Model Answer
0 min readIntroduction
Bowen’s Reaction Series, proposed by Norman L. Bowen in the 1920s, is a fundamental concept in igneous petrology that describes the order in which minerals crystallize from a cooling magma. It’s based on experimental petrology, where Bowen meticulously studied the melting and crystallization behavior of various silicate minerals. This series isn’t a rigid sequence but rather a guide to understanding how magma composition changes as it cools and how different igneous rocks are formed. Understanding this series is crucial for deciphering the origin and evolution of igneous rocks, and consequently, the geological history of a region.
Bowen’s Reaction Series: A Detailed Explanation
Bowen’s Reaction Series is divided into two main branches: the Discontinuous Series and the Continuous Series. These series operate due to changes in temperature and the stability of different silicate structures.
1. The 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. This series primarily involves ferromagnesian minerals (those rich in iron and magnesium). The sequence is as follows:
- Olivine: Crystallizes at the highest temperatures (around 1600°C). It’s unstable at lower temperatures.
- Pyroxene: Forms as olivine becomes unstable, typically between 1450°C and 1000°C.
- Amphibole: Crystallizes as pyroxene reacts with water and other volatiles, generally between 1000°C and 600°C.
- Biotite Mica: Forms at the lowest temperatures in the discontinuous series (below 600°C).
Each mineral in this series is incompatible with the next at the same temperature, leading to a distinct reaction and the formation of a new mineral phase.
2. The Continuous Series
The Continuous Series involves the plagioclase feldspars, which exhibit a continuous change in composition from calcium-rich (anorthite) to sodium-rich (albite) as the temperature decreases. This series operates concurrently with the discontinuous series.
- Calcium-rich Plagioclase (Anorthite): Crystallizes at high temperatures (around 1500°C) along with olivine.
- Sodium-rich Plagioclase (Albite): Forms as the magma cools, with the calcium content decreasing and the sodium content increasing.
The continuous series doesn’t involve distinct reactions like the discontinuous series; instead, it’s a gradual change in the chemical composition of the plagioclase feldspar.
Significance in the Process of Crystallization of Magma
Bowen’s Reaction Series has profound implications for understanding magma crystallization and igneous rock formation:
- Magma Differentiation: The series explains how a single magma can give rise to a variety of igneous rocks. As minerals crystallize and are removed from the melt, the remaining magma changes in composition. This process, known as magma differentiation, leads to the formation of different mineral assemblages and ultimately, different rock types.
- Formation of Layered Intrusions: In large magma chambers, minerals crystallizing at different temperatures can settle out due to gravity, forming layered intrusions. For example, the Bushveld Complex in South Africa exhibits distinct layers of chromitite, pyroxenite, and gabbro, reflecting the sequential crystallization of minerals according to Bowen’s Reaction Series.
- Geochemical Evolution of Magmas: The series helps trace the geochemical evolution of magmas. By analyzing the mineral composition of igneous rocks, geologists can infer the temperature and pressure conditions under which the magma crystallized and its original composition.
- Understanding Igneous Rock Associations: The series explains why certain minerals are commonly found together in igneous rocks. For instance, olivine and calcium-rich plagioclase are often found in ultramafic rocks, while quartz and sodium-rich plagioclase are common in felsic rocks.
| Series | Minerals | Temperature Range (°C) | Process |
|---|---|---|---|
| Discontinuous | Olivine, Pyroxene, Amphibole, Biotite | 1600 - 600 | Sequential crystallization with distinct reactions |
| Continuous | Calcium-rich Plagioclase (Anorthite) to Sodium-rich Plagioclase (Albite) | 1500 - 1000 | Gradual change in composition |
Conclusion
Bowen’s Reaction Series remains a cornerstone of igneous petrology, providing a framework for understanding the complex processes of magma crystallization and differentiation. It explains the origin of diverse igneous rocks and helps decipher the geological history of volcanic and plutonic environments. While simplified, the series provides a powerful tool for interpreting the formation and evolution of magmatic systems, and continues to be refined with advancements in experimental petrology and geochemical analysis.
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.