Superposition and Disturbing Rock Layers

1. Superposition and Disturbing Rock Layers Pgs 59 - 63

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.