Relative Dating: Principles and Examples

Relative dating is all about figuring out the order in which things happened in the past, without necessarily knowing exactly when they happened. It's like putting things in chronological order, but without knowing exactly how many years ago they happened.

What is relative dating?

Relative dating is like figuring out the order in which rocks formed, like putting them in first, second, third place. It doesn't tell us exactly when each rock formed, only that one rock is older or younger than another. These techniques are still important and used today, even alongside methods that give specific dates. To understand the order of rock layers, scientists had to develop some basic rules. These rules seem obvious now, but they were major advances in science at the time.

Relative dating relies on a set of key principles to establish the order of geological events and rock formations. These principles don't provide specific ages but determine which rocks are older or younger relative to each other.

Interpreting a geologic cross-section of a hypothetical region using Relative Dating Principles

Relative Dating Principles

Relative dating relies on a set of core principles to establish the order of geological events and rock formations without pinpointing specific ages. Here are the main ones:

Principle of Superposition

Superposition: in undisturbed rock layers, the oldest layer is on the bottom.

The principle of superposition is the foundation of relative dating. It states that in a sequence of undisturbed sedimentary rock layers, the oldest layers are at the bottom, and the youngest layers are at the top. This principle was formulated by Danish scientist Nicolaus Steno in the 17th century.

The reasoning behind the Principle of Superposition is based on the process of sedimentary layering. Sediments, such as sand, silt, and clay, are deposited by various geological processes like erosion, transport, and deposition. As new sediments accumulate on the Earth's surface over time, they settle on top of previously deposited layers. This continual process creates a stack of sedimentary layers, with the oldest layers at the bottom and the youngest layers at the top.

Therefore, when examining a sequence of sedimentary rock layers, if the layers have not been disturbed by tectonic activity or other geological processes, the lower layers are older than the layers above them. This principle provides a basic framework for interpreting the relative ages of rock layers and reconstructing the geological history of an area.

The Principle of Original Horizontality

Sedimentary layers are initially deposited in flat, horizontal beds.

The Principle of Original Horizontality is one of the fundamental principles used in relative dating within geology. It states that sedimentary layers of rock are originally deposited in horizontal or nearly horizontal layers. This principle was first proposed by Danish scientist Nicolaus Steno in the 17th century.

The concept behind this principle is that when sedimentary particles settle out of water or air to form rock layers, they do so under the influence of gravity. This process typically results in the formation of horizontal layers because particles settle evenly on top of each other on the Earth's surface. Therefore, when observing undisturbed sedimentary rock layers, they are usually found lying horizontally, or nearly so.

If you encounter sedimentary rocks that are not horizontal, it suggests that some geological process has occurred after their formation, such as folding, faulting, or tilting. These processes can occur due to tectonic forces, volcanic activity, or other geological events that disrupt the original horizontal orientation of the rock layers.

Lateral Continuity

Lateral continuity, where rock layers extend horizontally with consistent properties.

The Principle of Lateral Continuity is another fundamental concept in relative dating within geology. This principle states that sedimentary rock layers extend laterally in all directions until they either thin out or encounter a barrier. It implies that when sediment is deposited, it tends to spread out horizontally in continuous sheets.

In simpler terms, if you find a sedimentary layer exposed at one location, you can reasonably infer that the layer once extended continuously in all directions, even if it is now interrupted or absent due to erosion, faulting, or other geological processes.

This principle is particularly useful in interpreting the relative ages of rock layers across large distances. By observing similar rock types, sedimentary structures, and fossil assemblages in different locations, geologists can correlate and match up rock layers that were once part of the same continuous deposit. This correlation allows them to reconstruct the original extent of sedimentary formations and understand the geological history of an area.

The principle of inclusions

Principle of inclusions

The principle of inclusions, also sometimes called the law of included fragments, is a cornerstone of relative dating in geology. It helps geologists determine the order in which rocks formed by focusing on fragments of rock trapped within another rock.

When one rock formation contains fragments or inclusions of another rock formation, it suggests that the included rocks must have existed before the rock unit that contains them formed. For example:

If a conglomerate rock contains pebbles of granite, the granite pebbles must be older than the conglomerate itself.
If a lava flow contains pieces of pre-existing rock that it engulfed as it flowed, those pieces are older than the lava flow.

The Principle of Inclusions is based on the idea that the rocks or materials being included must have been formed or existed before the rock unit that contains them.

The Principle of Cross-Cutting Relationships

Principle of Cross-Cutting Relationships

The principle of cross-cutting relationships is a fundamental concept in relative dating. It states that any geological feature that cuts across another geological feature must be younger than the feature it cuts across. This principle helps geologists establish the relative timing of geological events by examining the relationships between different rock units and structures.

In simpler terms, if you see one geological feature, such as a fault, cutting across another feature, like a layer of sedimentary rock, you can infer that the fault is younger than the sedimentary layer it crosses. This is because the fault must have formed after the deposition of the sedimentary layer.

Cross-cutting relationships can involve various geological features can include rock layers, faults, igneous intrusions (such as dykes or sills), veins, erosional surfaces, and other structures. By analyzing these relationships in the field or in geological maps, geologists can create a relative timeline of events, determining which features are older or younger relative to one another.

Faunal Succession

Faunal succession

The Principle of Faunal Succession is a fundamental concept in both geology and paleontology. It states that fossil organisms succeed one another in a definite and determinable order, and therefore, any time period can be recognized by its fossil content. This principle was developed in the early 19th century by geologists and paleontologists who observed patterns in the distribution of fossils in sedimentary rocks.

The key aspects:

Vertical Succession: As you dig deeper into sedimentary rock layers (going down vertically), the fossils you find will represent progressively older life forms. This reflects the history of life on Earth, where organisms have evolved and changed over vast stretches of time.
Horizontal Succession: Over wide horizontal distances, sedimentary rock layers of the same age will contain similar assemblages of fossils. This allows geologists to correlate rock layers from different locations based on the fossils they contain.

The key idea behind the Principle of Faunal Succession is that different species of organisms have evolved and become extinct at different times throughout Earth's history. As a result, the fossils found in sedimentary rocks can be used to establish a relative chronological order of the rock layers. Specifically:

Younger rock layers typically contain fossils of more recent species that have evolved more recently.
Older rock layers contain fossils of species that lived during earlier geological time periods.

By studying the fossil content of sedimentary rock layers, geologists can correlate and match up rock layers from different locations based on the similarity of their fossil assemblages. This allows them to create a relative timeline of geological events and the history of life on Earth.

Unconformities

An unconformity is a discontinuity in the rock record, representing a missing interval of geologic time. It's essentially a gap between layers of rock, indicating a period where sediment deposition wasn't happening or existing layers were eroded away. These gaps can range from a few thousand to billions of years!

Types of Unconformities

There are different ways these gaps appear depending on the geological processes involved. Here are main three types:

Disconformity: This is a relatively short gap where the rock layers above and below are parallel. It suggests a pause in deposition, like a shallow sea, before new sediments accumulated.
Nonconformity: This is where igneous or metamorphic rocks (older, non-sedimentary rocks) underlie younger sedimentary layers. This indicates a significant period of erosion that exposed the older rocks before new sediments were deposited.
Angular Unconformity: This is where tilted layers are overlain by horizontal layers. It suggests a period of deformation (like mountain building) and erosion, followed by deposition of new, flat layers.

Relative Dating Example

Relative dating is a method geologists use to determine the chronological order of rock layers and events without exact ages. Here's a concise example using the Grand Canyon:

Relative Dating in the Grand Canyon

1. Law of Superposition

Principle: In undisturbed sedimentary layers, the oldest layers are at the bottom.
Application: In the Grand Canyon, the Vishnu Schist is at the bottom, making it the oldest, while the Kaibab Limestone at the top is the youngest.

2. Principle of Original Horizontality

Principle: Sediments are originally deposited horizontally.
Application: The horizontal layers in the Grand Canyon suggest they haven't been significantly disturbed since they were deposited.

3. Principle of Cross-Cutting Relationships

Principle: Features that cut through rocks, like faults or igneous intrusions, are younger than the rocks they cut.
Application: Igneous dikes cutting through the Vishnu Schist indicate these dikes are younger than the schist.

4. Fossil Succession

Principle: Fossils within rock layers help determine relative ages.
Application: Trilobite fossils in the Bright Angel Shale indicate these rocks are from the Cambrian period.

Some of the rock layers of the Grand Canyon. The youngest layer is the Kaibab limestone (aged 270 million years) and the oldest is the Vishnu schist basement rock layer (roughly 1.8 billion years old).

Applying Relative Dating in the Grand Canyon

Sequence of Layers: From oldest to youngest:

Vishnu Schist
Tapeats Sandstone
Bright Angel Shale
Muav Limestone
Kaibab Limestone

Determine Relative Ages

Superposition: Vishnu Schist is older than the Tapeats Sandstone above it.
Original Horizontality: Layers are mostly horizontal, suggesting minimal disturbance.
Cross-Cutting Relationships: Dikes cutting the Vishnu Schist are younger than the schist.
Fossil Succession: Trilobites in Bright Angel Shale confirm it’s Cambrian in age.

By using these principles, geologists can piece together the sequence of geological events that shaped the Grand Canyon, even without knowing the exact numerical ages of the rocks.

Relative Dating Vs. Absolute Dating

Both relative dating and absolute dating are techniques used to determine the age of things in the past, but they go about it in fundamentally different ways:

Relative Dating

Focuses on order: Tells you if something is older or younger than something else, but not its specific age in years.
Think of it as sequencing: Like putting historical events in chronological order without knowing the exact year.
Methods: Relies on principles like superposition (deeper layers are older), fossil succession (certain fossils appear in a predictable order), and cross-cutting relationships (features that cut through layers are younger).
Example: Finding a stone tool under a layer of volcanic ash tells you the tool is older than the eruption.
Useful for: Building a relative timeline of events in archaeology, geology, and paleontology.

Absolute Dating

Gives specific ages: Uses scientific techniques to determine the actual age of an object or event in years.
Think of it as pinpointing: Like figuring out the exact year a fossil or artifact is from.
Methods: Relies on radioactive isotopes in materials that decay at a predictable rate (e.g., carbon-14 dating).
Example: Carbon-14 dating a bone fragment can tell you it's 10,000 years old.
Useful for: Precise dating in archaeology, geology, and understanding past climatic changes.