Earth’s History: The Law of Superposition Guided Notes

The Principle of Superposition 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. The process of determining whether an event or object is older or younger than other events or objects is called relative dating.

The Principle of Superposition, continued Layers of sedimentary rock, such as the ones shown below, are stacked liked pancakes.

The Principle of Superposition, continued As you move from the top to the bottom in layers of sedimentary rock, the lower layers are older. Superposition is a principle that states that younger rocks lie above older rocks, if the layers have not been disturbed.

The Principle of Superposition, continued 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.

The Principle of Superposition, continued These forces can push other rocks into a sequence, tilt or fold rock layers, and break sequences into moveable parts.

The Geologic Column The geologic column is 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 interpret rock sequences and to identify the layers in puzzling rock sequences.

Relative Dating: Which Came First?

Other Clues to Relative Age Clues From Igneous Rock Extrusion Lava that cools at the surface is called an extrusion. Rock below an extrusion is always older.

Disturbed Rock Layers Geologists often find features that cut across existing layers of rock. Geologists use the relationships between rock layers and the features that cross them to assign relative ages to the features and the layers. .

Disturbed Rock 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. 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.

Disturbed Rock Layers, continued If rock layers are not horizontal, something must have disturbed them after they formed. The next slides describe four ways that rock layers may become disturbed.

Disturbed Rock Layers, continued A fault is a break in the Earth’s crust along which blocks of the crust slide relative to one another.

Disturbed Rock Layers, continued An intrusion is molten rock from the Earth’s interior that squeezes into existing rock and cools.

Disturbed Rock Layers, continued Folding occurs when rock layers bend and buckle from Earth’s internal forces.

Disturbed Rock Layers, continued Tilting occurs when internal forces in the Earth slant rock layers.