Law Of Superposition & Index Fossils
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. The relative age of rocks and fossils can be determined using two basic methods: ordering of rock layers and index fossils:
Ordering of Rock Layers
- Scientists read the rock layers knowing that each layer is deposited on top of other layers.
- The law of superposition states that each rock layer is older than the one above it. So, the relative age of the rock or fossil in the rock or fossil in the rock is older if it is farther down in the rock layers.
- Relative dating can be used only when the rock layers have been preserved in their original sequence.
- Certain fossils, called index fossils, can be used to help find the relative age of rock layers. To be an index fossil -
- an organism must have lived only during a short part of Earth's history;
- many fossils of the organisms 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 it is.
- Fossils that are found in many rock layers, therefore living long periods of time, do not qualify as index fossils.
Law of Superposition
Knowing the relative age of index fossils in one location helps scientists infer the relative age of fossils and rock layers in other locations.
Trilobites are a good example of an index fossil of the Paleozoic Era
Some Common Index Fossils over Geologic Time
- Complex layering due to intrusions and extrusions, faults, or unconformities can make dating rocks and fossils challenging. How do scientists accurately determine the age of rocks and fossils when each of these factors occur?
- Radioactive element decay can also be used to tell the age of fossils and rocks. How does this process work? Is it reliable? Why or why not?