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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