular organisms.

• Arrival of modern humans.

• The Cambrian Explosion.