Relative Dating

Relative Dating

Middle School

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Learning Objectives

• Define relative dating and explain how it differs from absolute dating.

• Explain Steno's Laws: Superposition, Original Horizontality, and Lateral Continuity.

• Describe Faunal Succession and the Law of Cross-Cutting Relationships.

• Identify the three main characteristics of a useful index fossil.

Key Vocabulary

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Relative Dating

A method of determining the age of rocks by comparing them to other rocks or fossils.

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Superposition

The principle that younger rock layers are deposited on top of older rock layers in sedimentary formations.

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Original Horizontality

This principle states that gravity initially deposits sediments in flat, horizontal layers over time.

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Cross-Cutting

This principle states a fault is younger than any rock layer that it cuts through.

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Faunal Succession

The concept that different fossils appear in a specific and predictable order through geologic time.

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Index Fossil

A fossil used to define and identify geologic periods because it was widespread and short-lived.

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What is Relative Dating?

• Relative dating compares the age of rocks or fossils to one another.

• It determines the sequence of events without providing an exact age in years.

• For example, we can tell if one rock layer is older than another.

• This method reads the history recorded in rock layers known as strata.

Multiple Choice

Which of these is the best description of relative dating?

• Determining the exact age of a rock in millions of years.

• Determining if one rock layer is older or younger than another.

• Calculating the chemical make-up of a fossil.

• Measuring the thickness of a sedimentary rock layer.

Steno's Laws of Stratigraphy

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Law of Superposition

• ​In undisturbed rock layers, the newest rocks are always on the top.

• ​​The oldest rocks are found at the very bottom of the layers.

• ​This helps scientists determine the relative age of different rock layers.

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Law of Horizontality

• ​Sediments are always deposited in flat, horizontal layers due to gravity.

• ​​If you find tilted layers, they were moved after they were formed.

• ​Geologic events like earthquakes can cause these layers to fold or tilt.

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Law of Lateral Continuity

• ​Sediment layers extend continuously until they thin out or hit a barrier.

• ​​Layers on opposite sides of a valley were most likely once connected.

• ​Erosion from a river can carve through once-continuous rock layers over time.

Multiple Choice

According to the Law of Superposition, where are the oldest rock layers found?

• At the bottom of the rock layers.

• At the top of the rock layers.

• Mixed in between other layers.

• Only in the middle of the sequence.

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More Principles of Relative Dating

Cross-Cutting Principle

• A geologic feature like a fault that cuts through rock is younger than the rock it cuts.

• This idea was proposed by James Hutton, a geologist who studied how rock formations are created.

• The original rock layers had to be present first before they could be cut by any new feature.

Faunal Succession

• Fossils appear in a definite order in rock layers, a principle discovered by William Smith.

• Specific fossils are unique to certain time periods, helping geologists to determine the age of rocks.

• For example, dinosaur fossils are never found in the same rock layers as human fossils.

Multiple Choice

A fault cuts through three layers of sedimentary rock. Which is younger?

• The fault

• The top layer of rock

• The bottom layer of rock

• They are all the same age

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Index Fossils: Markers in Time

• Index fossils are special fossils used to define geologic periods.

• An index fossil must be easy to recognize and distinctive from others.

• It must be widespread and found in many different geographic locations.

• It must have lived for a short geologic period of time.

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Multiple Choice

Which characteristic is NOT required for a fossil to be a good index fossil?

• It must come from an organism that lived for a very long time.

• It must be easy to recognize.

• It must be geographically widespread.

• It must come from an organism that lived for a short geologic time.

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Common Misconceptions

Misconception	Correction
The oldest rock layer is always at the bottom.	This is only true for undisturbed rock layers (Law of Superposition).
A fault is older than the rock layers it cuts through.	A fault is younger than the rock it cuts (Law of Cross-Cutting Relationships).
Any fossil can be an index fossil.	Index fossils are widespread, from short-lived species, and easy to recognize.
Relative dating gives a rock's exact age in years.	Relative dating only determines the sequence of events (older vs. younger).

Multiple Choice

A geologist finds a fossil in a layer of rock. According to the Principle of Faunal Succession, what does this fossil primarily indicate?

• The geologic time period when the rock was deposited.

• The exact age of the rock in years.

• The direction the ancient river was flowing.

• The climate of the area when the rock formed.

Multiple Choice

Why would a fossil of an organism that existed for 200 million years be a poor choice for an index fossil?

• It lived for too long, so it appears in too many rock layers to pinpoint a specific time.

• It is too old to be useful for dating more recent rocks.

• It is not widespread enough to be found in different locations.

• It would be too difficult to recognize.

Multiple Choice

Imagine a rock outcrop with five horizontal layers (1-5, from bottom to top). A fault cuts through layers 1, 2, and 3. An igneous intrusion cuts through layers 1-4 and the fault. What is the correct sequence of events from oldest to youngest?

• Layers 1-5 deposited, then the fault, then the intrusion.

• Layers 1-3 deposited, then the fault, then layer 4, then the intrusion, then layer 5.

• The fault, then the intrusion, then all five layers were deposited.

• Layers 1-4 deposited, then the intrusion, then the fault, then layer 5.

Multiple Choice

Geologists in North America and Europe find the same type of index fossil in a rock layer in each continent. What can they conclude about those rock layers?

• The rock layers on both continents formed during the same, specific geologic time period.

• The rock layers were once physically connected in a single landmass.

• The environments on both continents were identical in every way.

• The index fossils must be from an organism that lived for a very long time.

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Summary

• Relative dating orders events in time without using exact numerical ages.

• Rock layers form horizontally, with the oldest layers at the bottom.

• Any feature that cuts across rock layers is younger than the layers.

• Fossils appear in a known order; index fossils help date specific layers.

Poll

On a scale of 1-4, how confident are you about the principles of relative dating?

• 1

• 2

• 3

• 4