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Principles of Relative Dating
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Principles of Relative Dating

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Undisturbed Strata

A layer of rock that is in its original position. No geologic forces have moved it or altered it from its original position.

Vocabulary

Relative Dating

a way of determining the relative age of a layer of rock by comparing it to layers around it

Strata

Layers of rock(Stratum is singular)

Youngest
Oldest
In undisturbed strata, the layer on the bottom is the oldest (meaning it was created first).

Principle of Superposition

Principle of Superposition

Each differently colored layer is a different type of rock formed during a different time period.
The rocks on the bottom are the oldest according to this principle.

Principle of Superposition

Each differently colored layer is a different type of rock formed during a different time period.
The rocks on the bottom are the oldest according to this principle.
Youngest
Magma
Oldest

Principle of Crosscutting

Events or layers of rock that cross other layers are younger than the layers they cut across. Common cross cutting layers / events: Igneous intrusions & Fault lines (breaks in layers of rock)

Principle of Crosscutting - example

These white lines across the darker rock are a different type of rock. They were once magma that creeped up into the darker rock and then cooler into igneous rock. This white rock is younger. They're called an igneous intrusion
Another example of an igneous intrusion. Here it is diabase cutting across a layer of shale.

Principle of Crosscutting - example

Rock strata that are not horizontal were originally horizontal when first formed. Geologic processes happened to the strata to make them curved, diagonal or wavy

Tectonic PlatePressure

Principle of Original Horizontality

Examples. These layers of rock were not always tilted, they were originally horizontal.

Principle of Original Horizontality

Examples. These layers of rock were not always tilted, they were originally horizontal.

Principle of Original Horizontality

Layers of rock initially extend laterally (side to side) in all directions until interrupted by geologic processes. In other words: Rock layers that look similar, but are now separated by a valley or other erosional feature, can be assumed to be all a part of the same layer / deposition event.

Principle of Lateral Continuity

Principle of Lateral Continuity

These layers match up and can be assumed they were deposited at the same time!
Layers are the same on both sides

Principle of Lateral Continuity

When a rock formation includes pieces of another rock, the included pieces are older than the rock they are found inside.

Principle of Inclusions

Principles of Relative DatingIndex

Superposition

Inclusions

Crosscutting

Lateral Continuity

Original Horizontality

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