# Relative Dating: Which Came First?

## Presentation on theme: "Relative Dating: Which Came First?"— Presentation transcript:

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 M is an intrusion; R is an intrusion that is a cross-cutting feature; F and H show a fault; J and G show folding; D, E, F, and G show tilting.

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
Here’s an example of what a geologic column would look like if you could stack rock layers up like Legos.

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
It is younger than the surrounding layers since those had to be there first for R to cut across them.

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 In the top picture: Is the rock solid? Do you see missing layers? This is an unconformity called a disconformity since the rock layers are discontinuous because part of them are missing. Second and third pictures: Do you see that the blue layer is missing? Part of it is there, so the other part is missing due to erosion. The slanted layers make angular unconformity.

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