Finding the Relative Age of Rocks

Standard 8-2.6 – Infer the relative age of rocks and fossils from index fossils and the ordering of the rock

layers.

Relative and Absolute Ages

Relative Age The age of a rock compared to the ages of rock layers.

Absolute Age The age of a rock given

as the number of years since the rock formed.

Position of Rock Layers

Its difficult to determine the absolute age so geologists use method to find a rock’s relative age. Use the:

LAW OF SUPERPOSITION: in horizontal sedimentary rock layers, the oldest layer is at the bottom. Each higher layer is younger than the layers below it.

Other Clues to Relative Age

A. Clues from Igneous Rocks:

1. Lava that hardens on the surface is called an Extrusion

(example – an eruption would put a layer of igneous rock on top of sedimentary rocks.

Rock layers below an extrusion are always older than the extrusion.

The extrusion is in black

Now the extrusion is the youngest layer.

A. Clues from Igneous Rocks:

2. Magma that cools and pushes into bodies of rock and hardens is called an Intrusion

An intrusion is always younger than the rock layers around and beneath it.

The intrusion (in red) is now younger than the surrounding rocks.

Draw this in your notes and label each of the parts listed below.

1. Intrusion

2. Extrusion

3. Oldest rock

4. Youngest rock

B. Clues from Faults:

Fault: is a break in the Earth’s crust.

• Forces inside the Earth cause movement of the rock on opposite sides of a fault.

• Fault is always younger than the rock it cuts through.

REVIEWREVIEW Geologists use the Relative and Absolute Geologists use the Relative and Absolute

Age of rocks to determine age.Age of rocks to determine age.

Using the Law of SuperpositionUsing the Law of Superposition

Other clues are from Igneous rocks Other clues are from Igneous rocks – Extrusion Extrusion – IntrusionIntrusion

Clues from FaultsClues from Faults

GAPS IN THE GEOLOGIC RECORD

- Record of sedimentary rock layers is not always complete

- Deposition slowly builds layers upon layer of sedimentary rock, BUT some of these layers may erode away, exposing an older rock surface.

Unconformity – is a gap in the geologic record. An unconformity shows where some rock layers have been lost because of erosion.

USING FOSSILS TO DATE ROCKS

To date rock layers, geologists first give a relative age to a layer of rock at one location. THEN they can give the same age to matching layers of rock at other locations.

Certain fossils, called Index Fossils help geologist match rock layers.

INDEX FOSSILS – Fossils of widely distributed organisms that lived during only one short period.

• Example of an Index Fossil:

• Trilobites (hard shelled animals whose bodies had three distinct parts.

• Trilobites evolved in shallow seas more than 500 million years ago.

• Over time, many types have appeared.

• They became extinct about 245 million years ago.

• They have been found in many different places.

To become a Index Fossil … a trilobite must be different in some way from other trilobites.

Example – type with large eyes

These large-eyed ..bites survived for a time AFTER other bites became extinct.

If a geologist finds large-eyed Trilobites in a rock layer, the geologist can infer that those rocks are younger than rocks containing other types of trilobites

•The World's Biggest Trilobite

•A team of Canadian paleontologists working along Hudson Bay in northern Manitoba has discovered the world's largest recorded complete fossil of a trilobite, a many-legged, sea-dwelling animal that lived 445 million years ago. The giant creature is more than 70 cm long (about 28 inches), 70 percent larger than the previous record holder. "This is an important and amazing find," says Bob Elias, a professor in the department of geological sciences at the University of Manitoba. "It looks like a huge bug!"

You can use INDEX FOSSILS to match rock layers.

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Figure 13

Adobe Info sheet on Trilobites