Model Answer
0 min readIntroduction
Magma, the molten rock beneath the Earth’s surface, is not a complete melt of the source rock. Instead, it is typically generated through a process called ‘partial melting’, where only a fraction of the source rock transitions into a liquid state. This process is fundamental to understanding the composition and origin of various igneous rocks, and consequently, the evolution of the Earth’s crust and mantle. Partial melting is a complex process influenced by several factors, and its understanding is crucial for deciphering the Earth’s internal dynamics and volcanic activity.
Defining Partial Melting
Partial melting refers to the melting of only a portion of a solid rock. This occurs because rocks are rarely composed of a single mineral; they are typically aggregates of different minerals, each with its own unique melting point. When a rock is heated, the minerals with the lowest melting points will melt first, while those with higher melting points remain solid. The resulting liquid, the magma, will have a different composition than the original rock.
Role of Partial Melting in Magma Generation
Partial melting plays a pivotal role in magma generation within the Earth’s mantle and crust. Several key processes contribute to this:
1. Decompression Melting
This occurs when the pressure on a hot rock is reduced, lowering its melting point. This commonly happens at mid-ocean ridges where mantle plumes rise and the overlying pressure decreases. As the mantle rock rises, it experiences decompression, leading to partial melting and the formation of basaltic magma.
2. Addition of Volatiles (Flux Melting)
The introduction of volatiles, such as water or carbon dioxide, lowers the melting point of rocks. This is particularly important in subduction zones, where water-rich oceanic crust descends into the mantle. The water released from the subducting slab lowers the melting point of the overlying mantle wedge, causing partial melting and the generation of arc magmas (andesitic to rhyolitic).
3. Heat Transfer Melting
This occurs when hot magma rises from the mantle and transfers heat to the surrounding crustal rocks. If the crustal rocks are already close to their melting point, the added heat can trigger partial melting, generating granitic magmas. This is common in continental volcanic arcs and rift valleys.
Factors Influencing Partial Melting
The extent and composition of partial melting are controlled by several factors:
- Temperature: Higher temperatures promote more extensive melting.
- Pressure: Increasing pressure generally increases the melting point, but decompression can induce melting.
- Water Content: The presence of water significantly lowers the melting point.
- Rock Composition: Different minerals have different melting points. Rocks rich in minerals with low melting points will melt more easily.
Types of Partial Melting and Magma Composition
The degree of partial melting significantly influences the composition of the resulting magma.
| Degree of Partial Melting | Magma Composition | Geological Setting |
|---|---|---|
| Low (5-20%) | Mafic (Basaltic) | Mid-Ocean Ridges, Mantle Plumes |
| Intermediate (20-50%) | Intermediate (Andesitic) | Subduction Zones |
| High (50-100%) | Felsic (Rhyolitic) | Continental Crust, Hotspots |
For instance, melting of peridotite (the main rock of the upper mantle) at low degrees of partial melting produces basaltic magma, enriched in iron and magnesium. Higher degrees of partial melting, or melting of more silica-rich rocks in the crust, produce more felsic magmas like rhyolite, enriched in silica, potassium, and sodium.
Bowen’s Reaction Series and Magma Evolution
Bowen’s Reaction Series describes the order in which minerals crystallize from a cooling magma. This series is crucial in understanding how magma composition evolves during partial melting and fractional crystallization. Minerals that crystallize early in the series are those with higher melting points and are less likely to be present in the initial melt produced by partial melting.
Conclusion
In conclusion, partial melting is a fundamental process in magma generation, driven by changes in temperature, pressure, and volatile content. The degree of partial melting, coupled with the composition of the source rock, dictates the resulting magma’s characteristics. Understanding these processes is vital for interpreting the Earth’s geological history, predicting volcanic eruptions, and comprehending the formation of diverse igneous rocks. Further research into the complexities of partial melting continues to refine our understanding of Earth’s dynamic interior.
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.