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Earth History (Geological Timeline)
Lecturer Dr. Sameh Abu El Soud

History of Early Geology
Catastro‐phism Tries to explain the presence of everything on earth and  their composition by catastrophes.  E.g. A catastrophe happened to  dinosaurs 80 million years ago.  A comet fell on earth surface leading to  volcanoes, leading to dust clouds in the atmosphere, preventing solar  rays penetration , the plants died and then the dinosaurs.

Uniformi‐tarianism (James Hu on, late 1700s) ‐ He proposed that the  same processes that are at work today were at work in the past.  Summarized by “THE PRESENT IS THE KEY TO THE PAST.” A gradual  continuous change that accumulates with time for thousands and  millions of years causing “significant change”.  Uniformitarianism is a  basic foundation of modern geology.

Age of the Earth
Estimated age for the Earth and the rest of the solar system is about 4.55 billion years. The oldest Earth rocks: 3.8 to 3.9 billion years Oldest Earth minerals (zircons): 4.2 billion years Oldest Moon rocks: 4.44 billion years

Early Earth Timeline
•About 5.5-6 billion years ago (BYA), the solar nebula begins to collapse •About 4.6 BYA, Sun begins fusion •About 4.5-4.56 BYA, Proto-Earth formed from planetesimals. •4.44+ BYA, Earth-Moon formed by giant impact. Earth melts, magma occurs. •4.2 BYA, Earth was completely differentiated. •4 BYA, earliest oceans formed, thick atmosphere exists •3.8 BYA, life develops •2.5-3 BYA, photosynthesis leads to O2 in ocean •2 BYA, O2 hits atmosphere

The Geologic Time Scale
Structure of the Time Scale
Based on their interpretations of the rock record, geologists have divided Earth’s 4.56-billion-year history into units that represent specific amounts of time. Taken together, these time spans make up the geologic time scale.

The Geologic Time Scale
Structure of the Time Scale
Eons represent the greatest expanses of time. Eons are divided into eras. Each era is subdivided into periods. Finally, periods are divided into smaller units called epochs. There are three eras within the Phanerozoic eon: the Paleozoic, which means “ancient life,” the Mesozoic, which means “middle life,” and the Cenozoic, which means “recent life.”

The Geologic Time Scale
Structure of the Time Scale
Each period within an era is characterized by somewhat less profound changes in life forms as compared with the changes that occur during an era.

The Geologic Time Scale
Precambrian Time
During Precambrian time, there were fewer life forms. These life forms are more difficult to identify and the rocks have been disturbed often.

• Geologic time is divided into different types of units. • Note that each Eon, Era or Period represents a different amount of time. For example, the Cambrian period encompasses ~50 million years whereas the Silurian period is only ~26 million years old. • The change in periods is related to the changing character of life on Earth and other changes in environment. • The beginning of the Phanerozoic represents the explosion of life. • The time before the Phanerozoic is commonly referred to as the PreCambrian and represents over 4 billion years of time. The Phanerozoic eon (abundant life) represents only the last 13% of Earth time.

The Geologic Time Scale
Difficulties With the Geologic Time Scale
A sedimentary rock may contain particles that contain radioactive isotopes, but these particles are not of the same age as the rock in which they exists. The age of a particular mineral in a metamorphic rock does not necessarily represent the time when the rock was first formed. Instead, the date may indicate when the rock was metamorphosed.

Discovering Earth’s History
Rocks Record Earth History
Rocks record geological events and changing life forms of the past. We have learned that Earth is much older than anyone had previously imagined and that its surface and interior have been changed by the same geological processes that continue today....
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