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 is the melting of only one component or a limited range of components in a solid rock. It occurs because rocks are rarely composed of a single mineral; they are 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. This results in a liquid phase coexisting with a solid residue.
Role of Partial Melting in Magma Generation
Partial melting plays a pivotal role in magma generation in several tectonic settings, including:
- Subduction Zones: As an oceanic plate subducts into the mantle, water released from the hydrated minerals in the subducting slab lowers the melting point of the overlying mantle wedge, inducing partial melting and generating arc magmas (e.g., Andes Mountains, Japan).
- Mid-Ocean Ridges: Decompression melting occurs as the mantle rises at mid-ocean ridges due to reduced pressure. This leads to partial melting of the asthenosphere, generating basaltic magma that forms new oceanic crust.
- Continental Hotspots: Mantle plumes, rising from deep within the Earth, experience decompression melting as they approach the surface, generating voluminous basaltic magmas (e.g., Hawaiian Islands, Yellowstone).
- Continental Collision Zones: While less common, partial melting can occur in the lower crust during continental collisions due to frictional heating and the presence of water.
Factors Influencing Partial Melting
Several factors control the extent and nature of partial melting:
1. Temperature
Increasing temperature is the most obvious driver of melting. Geothermal gradient and heat transfer from the core contribute to the temperature increase with depth. However, simply increasing temperature isn't always sufficient; other factors play a crucial role.
2. Pressure
Pressure has a complex effect. Increasing pressure generally increases the melting point of rocks. However, decreasing pressure (decompression melting) can lower the melting point, facilitating partial melting, as seen at mid-ocean ridges and hotspots.
3. Water Content
The presence of water (or other volatile substances like carbon dioxide) significantly lowers the melting point of rocks. This is particularly important in subduction zones, where water released from the subducting slab promotes melting in the mantle wedge. This phenomenon is known as ‘flux melting’.
4. Rock Composition
Different minerals have different melting points. Rocks with a higher proportion of minerals with lower melting points will melt more readily. For example, ultramafic rocks (high in magnesium and iron) require higher temperatures to melt compared to basaltic rocks.
Types of Partial Melting and Magma Composition
The degree of partial melting and the composition of the source rock significantly influence the composition of the resulting magma. Different types of partial melting lead to different magma characteristics:
- Equilibrium Partial Melting: Occurs when the melting process is slow enough to allow the melt and solid residue to remain in chemical equilibrium. This results in a magma composition that is representative of the source rock.
- Disequilibrium Partial Melting: Occurs when melting is rapid, preventing equilibrium. Certain elements preferentially partition into the melt or solid residue, leading to magma compositions that are different from the source rock. This is common in subduction zones.
The degree of partial melting is also critical. A small degree of partial melting (e.g., <1%) typically produces magma enriched in incompatible elements (elements that do not readily fit into the crystal structure of common mantle minerals, like potassium, sodium, and uranium). Higher degrees of partial melting produce magmas that are closer in composition to the source rock.
| Factor | Effect on Melting Point | Tectonic Setting Example |
|---|---|---|
| Temperature | Increases melting point | Deep mantle plumes |
| Pressure | Increases melting point (generally) | Deep within the Earth |
| Water Content | Decreases melting point | Subduction zones |
| Rock Composition | Varies depending on mineralogy | Ultramafic vs. Basaltic rocks |
Conclusion
Partial melting is a fundamental process in the Earth’s interior, responsible for generating the vast majority of magmas that drive volcanism and contribute to the formation and evolution of the Earth’s crust. The process is intricately linked to tectonic settings and is governed by a complex interplay of temperature, pressure, fluid content, and rock composition. Understanding partial melting is therefore essential for comprehending the dynamic processes shaping our planet and the distribution of resources within it.
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.