Relative Dating : Which Came First?

Beginning with a History of Geology

Theories of Geology

1. Catastrophism
1. Earth was shaped by dramatic events called catastrophes
2. Like MASSIVE earthquakes, floods, and volcanoes

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• Uniformitarianism
• Proposed by JAMES HUTTON (hint!) in 1788
• That the earth was formed and operates on the same principles that we see today.
• Supported by CHARLES LYELL in 1833 in his books “Principles of Geology”

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Modern Geology

• Mixture of the two, yes most of the time it is very gradual and constant but there can be sudden changes

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• Combine this with Paleontology, those who study the fossils of long gone organisms and we start to get a complete picture

Principles of Geology

• Geologists try to determine the order in which events have happened during Earth’s history.
• They rely on rocks and fossils to help them in their investigation.
• Relative dating
• The process of determining whether an event or object is older or younger than other events or objects.

• Layers of sedimentary rock, such as the ones shown below, are stacked like pancakes.
• As you move from the top to the bottom in layers of sedimentary rock, the lower layers are older.
• Superposition
• Principle that states that younger rocks lie above older rocks, if the layers have not been disturbed.

• Disturbing Forces
• Not all rock sequences are arranged with the oldest layers on the bottom and the youngest layers on top.
• Some rock sequences have been disturbed by forces within the Earth.
• These forces can:
• Push other rocks into a sequence
• Tilt or fold rock layers
• Break sequences into moveable parts.

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The Geologic Column

• Geologic Column
• an ideal sequence of rock layers that contains all the known fossils and rock formations on Earth, arranged from oldest to youngest.
• Geologists use the geologic column to:
1. Interpret rock sequences
2. Identify the layers in puzzling rock sequences.

Disturbed Rock Layers

• Geologists
• often find features that cut across existing layers of rock.
• assign relative ages to the features and the layers.
• The features must be younger than the rock layers because the rock layers had to be present before the features could cut across them.

fault

intrusion

• Events That Disturb Rock Layers
• Geologists assume that the way sediment is deposited to form rock layers — in horizontal layers — has not changed over time.
• If rock layers are not horizontal
• something must have disturbed them after they formed.
• Four ways that rock layers may become disturbed.
• A fault
• break in the Earth’s crust along which blocks of the crust slide relative to one another.
• An intrusion
• molten rock from the Earth’s interior that squeezes into existing rock and cools.

3. Folding
• occurs when rock layers bend and buckle from Earth’s internal forces.
4. Tilting
• occurs when internal forces in the Earth slant rock layers.

Gaps in the Record -- Unconformities

• Missing Evidence
• Sometimes, layers of rock are missing, creating a gap in the geologic record.
• Unconformity
• break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.

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Types of Unconformities

• Most unconformities form by both erosion and nondeposition, but other factors may be involved.
• Geologists place them into three major categories:
1. Disconformities
2. Nonconformities
3. Angular unconformities
• Disconformities exist where part of a sequence of parallel rock layers is missing.

• Nonconformities
• exist where sedimentary rock layers lie on top of an eroded surface of nonlayered igneous or metamorphic rock.

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• Angular Unconformities
• exist between horizontal rock layers and rock layers that are tilted or folded.

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Rock-Layer Puzzles

• Rock-layer sequences often have been affected by more than one geological event or feature.
• For example, intrusions may squeeze into rock layers that contain an unconformity
• Determining the order events
• is like solving a jigsaw puzzle.
• piece together the history of the Earth.

Fossils can help us to fill in the gaps as well!