4.3 Absolute Vs. Relative Dating

Relative ages.

In most families a person’s age fits into his or her generation: Siblings are around the same age as are first cousins. But in some families, multiple marriages, delayed childbearing, extended childbearing or other variations mixes up generations so that Aunt Julia may be five years younger than her nephew. In a family like this it’s hard to tell how people are related simply by age. With rock units we use certain principles to tell their ages relative to each other.

Relative Age Dating

Early geologists had no way to determine the absolute age of a geological material. If they didn’t see it form, they couldn’t know if a rock was one hundred years or 100 million years old. What they could do was determine the ages of materials relative to each other. Using sensible principles they could say whether one rock was older than another and when a process occurred relative to those rocks.

Steno’s Laws

Remember Nicholas Steno, who determined that fossils represented parts of once-living organisms? Steno also noticed that fossil seashells could be found in rocks and mountains far from any ocean. He wanted to explain how that could occur. Steno proposed that if a rock contained the fossils of marine animals, the rock formed from sediments that were deposited on the seafloor. These rocks were then uplifted to become mountains.

This scenario led him to develop the principles that are discussed below. They are known as Steno’s laws. Steno’s laws are illustrated below in (Figure below).

  • Original horizontality: Sediments are deposited in fairly flat, horizontal layers. If a sedimentary rock is found tilted, the layer was tilted after it was formed.
  • Lateral continuity: Sediments are deposited in continuous sheets that span the body of water that they are deposited in. When a valley cuts through sedimentary layers, it is assumed that the rocks on either side of the valley were originally continuous.
  • Superposition: Sedimentary rocks are deposited one on top of another. The youngest layers are found at the top of the sequence, and the oldest layers are found at the bottom.

(a) Original horizontality. (b) Lateral continuity. (c) Superposition.

More Principles of Relative Dating

Other scientists observed rock layers and formulated other principles.

Geologist William Smith (1769-1839) identified the principle of faunal succession, which recognizes that:

  • Some fossil types are never found with certain other fossil types (e.g. human ancestors are never found with dinosaurs) meaning that fossils in a rock layer represent what lived during the period the rock was deposited.
  • Older features are replaced by more modern features in fossil organisms as species change through time; e.g. feathered dinosaurs precede birds in the fossil record.
  • Fossil species with features that change distinctly and quickly can be used to determine the age of rock layers quite precisely.

Scottish geologist, James Hutton (1726-1797) recognized the principle of cross-cutting relationships. This helps geologists to determine the older and younger of two rock units (Figure below).

If an igneous dike (B) cuts a series of metamorphic rocks (A), which is older and which is younger? In this image, A must have existed first for B to cut across it.

The Grand Canyon

The Grand Canyon provides an excellent illustration of the principles above. The many horizontal layers of sedimentary rock illustrate the principle of original horizontality (Figurebelow).

  • The youngest rock layers are at the top and the oldest are at the bottom, which is described by the law of superposition.
  • Distinctive rock layers, such as the Kaibab Limestone, are matched across the broad expanse of the canyon. These rock layers were once connected, as stated by the rule of lateral continuity.
  • The Colorado River cuts through all the layers of rock to form the canyon. Based on the principle of cross-cutting relationships, the river must be younger than all of the rock layers that it cuts through.

The Grand Canyon, with the Kaibab Limestone marked with arrows.

Summary

  • Sediments are deposited horizontally with the oldest at the bottom. Any difference in this pattern means that the rock units have been altered.
  • The principle of faunal succession recognizes that species evolve and these changes can be seen in the rock record.
  • The Grand Canyon exhibits many of the principles of relative dating and is a fantastic location for learning about the geology of the southwestern U.S.

Practice

Use this resource to answer the questions that follow.

Absolute vs. Relative Dating

http://www.youtube.com/watch?v=JNOmpXo2xlU

1. What is superposition?

2. How can the age of the layers be determined?

3. How does volcanic ash help with relative dating?

4. What are the radioactive elements?

5. What is the clock for determining relative age?

Review

1. How do Steno’s laws help geologists to decipher the geological history of a region?

2. What is the principle of faunal succession?

3. Why does just about every geology textbook use the Grand Canyon as the example in the sections on geological history?

Vocabulary

Language:

lateral continuity
original horizontality
principle of cross-cutting relationships
principle of faunal succession
superposition
Skip Navigation

 

Where’s a good place to see geology?

The Southwestern United States is a fantastic place to see geology. The arid climate means that that the rocks are not covered by vegetation. In many places, especially the national parks, the formations are fantastic. The rocks themselves are very interesting. The principles discussed below are easily seen around the Southwest.

Relative Age Dating

Early geologists had no way to determine the absolute age of a geological material. If they didn’t see it form, they couldn’t know if a rock was one hundred years or 100 million years old. What they could do was determine the ages of materials relative to each other. Using sensible principles they could say whether one rock was older than another. They could also determine when a process occurred relative to those rocks.

Laws of Stratigraphy

The study of rock strata is called stratigraphy. The laws of stratigraphy can help scientists understand Earth’s past. The laws of stratigraphy are usually credited to a geologist from Denmark named Nicolas Steno. He lived in the 1600s. The laws are illustrated in the Figurebelow; refer to the figure as you read about Steno’s laws below.

(a) Original horizontality. (b) Lateral continuity. (c) Superposition.

Law of Superposition

Superposition refers to the position of rock layers and their relative ages (Figure below).Relative age means age in comparison with other rocks, either younger or older. The relative ages of rocks are important for understanding Earth’s history. New rock layers are always deposited on top of existing rock layers. Therefore, deeper layers must be older than layers closer to the surface. This is the law of superposition.

Superposition. The rock layers at the bottom of this cliff are much older than those at the top. What force eroded the rocks and exposed the layers?

Law of Lateral Continuity

Rock layers extend laterally, or out to the sides. They may cover very broad areas, especially if they formed at the bottom of ancient seas. Erosion may have worn away some of the rock, but layers on either side of eroded areas will still “match up.”

The Grand Canyon (Figure below is a good example of lateral continuity. You can clearly see the same rock layers on opposite sides of the canyon. The matching rock layers were deposited at the same time, so they are the same age.

Lateral Continuity. Layers of the same rock type are found across canyons at the Grand Canyon.

Law of Original Horizontality

Sediments were deposited in ancient seas in horizontal, or flat, layers. If sedimentary rock layers are tilted, they must have moved after they were deposited.

Law of Cross-Cutting Relationships

Rock layers may have another rock cutting across them, like the igneous rock pictured below (Figure below). Which rock is older? To determine this, we use the law of cross-cutting relationships. The cut rock layers are older than the rock that cuts across them.

Cross-cutting relationships in rock layers. The rock layers at the bottom are cut off by the rock layers that are higher up. Which layers are older and which are younger?

Unconformities

Geologists can learn a lot about Earth’s history by studying sedimentary rock layers. But in some places, there’s a gap in time when no rock layers are present. A gap in the sequence of rock layers is called an unconformity.

Look at the rock layers pictured below (Figure below); they show a feature called Hutton’s unconformity. The unconformity was discovered by James Hutton in the 1700s. Hutton saw that the lower rock layers are very old. The upper layers are much younger. There are no layers in between the ancient and recent layers. Hutton thought that the intermediate rock layers eroded away before the more recent rock layers were deposited.

Hutton’s discovery was a very important event in geology! Hutton determined that the rocks were deposited over time. Some were eroded away. Hutton knew that deposition and erosion are very slow. He realized that for both to occur would take an extremely long time. This made him realize that Earth must be much older than people thought. This was a really big discovery! It meant there was enough time for life to evolve gradually.

Hutton’s unconformity, in Scotland.

Vocabulary

  • relative age: Age of something relative to the age of something else; one rock is older than another rock, for example.
  • stratigraphy: Study of rock strata.
  • unconformity: Gap in a sequence of rocks layers; this represents a loss of time.

Summary

  • Sediments are deposited horizontally. This is original horizontality.
  • The oldest sedimentary rocks are at the bottom of the sequence. This is the law of superposition.
  • Rock layers are laterally continuous.This is the law of lateral continuity.
  • Rock B cuts across rock A. Rock A must be older. This is the principle of cross-cutting relationships.
  • A gap in a rock sequence is an unconformity.

Practice

Use the resource below to answer the questions that follow.

  1. What is superposition?
  2. How can the age of the layers be determined?
  3. How does volcanic ash help with relative dating?
  4. What are the radioactive elements?
  5. What is the clock for determining relative age?

Review

  1. How do Steno’s laws help geologists to decipher the geological history of a region?
  2. Why does the principle of lateral continuity work?
  3. How could you recognize the existence of an unconformity?

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s