Discovering Earth’s History

Rocks Record Earth’s History

1. Rocks record geological events and changing life forms of the past.

2. Scientists have learned that the Earth is much older than previously thought and the Earth’s surface and interior have been changed by the same geological processes that go on today.

A Brief History of Geology

1. In the mid 1600’s James Ussher made a timeline of both human and Earth history and determined the Earth was more than 5,000 years old.

2. In the late 1700’s, James Hutton, a Scottish physician, published a book called Theory of the Earth and established the principle of uniformitarianism.

Uniformitarianism means that the forces and processes that have shaped Earth in the past are essentially the same as those operating today.

Relative Dating - Key Principles

1. Relative dating is a process by which rocks are placed in their proper sequence or order. Only the chronological order of events are determined, not the absolute age in years.

2. Nicolaus Steno proposed the Law of Superposition which states in an undeformed sequence of sedimentary rocks, each layer is older than the one above it and younger than the layer below it.

A

B

C

D Oldest

Youngest

3. Steno also posed the Principle of Original Horizontality which says the layers of sediment are generally deposited in a horizontal position.

Original Horizontality

4. The Principle of Cross-Cutting Relationships states that a rock or fault is younger than any rock or fault through which it cuts.

inclusion is any material that is trapped inside a mineral during its formation.

fragments included in a host rock are older than the host rock itself.

Quartz Crystal with Sand Inclusions

5. No place on Earth is geologically complete. Unconformities are surfaces that represent a break in the rock record, caused by erosion or lack of deposition.

Correlation of Rock Layers

1. To develop a geologic time scale that can be applied to the entire Earth, rocks of similar age in different regions must be matched up- a task called correlation.

2. By correlating rocks, it is possible to create a more complete view of the geologic history of the region.

Angular Unconformity• The boundary

between the tilted layers and the horizontal layer.

Disconformity• The boundary

between the older, eroded surface and the younger, overlying layers.

Nonconformity• Stratified rock

rests upon unstratified rock.