Geologic Time Scale:
Geologic Time Scale:
• The Earth is over 4.7 billion years old.
• Life started as single-celled organisms and now has a great variety of multi-celled organisms.
• Scientists study life’s history from fossil records arranged in a Geologic Time Scale.
Organizing Earth’s History
• The Geologic Time Scale is a timeline that organizes the events in Earth’s history, like a calendar.
• Single-celled organisms dominated most of Earth’s history.
• Plants and fish have only evolved within the past 500 million years.
• Humans evolved about 110,000 years ago.
• If the entire history of Earth were squeezed into a single day, humans would not evolve until the last few seconds.
• The scale is divided into eons, era, periods, and epochs.
• An Eon, billions of years ago, is divided by the Cambrian boundary when the variety of life forms exploded
• An Era includes mass extinctions that mark the boundaries between eras.
• A Period includes blocks of time when a unique rock series was laid down.
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Fossils
• Fossils provide evidence for the geologic time scale.
• They are imprints or remains of organisms that were once alive.
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Fossil Formation:
• Fossils form when layers of sediment build up over a dead organism.
• Over time, pressure can cause these layers to turn to rock and the remains are preserved.
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• Fossils form in different ways, depending on the material the fossil is forming in and the climate.
|Fossil |Process |Notes |
|Molds and Casts |Acids eat away the skeleton or shell and leaves |If the mold fills with minerals, it becomes a |
| |an impression (mold) in the rock |cast |
|Tracks and Trails |Imprints are left in mud, which later hardens |Dinosaur tracks are fairly common |
|Petrifaction |Minerals in plant cells crystallize; minerals |Common with plants |
| |enter openings or cavities in shells or bones. | |
|Soft tissue preservation | |Fossils do not last long |
| |Mummification or frozen in ice | |
|Traps |Whole organisms locked in tar pits, asphalt, | |
| |amber, etc. | |
Fossils and Geologic Time Scale
• Most fossils form in sedimentary rock.
• Positions of fossils with in the rock help construct the geologic time scale because the lowest layer of rocks are the oldest, meaning the fossils would be
the oldest too.
• Therefore, fossils on the top are from organisms that lived in more recent times.
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Review Questions:
1. Which division in the geologic time scale is billions of years long?
- Eon
2. During which period in the geologic time scale was there a great increase in the variety of life forms on Earth?
- Cambrian Boundary
3. During which epoch in the geologic time scale did humans evolve?
- Holocene
4. Which type of organism evolved first?
|Millions of Years Ago |Event |
|360-286 |First reptiles |
|410-360 |First amphibians |
|440-410 |First insects |
|505-440 |First fish |
- first fish
5. How can the Grand Canyon be used to construct the geologic time scale?
- Fossil algae at the bottom lived more than 2.5 billion years ago, seed plants in the middle layers lived more than 65 million years ago.
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