Kara Sears
Stratigraphy is a branch of geology that studies the layers of rock in the Earth's crust and the history they record. Nicolaus Steno, a 17th-century Danish geologist, formulated the first four laws of stratigraphy: the law of superposition, original horizontality, cross-cutting relationships, and lateral continuity. The law of superposition states that any sedimentary layer can be assigned a relative age based on its position in the stack, with the oldest layer at the bottom and the youngest at the top. The law of original horizontality states that sedimentary rocks are deposited in flat horizontal layers and may later be tilted by faults and folding. The law of cross-cutting relationships states that a dike or sill is younger than the rocks it cuts across. The law of lateral continuity states that sedimentary layers are originally continuous and may be faulted or removed by erosion after their deposition. The fifth and sixth laws were proposed by Scottish geologist Charles Lyell and English geologist William Smith, respectively. The fifth law is the law of inclusions, which states that fragments of rock contained in another rock are older than the layer they are contained in. The sixth law is the law of faunal succession, which states that fossils occur in sedimentary rock layers and can be used to predict the relative age of those layers.
| Characteristics | Values |
|---|---|
| Number of Principles | 6 |
| Name of the Proposer | Nicolaus Steno (also known as Niels Stensen) |
| Profession of the Proposer | 17th-century Danish geologist or scientist |
| 1st Principle | Original Horizontality |
| 1st Principle Description | Sedimentary rocks are originally deposited in flat horizontal layers and then may later be tilted by faults and folding |
| 2nd Principle | Lateral Continuity |
| 2nd Principle Description | Layers are originally deposited as continuous layers that may be faulted or removed by erosion after their deposition |
| 3rd Principle | Superposition |
| 3rd Principle Description | Any sedimentary layer or bed can be assigned an "older" or "younger" age relative to the layers below and above it |
| 4th Principle | Cross-Cutting Relations |
| 4th Principle Description | A dike or sill is younger than the rocks they cut across |
| 5th Principle | Inclusions |
| 5th Principle Description | Fragments of rocks contained in another rock are older than the layer that they are contained in |
| 6th Principle | Faunal Succession |
| 6th Principle Description | Fossils occur in sedimentary rock layers and fossils succeed each other in time and this can be used to predict the relative age of sedimentary rock layers and to correlate rock layers |
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What You'll Learn
- Law of Superposition: Sedimentary layers are younger/older relative to layers above/below
- Law of Original Horizontality: Sedimentary rocks are deposited in flat horizontal layers
- Law of Lateral Continuity: Sedimentary layers are continuous for long distances when deposited
- Law of Cross-Cutting Relationships: A dike/sill is younger than rocks it cuts across
- Law of Faunal Succession: Fossils in sedimentary rock layers succeed each other over time
Law of Superposition: Sedimentary layers are younger/older relative to layers above/below
Stratigraphy is a branch of geology that deals with the order and relative position of strata and their relationship to the geological time scale. Nicolaus Steno, a 17th-century Danish geologist, formulated the laws of stratigraphy, also known as Steno's laws.
The Law of Superposition is one of the six principles of stratigraphy and states that within a sequence of layers of sedimentary rock, the oldest layer is at the base, and the layers above are progressively younger. In other words, any sedimentary layer can be assigned an "older" or "younger" age relative to the layers below and above it. This principle is critical as the basis for the relative geologic time scale.
The Law of Superposition is based on the idea that sedimentary rocks are formed in continuous, horizontal layers, with younger layers on top of older layers. This is because sediments are deposited underwater, forming flat, horizontal layers. If a sedimentary rock is found tilted, it means that the layer was tilted after its formation. For example, if a valley cuts through sedimentary layers, it can be assumed that the rocks on either side of the valley were originally continuous.
It is important to note that the Law of Superposition assumes that the layers formed from sedimentation. In the case of metamorphic rocks, the relative ages of the layers can be different because these layers developed from pressure rather than deposition. Additionally, sedimentary rock layers may be out of sequence if they have been altered by folding, intrusions, or cross-cutting.
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Law of Original Horizontality: Sedimentary rocks are deposited in flat horizontal layers
Stratigraphy is the branch of geology concerned with the order and relative position of strata and their relationship to the geological time scale. Nicolaus (or Nicholas) Steno, a 17th-century Danish scientist, first developed the six basic principles or laws of stratigraphy that are used to determine the relative age of a sequence of layered rocks.
The Law of Original Horizontality is one of the six basic principles of stratigraphy. This law states that sedimentary rocks are deposited in flat horizontal layers. In other words, sediments are deposited in horizontal layers that are parallel to the surface on which they were deposited. This implies that tilted or folded layers indicate that the crust has been deformed.
The principle of original horizontality is based on the idea that sedimentary rocks are formed by the accumulation of sediment particles. Over time, these particles settle and accumulate in horizontal layers due to the force of gravity. This results in the formation of flat, horizontal strata or layers of sedimentary rock.
However, it's important to note that while sedimentary rocks may be initially deposited in horizontal layers, they can subsequently be tilted or folded by geological processes such as mountain-building, faults, or plate tectonics. Thus, the presence of folded or inclined strata suggests that the Earth's crust has been disturbed or deformed after the initial deposition of the sedimentary layers.
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Law of Lateral Continuity: Sedimentary layers are continuous for long distances when deposited
Stratigraphy is a branch of geology that deals with the order and relative position of strata and their relationship to the geological time scale. Nicolaus Steno, a 17th-century Danish geologist, formulated the six basic principles used to determine the relative age of a sequence of layered rocks. One of these principles is the Law of Lateral Continuity, which states that sedimentary rock layers are continuous for long distances when deposited.
The Law of Lateral Continuity, also known as Steno's Law, asserts that sedimentary rock layers extend horizontally in all directions, forming continuous sheets. These layers continue until they thin out or encounter a barrier, such as a canyon, mountain range, or fault. This principle is essential in geology because it helps geologists understand the distribution and relationship of sedimentary rock layers across vast distances. By examining the continuity of rock layers, geologists can reconstruct the geological history of an area, providing valuable insights into past environments and conditions.
A classic example of the Law of Lateral Continuity is the Grand Canyon, where sedimentary rock layers are visible on both sides of the canyon. This demonstrates that these layers were once continuous before the canyon was formed through erosion. Additionally, the principle allows geologists to correlate rock layers found in different locations based on their continuity. For instance, similar rock layers found on opposite sides of a canyon or valley can be inferred to be part of the same continuous deposit, later separated by erosion or geological activity.
The Law of Lateral Continuity is closely related to the Principle of Original Horizontality, which states that sedimentary rocks are initially deposited in flat horizontal layers. While original horizontality focuses on the vertical aspect, lateral continuity deals with the horizontal extent. It is important to note that sedimentary layers do not extend indefinitely; their limits are controlled by factors such as the amount and type of sediment available and the size and shape of the sedimentary basin. As sediment is transported, it will eventually be deposited, but the layer may become thinner as the distance from the source increases.
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Law of Cross-Cutting Relationships: A dike/sill is younger than rocks it cuts across
Stratigraphy is a branch of geology that deals with the order and relative position of strata and their relationship to the geological time scale. Nicolaus Steno, a 17th-century Danish geologist, formulated four laws of stratigraphy: the law of superposition, the law of original horizontality, the law of cross-cutting relationships, and the law of lateral continuity.
The Law of Cross-Cutting Relationships, also known as the Principle of Cross-Cutting Relationships, states that a geologic feature that cuts across another is younger than the rock it cuts across. This principle was first developed by Steno in 1669, and later elaborated on by James Hutton in 1795 and Charles Lyell in 1830. It is a relative dating technique used in geology.
The principle applies to various types of cross-cutting relationships, including structural, intrusional, stratigraphic, sedimentological, paleontological, and geomorphological relationships. Structural relationships involve faults or fractures cutting through older rocks, while intrusional relationships occur when an igneous intrusion, such as a dike or sill, intrudes into pre-existing rocks. In the case of stratigraphic relationships, an erosional surface or unconformity cuts across older rock layers or other geological features. Sedimentological relationships are formed when currents erode or scour older sediments in a local area, creating features like channels filled with sand. Paleontological relationships are observed when animal activity or plant growth produces truncation. Geomorphological relationships involve surficial features such as rivers flowing through gaps in ridges of rock or impact craters excavating into subsurface layers.
The Law of Cross-Cutting Relationships helps geologists determine the relative ages of rock units and geologic structures. For example, a dike or sill that cuts across rock layers is younger than those layers. This principle allows for a better understanding of the sequence of geological events and can be observed at different scales, from cartographic to microscopic.
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Law of Faunal Succession: Fossils in sedimentary rock layers succeed each other over time
Stratigraphy is a branch of geology that deals with the order and relative position of rock strata and their relationship to the geological time scale. Nicolaus Steno, a 17th-century Danish geologist, formulated four laws of stratigraphy: the Law of Superposition, the Law of Original Horizontality, the Law of Cross-Cutting Relationships, and the Law of Lateral Continuity. Later, two more principles were added: the Law of Inclusions and the Law of Faunal Succession.
The Law of Faunal Succession, also known as the Principle of Faunal Succession, is based on the observation that sedimentary rock layers contain fossils, and these fossils succeed each other over time in a specific, reliable order. This means that fossils of older organisms are found in older rock layers, while fossils of more recent organisms are found in younger rock layers. This law was first documented in the early 19th century in England by William Smith, and concurrently in France by Cuvier with the help of the mineralogist Alexandre Brongniart.
The law is significant because it allows geologists to determine the relative age of rocks and strata. For example, a fossilised Neanderthal bone will never be found in the same stratum as a fossilised Megalosaurus because Neanderthals and megalosaurs lived during different geological periods, separated by millions of years. This principle, along with the Law of Superposition, helps establish the sequence in which sedimentary rocks were formed.
The study of fossils to determine the age of rocks is called biostratigraphy. Faunal succession played a crucial role in developing our understanding of biological evolution. It also helped establish a method for assigning a stratigraphic classification based on the time relations of strata rather than rock types, which was previously used to indicate age.
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