Model Answer
0 min readIntroduction
Magma, the molten rock beneath the Earth’s surface, is not typically formed by complete melting of the source rock. Instead, it is predominantly generated through a process called ‘partial melting’. This process, crucial to understanding igneous petrology and plate tectonics, involves the melting of only a fraction of the solid rock, leaving a solid residue behind. The composition of the resulting magma is significantly different from the original source rock, and understanding the factors controlling partial melting is fundamental to deciphering the Earth’s magmatic systems and the evolution of the crust and mantle.
Defining Partial Melting
Partial melting occurs when a rock is heated, but not to the temperature at which it completely liquefies. This is due to the fact that rocks are typically composed of multiple minerals, each with a different melting point. The minerals with the lowest melting points will melt first, forming a liquid phase, while the higher melting point minerals remain solid. This liquid phase, enriched in the elements that lower the melting point, is the magma.
Factors Influencing Partial Melting
Several factors control the degree of partial melting and the composition of the resulting magma:
- Temperature: Increasing temperature promotes melting. Geothermal gradient and heat from radioactive decay are primary heat sources.
- Pressure: Increasing pressure generally increases the melting point. However, at certain depths, pressure can *decrease* the melting point, particularly in the presence of volatiles.
- Water Content: The addition of water (or other volatiles like CO2) significantly lowers the melting point of rocks. This is particularly important in subduction zones.
- Rock Composition: The mineral composition of the source rock dictates its melting behavior. Rocks rich in silica (like granite) generally have lower melting points than those rich in magnesium and iron (like peridotite).
Mechanisms of Partial Melting
Partial melting doesn’t occur uniformly. Several mechanisms contribute to its initiation and progression:
- Flux Melting: Occurs when volatiles (water, CO2) are added to the mantle, lowering the melting point. Common in subduction zones.
- Decompression Melting: Occurs when pressure is reduced on hot mantle rock, allowing it to melt without adding heat. This happens at mid-ocean ridges and continental rift zones.
- Heat Transfer Melting: Occurs when hot magma intrudes into cooler crustal rocks, transferring heat and causing localized melting.
Partial Melting in Different Tectonic Settings
The role of partial melting varies significantly depending on the tectonic environment:
Mid-Ocean Ridges (MORs)
Decompression melting of the asthenosphere (upper mantle) is the dominant mechanism. As plates diverge, the pressure on the upwelling mantle decreases, leading to partial melting of peridotite. This generates basaltic magma, which forms the oceanic crust.
Subduction Zones
Flux melting is the primary process. As the subducting slab descends, it releases water into the overlying mantle wedge. This water lowers the melting point of the mantle peridotite, generating magma. The magma composition varies from basaltic to andesitic, depending on the degree of partial melting and the contribution from the subducting slab.
Continental Hotspots
Mantle plumes, rising from deep within the mantle, undergo decompression melting as they approach the surface. This generates voluminous basaltic magma, forming hotspot volcanoes like Hawaii and Yellowstone.
Continental Collision Zones
Partial melting can occur due to heat generated by friction and radioactive decay in the thickened crust. This often results in the formation of granitic magmas, contributing to the growth of continental crust.
Types of Partial Melts and Magma Compositions
The composition of the magma generated by partial melting depends on the composition of the source rock and the degree of melting. Higher degrees of melting produce magmas closer in composition to the original source rock. Lower degrees of melting result in magmas that are enriched in incompatible elements (elements that prefer to be in the melt phase) and depleted in compatible elements (elements that prefer to stay in the solid phase). This process of fractional melting leads to chemical differentiation of the mantle and crust.
| Tectonic Setting | Dominant Melting Mechanism | Source Rock | Typical Magma Composition |
|---|---|---|---|
| Mid-Ocean Ridge | Decompression Melting | Peridotite | Basalt |
| Subduction Zone | Flux Melting | Peridotite, Hydrated Peridotite | Basalt to Andesite |
| Continental Hotspot | Decompression Melting | Mantle Plume | Basalt |
Conclusion
Partial melting is a fundamental process in the generation of magmas and the evolution of the Earth’s crust and mantle. The interplay of temperature, pressure, volatile content, and rock composition dictates the degree of melting and the resulting magma composition. Understanding these factors is crucial for interpreting volcanic activity, the formation of different igneous rocks, and the long-term evolution of our planet. Further research into the complexities of partial melting continues to refine our understanding of Earth’s dynamic 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.