Model Answer
0 min readIntroduction
Igneous rocks, formed from the cooling and solidification of magma or lava, often intrude into pre-existing rock formations. These intrusions can exhibit varying relationships with the surrounding strata, categorized as either concordant or discordant. Understanding these relationships is crucial for deciphering the geological history of a region. Concordant intrusions form parallel to the existing layering, while discordant intrusions cut across it. This distinction significantly impacts the resulting geological structures and provides insights into the processes governing magma movement and emplacement. This answer will delineate these two types and illustrate laccoliths and lopoliths, two prominent examples of concordant intrusions, with accompanying block diagrams.
Concordant and Discordant Igneous Bodies: A Distinction
The classification of igneous intrusions into concordant and discordant bodies is based on their orientation relative to the bedding planes of the surrounding sedimentary or metamorphic rocks.
- Concordant Intrusions: These intrusions are emplaced parallel to the existing layering of the host rocks. They typically form when magma exploits pre-existing weaknesses or bedding planes. Examples include sills, laccoliths, and lopoliths.
- Discordant Intrusions: These intrusions cut across the existing layering of the host rocks. They are often associated with fractures, faults, or zones of weakness. Examples include dykes, batholiths, and stocks.
The mode of emplacement significantly influences the shape and size of the resulting igneous body. Concordant intrusions tend to be more laterally extensive, while discordant intrusions are often more vertical and localized.
Illustrating Concordant Intrusions: Laccolith and Lopolith
(i) Laccolith
A laccolith is a sheet-like igneous intrusion that has uplifted the overlying strata, forming a dome-shaped structure. It is typically concordant, emplaced between sedimentary layers. The pressure exerted by the magma causes the overlying rocks to bulge upwards. The magma doesn't penetrate the entire layer but accumulates, creating a lens-shaped body.
(Image Source: Wikimedia Commons - Illustrative representation of a laccolith. Note: Actual diagrams may vary in detail.)
- Formation: Magma rises along bedding planes, accumulating and exerting upward pressure.
- Shape: Lens-shaped with a flat base and a domed upper surface.
- Location: Often found in sedimentary basins.
- Example: The Henry Mountains in Utah, USA, are classic examples of laccoliths.
(ii) Lopolith
A lopolith is a large, saucer-shaped igneous intrusion that has caused the downward sagging of the overlying strata. It is also concordant, forming when magma intrudes between layers and causes the central portion of the overlying rocks to subside. Unlike laccoliths, lopoliths are characterized by a concave-upward shape.
(Image Source: Wikimedia Commons - Illustrative representation of a lopolith. Note: Actual diagrams may vary in detail.)
- Formation: Magma intrudes, causing the overlying rocks to subside due to their weight and the removal of support.
- Shape: Saucer-shaped with a concave-upward profile.
- Location: Often found in areas with thick sedimentary sequences.
- Example: The Sudbury Igneous Complex in Ontario, Canada, is a prominent example of a lopolithic intrusion, associated with a major impact structure.
The difference between laccoliths and lopoliths lies in the direction of deformation of the overlying strata. Laccoliths cause uplift, while lopoliths cause subsidence. Both are indicative of relatively shallow emplacement depths and significant magma volumes.
| Feature | Laccolith | Lopolith |
|---|---|---|
| Shape | Lens-shaped, domed | Saucer-shaped, concave-upward |
| Overlying Strata | Uplifted | Downwarped |
| Emplacement | Between layers, causing bulging | Between layers, causing sagging |
| Example | Henry Mountains, Utah | Sudbury Igneous Complex, Canada |
Conclusion
In conclusion, the distinction between concordant and discordant igneous intrusions is fundamental to understanding the processes of magma emplacement and the resulting geological structures. Laccoliths and lopoliths, as examples of concordant intrusions, demonstrate how magma can interact with and deform the surrounding strata, creating unique and informative geological features. Studying these intrusions provides valuable insights into the Earth’s dynamic processes and the evolution of its crust. Further research into the mechanics of magma movement and the rheological properties of host rocks will continue to refine our understanding of these fascinating geological phenomena.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.