Relative Dating Principles

Principles of Relative Dating

Vocabulary

Strata

Relative Dating

Undisturbed Strata

Lateral

Layers of rock (Stratum is singular)

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

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

Side to Side

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Principle of Superposition

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

Youngest

Oldest

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.

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)

Youngest

Magma

Oldest

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

Principle of Crosscutting - example

Another example of an igneous intrusion. Here it is diabase cutting across a layer of shale.

Principle of Original Horizontality

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 Plate Pressure

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 Lateral Continuity

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

These layers match up and can be assumed they were deposited at the same time!

Principle of Lateral Continuity

Layers are the same on both sides

Principle of Inclusions

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

Principles of Relative DatingIndex

Superposition

Crosscutting

Original Horizontality

Inclusions

Lateral Continuity