Dating Rock Layers and Fossils

Relative Age & Ordering of Rock Layers

What is Relative Age?

• The relative age means the age of one object compared to the age of another object.

• Relative age does not tell the exact age of an object.

How do we find Relative Age?

• The relative age of rocks and fossils can be determined using two basic methods: ordering of rock layers and index fossils.

Multiple Choice

Relative age does not tell the exact age of an object.

• True

• False

Multiple Choice

The relative age of rocks and fossils can be determined using two basic methods ordering of rock layers and shape of fossils.

• True

• False

Ordering of Rock Layers

• Scientists read the rock layers knowing that each layer is deposited on top of other layers.

Ordering of Rock Layers

The law of superposition states that each rock layer is older than the one above it. So using this layering, the relative age of the rock or fossil in the rock is older if farther down in the rock layers. Relative dating can be used only when the rock layers have been preserved in their original sequence.

Index Fossils

• These can be used to help find the relative age of rock layers.

Index Fossils

• An organism must have lived only during a short part of Earth’s history; many fossils of the organism must be found in rock layers; the fossil must be found over a wide area of Earth; the organism must be unique.

• The shorter time period a species lived, the better an index fossil it is. 

Example:

• A key example of an organism used as an index fossil are trilobites, a group of  hard-shelled animals whose body had three sections, lived in shallow seas, and became extinct about 245 million years ago. 

• Therefore, if a trilobite is found in a particular rock layer, it can be compared with trilobites from other layers to estimate the age of the layer in which it was found.  

• Fossils that are found in many rock layers, therefore living long periods of time, do not qualify as index fossils.

Multiple Choice

Which answer choice describes the Law of Superposition?

• Each rock layer is older than the one above it.

• Each rock layer is younger than the one above it.

• Rock layers have no age based on other rock layers.

Multiple Choice

Relative dating can be used ONLY when...

• The rock layers are the same type.

• The rock layers are not in their original sequence.

• The rock layers have been preserved in their original sequence.

Multiple Choice

Index Fossils can be used to help find the exact age of rock layers.

• True

• False

Multiple Select

Check all that apply to an Index Fossil -

• An organism must have lived only during a short part of Earth’s history.

• Many fossils of the organism must be found in rock layers.

• The fossil must be found over a wide area of Earth.

• The organism must be unique.