Model Answer
0 min readIntroduction
Alkaline rocks are a fascinating group of igneous rocks characterized by a high proportion of alkali metals (sodium and potassium) relative to other elements. They represent a significant portion of continental crust and are often associated with intraplate volcanism. The Deccan Traps, a large igneous province in India, represent one of the largest volcanic features on Earth, and understanding its formation is crucial for comprehending the geological evolution of the Indian subcontinent. The Deccan volcanism, occurring around the Cretaceous-Paleogene boundary, is linked to significant global events, including the extinction of dinosaurs.
Classification of Alkaline Rocks
Alkaline rocks are classified based on their mineralogical composition, alkali content, and geochemical characteristics. They are broadly divided into several categories:
- Ultramafic Alkaline Rocks: These are characterized by a high content of magnesium and iron, with minerals like olivine, pyroxene, and amphibole. Examples include kimberlites and lamproites, often associated with diamond deposits.
- Mafic Alkaline Rocks: These rocks contain moderate amounts of magnesium and iron, along with alkali feldspars and nepheline. Basanites and tephrites fall into this category.
- Intermediate Alkaline Rocks: These rocks have an intermediate composition, with minerals like alkali feldspar, pyroxene, and amphibole.
- Felsic Alkaline Rocks: These are rich in silica and alkali elements, containing minerals like alkali feldspar, quartz, and feldspathoids (like nepheline and leucite). Examples include phonolites and trachytes.
Further classification considers the presence or absence of feldspathoids. Rocks with feldspathoids are termed ‘feldspathoid alkaline rocks’ while those without are ‘feldspar alkaline rocks’. Geochemically, alkaline rocks are characterized by high ratios of Na2O + K2O to SiO2.
Age and Duration of Deccan Volcanism
The Deccan Traps volcanism occurred during the late Cretaceous and early Paleogene periods, approximately 66 to 62 million years ago. Determining the precise age and duration has been a subject of intense research.
Dating Methods
- Radiometric Dating: Argon-argon (40Ar/39Ar) dating of basalt flows has been the primary method used to determine the age of the Deccan Traps. Initial dating suggested a relatively short duration, around 30,000 years, centered around the Cretaceous-Paleogene (K-Pg) boundary.
- Paleomagnetic Stratigraphy: Analysis of the magnetic polarity of the lava flows provides a chronological framework. The Deccan Traps exhibit a sequence of normal and reversed polarity zones, which can be correlated with the global geomagnetic polarity timescale.
- Geochemical Correlation: Strontium isotope ratios (87Sr/86Sr) and other trace element analyses help correlate lava flows and refine the age estimates.
Duration and Phases of Volcanism
Recent high-resolution 40Ar/39Ar dating studies suggest a more complex history than initially thought. The Deccan volcanism can be broadly divided into three phases:
| Phase | Age (Ma) | Characteristics |
|---|---|---|
| Phase 1 | 66.2 – 65.8 Ma | Initial phase, characterized by relatively low eruption rates. |
| Phase 2 | 65.8 – 65.2 Ma | Main phase, with the highest eruption rates and largest volumes of lava. Coincides closely with the K-Pg boundary. |
| Phase 3 | 65.2 – 62 Ma | Decline phase, with decreasing eruption rates and a shift in magma composition. |
The total duration of the Deccan volcanism is estimated to be around 3-4 million years, with the most intense phase lasting approximately 300,000-500,000 years. The total volume of lava erupted is estimated to be over 1.5 million cubic kilometers.
Proposed Causes
The cause of the Deccan volcanism is linked to the Reunion hotspot, a mantle plume currently located beneath Réunion Island in the Indian Ocean. The Indian plate moved over this hotspot, leading to decompression melting and extensive volcanism. The timing of the Deccan Traps volcanism, coinciding with the K-Pg boundary, has led to speculation about a possible link to the Chicxulub impact event, although the exact relationship remains debated. Some researchers suggest the impact may have triggered or enhanced the volcanism, while others argue that the Deccan Traps were already erupting and the impact had a minimal effect.
Conclusion
In conclusion, alkaline rocks are classified based on their mineralogical and geochemical composition, with the Deccan Traps exhibiting a diverse range of alkaline basaltic compositions. The Deccan volcanism, spanning approximately 3-4 million years around the K-Pg boundary, was likely driven by the Reunion hotspot. While the precise relationship between the Deccan Traps and the K-Pg extinction event remains a topic of ongoing research, understanding this massive volcanic province is crucial for unraveling the geological and biological history of our planet.
Answer Length
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