Relative Dating: Which Came First?

(No we aren’t talking about dating your cousin even though we live in

Alabama. )

What is relative dating?• It is determining the age of a rock or

fossils based upon the rocks or fossils around the fossil or rock in question.– Example:

• If you go visit a preschool, even at 11 or 12 years of age the kids at the preschool will think you are old. In this case your age, relative to your surroundings is old.

• If you go visit a nursing home, the people there generally being elderly, will think you are just “babies” because you are so much younger than they are. In this case your age, relative to your surroundings, is young.

Relative dating of rock layers

The Principle of Superposition

• Rock layers are put down one on top of the other over millions of years.

• The oldest layers are at the bottom, and the youngest layers are at the top if the rock layer is undisturbed.

Notice the different

color bands that

indicate different layers of

rock.

Which layer is the oldest according to superposition? How can you tell?

Disturbing Forces• Not all rock sequences are youngest to

oldest.

• Natural forces can move these layers.– Things such as faulting, folding, tilting, and

intrusions. These things disrupt the rock layer the same way tearing pages from a book disrupts the book.

• Geologists, like crime scene investigators, must use clues trapped in the rock layers to put the rocks back in sequence and get a true look at Earth’s history.

Disturbing Forces (See page 158)• Fault: break in the Earth’s crust where a

hanging wall is pushed up or down.

• Folding: rock layers are bent into synclines or anticlines.

• Intrusion: molten rock from Earth’s interior squeezes into existing rock layer, cools, and appears to have cut across it.

• Tilting: occurs when forces inside the Earth slants the rock layers.

Disturbing Forces

Geologic Column• To solve the mystery trapped in disturbed

rock layers, scientists combine data from undisturbed rock layers around the world to form the ideal rock sequence that contains all known fossils and rock formations on Earth arranged from oldest to youngest.

• This sequence is the geologic column and does NOT exist naturally on Earth.

An Example of a Geologic Column

Disturbed Rock Layers

• Another tool aiding geologists in age rocks is cross cutting features, such as an intrusion.

• The intrusion or cross-cutting feature is the youngest thing present since the other layers had to be present before it could cut across them.

Cross-Cutting Feature: Look at Letter R

Gaps in the Record: Unconfomities

• Dating rock layers with folds, faults, etc. is difficult, but imagine dating rocks that have missing layers.

• The missing layers are caused by erosion or nondeposition—they weren’t put there to start with—and they are called unconformities.

Unconformities

Three types of Unconformities

• Disconformities: part of a sequence of parallel rock layers is missing.– This is common in the Grand Canyon.

• Nonconformities: sedimentary rock layers lie on top of an eroded surface of non-layered igneous or metamorphic rock.

• Angular unconformity: found b/w horizontal layers of rock that have been tilted or folded.

Disconformity

Disconformities

Nonconformities

Nonconformity

Angular Unconformity

Angular Unconformity