Superposition and disturbing rock layers
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Superposition and Disturbing Rock Layers. Pgs 59 - 63. The Principle of Superposition. Rock layers are formed from the bottom up. Each layer is believed to be created laying flat. The principle of superposition states that younger rocks lie above older rocks in undisturbed sequence.

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Superposition and Disturbing Rock Layers

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Superposition and disturbing rock layers

Superposition and Disturbing Rock Layers

Pgs 59 - 63


The principle of superposition

The Principle of Superposition

  • Rock layers are formed from the bottom up.

  • Each layer is believed to be created laying flat.

  • The principle of superposition states that younger rocks lie above older rocks in undisturbed sequence.

  • Just like a sky scraper is built from the bottom up.


Disturbing forces

Disturbing Forces

  • Rock layers are disturbed by forces within the Earth and from tectonic forces.

  • The forces tilt or fold rock layers.

  • Sometimes older rock layers get on top of younger layers.

  • To help solve this dilemma, geologists construct a geologic column to help them.


Geologic column

Geologic Column

  • Geologists gather information from rock sequences around the world to construct and accurate geologic column.

  • The geologic column is the ideal sequence of rock layers that contain all the known fossils and rock formations on Earth arranged from oldest to youngest.

  • Geologic columns help scientists interpret rock sequences.

  • Sometimes they use the column to put rock sequences in order.


Rock layer disturbances

Rock layer disturbances

  • Some features cut across rock layers that make them difficult to interpret.

  • Faults – a break in the Earth’s crust

  • Intrusion – molten rock from the Earth’s interior that squeezes into existing rock and cools.

  • Folding – when rock layers bend and buckle from Earth’s internal forces.

  • Tilting – when internal forces slant rock layers without folding them.


Gaps in the record

Gaps in the Record

  • The effects of disturbances make it difficult to date the rock layers.

  • Sometimes there are layers that are missing all together.

  • Missing layers cause unconformities.

  • Unconformity – a surface that represents a missing part of the geologic column.


Unconformities

Unconformities

  • When geologists find unconformities, they must figure out if the missing layer was actually there or somehow removed.

  • Nondeposition – the deposition process stops and then resumes again later.

  • Erosion – sometimes parts of layers or entire layers of rock are eroded away before deposition restarts.


Types of unconformities

Types of Unconformities

  • There are three types of unconformities.

  • Disconformity

    • Where part of a sequence of parallel rock layers is missing

    • Often hard to see and the most common.

  • Nonconformity

    • Sedimentary rock layers lie on top of an eroded surface of igneous or metamorphic.

  • Angular unconformity

    • Exists between horizontal rock layers and tilted or folded rock layers.


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