Principle of Original Horizontality
Principle of Original Horizontality
The Principle of Original Horizontality states that layers of sediment are originally deposited horizontally under the action of gravity. This principle was proposed by the Danish geological pioneer Nicholas Steno (1638–1686).
The principle is important to the analysis of folded and tilted strata. From these observations is derived the conclusion that the Earth has not been static and that great forces have been at work over long periods of time, further leading to the conclusions of the science of plate tectonics; that movement and collisions of large plates of the Earth's crust is the cause of folded strata.
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| Folds of alternate layers of limestone and chert occur in Greece. The limestone and chert were originally deposited as flat layers on the floor of a deep sea basin. These folds were created by Alpine deformation. |
As one of Steno's Laws, the Principle of Original Horizontality served well in the nascent days of geological science. However, it is now known that not all sedimentary layers are deposited purely horizontally.
Exceptions: While the principle holds true for many sedimentary environments, there are exceptions where sediments might not deposit horizontally:
- Currents: In environments like sand dunes or certain river conditions, sediment can be deposited at an angle like create cross-bedding where layers are not horizontal.
- Slopes: Sediments on the slopes of underwater features might not settle horizontally.
- Tectonic Activity: Immediate or soon after deposition, tectonic forces can distort layers.
- Reefs: Organic reefs can grow in various shapes not adhering to horizontality.
- Impact Craters or Volcanic Deposits: Here, materials might be laid down in a non-horizontal fashion due to the immediate topography created by the event.
The law or principle of original horizontal states that sediments are always deposited in horizontal or near horizontal layers or strata under gravity action. Any folding or steep tilting of these rock layers happens after formation due to the Earth’s movements.
Some of the Earth’s movements that could make nearly or horizontal/flat rock layers fold, incline, or deform rocks include mountain building, earthquakes, faulting, or tectonic plate movements. These forces will break, uplift, or buckle layers, making some younger one goes beneath older ones, steep contours, or inclination.
Steno observed that since sediments settle from water or wind due to gravity, they will be deposited horizontally on the sediment basin or Earth’s surface and then cement with time. Otherwise, the sediments would have slithered under gravity if on a steeply inclined surface to the lowest point.
Use in Relative Dating: This principle allows geologists to infer the relative ages of rock layers and understand the sequence of geological events. If layers are horizontal, one can assume they remain in their original sequence (unless there's evidence of overturning or other major disruptions). Folded or tilted layers suggest a disturbance post-deposition.
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Principle of Original Horizontality: undisturbed sedimentary layers are flat |
Related Concepts:
Principle of Superposition: In undisturbed sequences, the oldest rocks are at the bottom and each successive layer above is younger.
Principle of Lateral Continuity: Originally, layers of sediment extend laterally in all directions until they thin out or reach the edge of their depositional basin.
This principle is crucial for interpreting the geological history of a region, allowing scientists to reconstruct past environments, understand tectonic activities, and even locate resources like oil and gas, which can be trapped in certain configurations of rock layers.
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