Model Answer
0 min readIntroduction
Basaltic magma, a common type of magma found at divergent plate boundaries and hotspots, is characterized by its relatively low silica content (45-52 wt%), high iron and magnesium content, and low viscosity. The evolution of this magma from its initial molten state to the formation of diverse igneous rocks is governed by the processes of crystallization and differentiation. These processes are elegantly explained by Norman L. Bowen’s Reaction Series, a cornerstone concept in igneous petrology, which describes the order in which minerals crystallize from a cooling magma. Understanding this series is fundamental to deciphering the origin and evolution of basaltic volcanic provinces worldwide.
Basaltic Magma: Composition and Characteristics
Basaltic magma typically originates from the partial melting of the Earth’s mantle. Its key characteristics include:
- Low Silica Content: 45-52 wt% SiO2, leading to lower viscosity.
- High Iron (Fe) and Magnesium (Mg) Content: Contributes to the formation of mafic minerals.
- High Temperature: Typically between 1000-1200°C.
- Low Gas Content: Compared to other magma types, though still significant.
Bowen’s Reaction Series: A Detailed Explanation
Bowen’s Reaction Series describes the order in which minerals crystallize from a cooling magma. It’s divided into two branches:
Discontinuous Series
The discontinuous series involves minerals that crystallize at distinct temperature ranges, with each mineral being replaced by the next as the temperature decreases. The sequence is:
- Olivine: Crystallizes first at the highest temperatures (around 1600°C).
- Pyroxene: Forms as the temperature drops below olivine’s crystallization point.
- Amphibole: Crystallizes at lower temperatures, replacing pyroxene.
- Biotite Mica: Forms at the lowest temperatures in the discontinuous series.
Continuous Series
The continuous series involves a gradual change in mineral composition with decreasing temperature. This series primarily concerns plagioclase feldspar:
- Calcium-rich Plagioclase (Anorthite): Crystallizes at high temperatures.
- Sodium-rich Plagioclase (Albite): As the magma cools, calcium is progressively replaced by sodium in the plagioclase structure, resulting in a continuous change in composition.
[Insert Diagram Here: A clear, labelled diagram of Bowen’s Reaction Series is essential. It should show both the discontinuous and continuous series, with temperature scales and mineral names clearly indicated.]
Crystallization and Differentiation of Basaltic Magma
The crystallization and differentiation of basaltic magma are driven by several processes:
- Fractional Crystallization: As minerals crystallize, they can be removed from the magma through settling (due to density differences) or flow. This changes the composition of the remaining magma, leading to the formation of different rock types. For example, early crystallization of olivine and pyroxene leaves a magma enriched in silica, iron, and sodium.
- Partial Melting: The initial magma is often not a complete melt, and the composition of the melt changes as crystallization progresses.
- Magma Mixing: Mixing of different magma batches can alter the overall composition and drive differentiation.
- Assimilation: Incorporation of surrounding country rock into the magma can also change its composition.
The following table illustrates the typical sequence of crystallization and the resulting rock types from basaltic magma:
| Temperature (°C) | Crystallizing Minerals | Resulting Rock Type | Magma Composition Change |
|---|---|---|---|
| 1200-1000 | Olivine, Ca-rich Plagioclase | Peridotite, Gabbro | Decreased Mg, Fe, Ca; Increased Si |
| 1000-900 | Pyroxene, Plagioclase (intermediate composition) | Diorite, Andesite | Decreased Mg, Fe; Increased Si, Na |
| 900-800 | Amphibole, Plagioclase (Na-rich) | Granodiorite, Rhyolite | Decreased Ca, Mg, Fe; Increased Si, Na, K |
| <800 | Biotite, Quartz, K-feldspar | Granite | Highly evolved, enriched in Si, Na, K |
Example: The Hawaiian Islands are a prime example of basaltic volcanism. The initial basaltic magma undergoes fractional crystallization, leading to the formation of a range of rock types, from olivine-rich peridotites to more silica-rich basalts and andesites. The Kilauea volcano exhibits ongoing differentiation processes.
Conclusion
In conclusion, the crystallization and differentiation of basaltic magma are fundamental processes in igneous petrology, governed by Bowen’s Reaction Series. Fractional crystallization, coupled with other processes like magma mixing and assimilation, leads to a diverse suite of igneous rocks from a single parent magma. Understanding these processes is crucial for interpreting the geological history of volcanic regions and the evolution of the Earth’s crust. Continued research into magma dynamics and crystallization kinetics will further refine our understanding of these complex processes.
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.