1. Geologic Time
Unit 8.5

2. Geologic Time Scale
Geologists have divided the whole of geologic history into units of varying magnitudes, called the Geologic Time Scale. The major units of the time scale were designated during the 19th century, mainly by scientists in Western Europe and Great Britain.

3. Geologic Time Scale
Because radiometric dating was not available at that time, the entire time scale was created using methods of relative dating. It was only in the 20th century that radiometric dating permitted numerical dates to be added.

4. Geologic Time
Geologic time is divided into time units of different sizes:
Eon Era Periods Epoch
*Fundamental time interval because each period represents a major change in life forms

5. Precambrian Time
90% of Earth’s history occurred during the Precambrian time (4.5 bya– 543 mya). Most rocks & fossils did not survive from this time. The Precambrian time is broken into Hadean, Archean and Proterzoic Eons.

6. Hadean Eon
The Hadean was a time of extreme volcanic activity and heavy meteorite impacts. There was no ocean due to the intense volcanic gases and steam – too hot for liquid water to exist on the surface and in the atmosphere.

7. Proterzoic Eon
In the Proterzoic Eon, which lasted 2 billion years, the landmasses joined & broke apart. The evolution of single-celled organisms and primitive multi-celled plants/animals took place. The accumulation of free O2 in the atmosphere occurred resulting in a primitive O3 layer – this reduced harmful UV light from reaching the Earth’s surface.

8. Archean Eon
During the Archean Eon, Earth’s surface started to cool, which allowed ocean formation. Islands were formed by volcanoes. These islands collided with one another forming continents. Simple life formed at this time. Oxygen became abundant due to photosynthesis of primitive stromatolites.

9. Paleozoic Era
In the Paleozoic Era (543 million to 295 mya), the sea levels rose and fell, influencing how life progressed and diversified.
The Paleozoic broken into 6 periods:
Cambrian Ordovician Siluarian
Devonian Carboniferous Permian

10. Cambrian Period
During the Cambrian Period, the Cambrian Explosion occurred due to high sea levels which resulted in a great diversity of life. Many organisms developed the ability to produce shells which provided them with protection. The dominant fossil of this time was the trilobites.

11. Ordovician Period
In the Ordovican Period, Gondwanaland (South America, Australia, Antarctica, and India) formed over the Southern Pole. Early on during this period, abundant and diverse marine life flourished. A surge of extinctions followed due to glaciation and lowered sea levels.

12. Silurian Period
During the Silurian Period, the climate started to stabilize as glaciers melted and sea levels rose. Terrestrial life emerged as plants first moved onto land, and animals followed.

13. Devonian Period
Laurasia (North America, Europe, and Siberia) formed around the equator. Plants, forests, and ferns flourished during the Devonian period. Fish diversified into many new groups. Bony, lobed-finned fishes gave rise to land- living vertebrates. Known as “The Age of the Fish”.

14. Carboniferous Period
Pangaea formed due to the joining of Laurasia and Gondawaland. Swampy conditions from the Carboniferous Period produced coal deposits as organisms’ remains settled and decayed. Most of the coal we used today come from swamps of this period. The first amniotes (shelled egg organisms) evolved, allowing a complete move onto land.

15. Permian Period
During the Permian Period, Pangaea caused climates to change. Glaciers decreased and continents became drier, and sea levels decreased. This is the period where the greatest extinction of all time occurred. - 95% of marine species & 70% land species Reptiles continued to evolve, giving rise to the dinosaurs in the Mesozoic Era.

16. Mesozoic Era
The Mesozoic Era contained the Triassic, Jurassic, and Cretaceous Periods. Animals that survived the Permian Extinction evolved into dinosaurs – Age of the Reptiles. Land plants diversified and flowering plants emerged.

17. Mesozoic Era
Pangaea started to break up at the rift between North America and Africa. The changes inland caused extensive coastlines, members of the same species were separated resulting in the evolution of new species. During the Mesozoic Era, Earth’s climate became warmer.

18. Mesozoic Era
In the Cretaceous Extinction (65 million years ago), 60% of all of Earth’s species were killed off. The theory currently accepted for the cause of the Cretaceous Extinction is that a large meteorite (10m in diameter) slammed into the Yucatan Peninsula. This is due to the large amounts of Iridium found in this layer of bedrock

19. Cenozoic Era
The Cenozoic Era can be broken into Tertiary and Quaternary Periods. Due to the Cretaceous Extinction, mammals started to become the dominant group – Age of the Mammals. During the era, Pangaea experienced its 3rd & final break up – North America/Greenland from Europe & Antarctica from Australia.

20. Quaternary Period
The Quaternary period (the Pleistocene and Holocene Epochs) is the time humans evolved. Climates cooled, causing an Ice Age responsible for dropping sea levels. Due to the drop of the sea level, land bridges were exposed, creating connections between land masses which allowed humans to migrate from Asia into North America.

21. Quaternary Period
A small extinction occurred in the Pleistocene Period, possibly due to extreme climate changes and hunting by humans. The Holocene period is known as the “Human Age” as it includes all of Humanity’s recorded history (past 10,000 years). Many scientists claim that we are actually at the dawn of a 3rd period, the Anthrocene Period, as humans have had a dramatic impact on the Earth.