Relative Age of Rocks & Law of Superposition: Sixth Grade Science
Relative dating establishes the chronological order of past geological events by analyzing rock strata and applying fundamental principles of historical geology.
Relative Dating Versus Absolute Dating
To unravel Earth's history, geologists use two distinct dating methodologies:
- Relative Dating: Establishes the sequential chronological order of past events without determining exact numerical years. Relative dating answers the question: "Which rock layer formed first, which formed second, and which is youngest?"
- Absolute Dating: Determines the exact numerical age in millions of years using radioactive decay measurements of unstable isotopes (e.g., "This basalt layer is 142 million years old").
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| RELATIVE DATING ANALOGY |
| |
| Relative Age: "My grandfather is older than my mother, |
| and my mother is older than me." |
| |
| Absolute Age: "My grandfather is 78 years old, |
| my mother is 45 years old, and I am 12 years old." |
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The Four Fundamental Principles of Relative Dating
Geologists determine the relative ages of rock layers using four foundational scientific principles established during the seventeenth and eighteenth centuries:
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| FOUR PRINCIPLES OF RELATIVE DATING |
+---------------------+-------------------+-----------------------------------+
| Geologic Principle | Pioneer Geologist | Core Scientific Rule |
+---------------------+-------------------+-----------------------------------+
| Law of Superposition| Nicolas Steno | In undeformed horizontal strata, |
| | (1669) | the oldest layer is at the bottom |
| | | and youngest layer is at the top. |
+---------------------+-------------------+-----------------------------------+
| Principle of | Nicolas Steno | Sedimentary rock layers are |
| Original | (1669) | originally deposited horizontally;|
| Horizontality | | tilted layers were tilted LATER. |
+---------------------+-------------------+-----------------------------------+
| Principle of Cross- | James Hutton | Any geological feature that CUTS |
| Cutting | (1788) | ACROSS a rock (fault, igneous |
| Relationships | | intrusion) is YOUNGER than rock. |
+---------------------+-------------------+-----------------------------------+
| Principle of | Charles Lyell | Chunks of rock (inclusions) found |
| Inclusions | (1830) | INSIDE another rock are OLDER |
| | | than the rock surrounding them. |
+---------------------+-------------------+-----------------------------------+
The Law of Superposition: Stacking History
The Law of Superposition is the fundamental bedrock of historical geology. When sedimentary particles settle out of water or air, gravity pulls them to the bottom of the basin. Each new layer of mud or sand accumulates on top of the previously deposited layer:
- The bottommost stratum was deposited first and is the oldest layer.
- Each succeeding layer stacked above is progressively younger.
- The uppermost stratum at the ground surface is the youngest layer.
This principle holds true as long as tectonic folding or faulting has not completely overturned the rock sequence upside down.
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| LAW OF SUPERPOSITION DIAGRAM |
| |
| +---------------------------------------------------------+ |
| | Layer D: Limestone (YOUNGEST LAYER) | |
| +---------------------------------------------------------+ |
| | Layer C: Siltstone (Younger than B) | |
| +---------------------------------------------------------+ |
| | Layer B: Sandstone (Younger than A) | |
| +---------------------------------------------------------+ |
| | Layer A: Conglomerate (OLDEST LAYER) | |
| +---------------------------------------------------------+ |
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The Principle of Cross-Cutting Relationships
The Principle of Cross-Cutting Relationships states that an igneous intrusion or fault fracture is always younger than the rock layers through which it cuts:
- You cannot cut a loaf of bread before the bread is baked!
- If an earthquake fault line slices through Layers A, B, and C, the rock layers must have existed first; the faulting event is younger than all three layers.
- If molten magma rises through an underground conduit, cutting through sedimentary layers to form a vertical igneous dike, the dike is younger than the sedimentary strata it cuts across.
- Furthermore, the rock immediately surrounding the intrusion experiences contact metamorphism (a baked zone); the rocks that are baked must be older than the hot intrusion.
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| CROSS-CUTTING INTRUSION AND FAULT |
| |
| +------------------------------------------+ |
| | Layer 3: Shale | |
| +------------------------------------------+ / Fault (Snaps all)|
| | Layer 2: Sandstone || Magma || | / |
| +----------------------|| Dike ||---------+ / |
| | Layer 1: Limestone || (Cuts)|| | / |
| +------------------------------------------+ / |
| |
| Chronology: Layers 1, 2, 3 -> Dike Intrudes -> Fault Breaks Everything |
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Unconformities: Missing Geologic Time
The geological rock record is rarely complete; erosion often destroys rock strata before new layers can be deposited. A gap in the geological rock record representing missing time caused by erosion or non-deposition is called an unconformity. Unconformities represent "missing pages" in Earth's history book.
Exercises and Solutions
Exercise 1: Sequencing a Complex Geological Cross-Section. A geological cross-section displays the following features: Horizontal sedimentary Layer A rests at the bottom, covered by horizontal sedimentary Layer B, which is covered by horizontal sedimentary Layer C. A vertical igneous dike (Dike D) cuts through Layer A and Layer B, but terminates abruptly at the base of Layer C without cutting into it. A normal fault (Fault F) cuts through all four features: Layer A, Layer B, Layer C, and Dike D. Determine the complete sequential chronological order of events from oldest to youngest.
Solution 1: Chronological sequence from oldest to youngest: (1) Deposition of sedimentary Layer A (oldest, at bottom by Superposition); (2) Deposition of sedimentary Layer B (above A); (3) Intrusion of igneous Dike D (cuts across Layers A and B by Cross-Cutting Relationships, but formed before C because it does not penetrate Layer C); (4) Deposition of sedimentary Layer C (deposited on top of B and Dike D); (5) Fault F ruptures (youngest, cuts across all previously existing features: Layers A, B, C, and Dike D).
Exercise 2: Applying the Principle of Inclusions. A granite magma chamber intrudes into ancient crustal gneiss rock. Along the boundary between the two rocks, geologists discover angular chunks of dark gneiss completely enclosed inside the solid pink granite. (a) Apply the Principle of Inclusions to determine whether the granite or the gneiss is older; (b) Explain the physical mechanism that formed these inclusions (xenoliths).
Solution 2: (a) The gneiss is older than the granite. According to the Principle of Inclusions, any rock fragments (inclusions) found embedded inside another rock mass must have existed first, making them older than the host rock surrounding them. (b) When the superheated molten granite magma intruded into the subterranean crust, thermal stress and hydraulic pressure shattered fractured blocks of the surrounding country rock (the gneiss). Chunks of gneiss broke off and fell into the liquid magma chamber in a process called stoping. The gneiss fragments (known as xenoliths) resisted complete melting and were frozen permanently inside the granite as it crystallized, preserving proof that the gneiss was already solid bedrock before the granite melted into existence.