Partial Melting:
* Process: A solid rock (typically in the Earth's mantle or crust) is heated to a point where only a portion of it melts. The minerals with lower melting points melt first, creating a magma that is chemically different from the original rock.
* Result: A melt that is enriched in low-melting point minerals and depleted in high-melting point minerals.
* Example: A peridotite (a rock rich in olivine) might partially melt to produce a magma enriched in basaltic minerals (like pyroxene and plagioclase).
Fractional Crystallization:
* Process: As a magma cools, different minerals crystallize out at different temperatures. These crystals can settle to the bottom of the magma chamber, separating from the remaining melt.
* Result: A magma that becomes progressively enriched in higher-melting point minerals, and the remaining melt becomes more mafic (rich in magnesium and iron) over time.
* Example: A basaltic magma might initially crystallize olivine and pyroxene, which settle to the bottom. The remaining melt will then be relatively enriched in silica, sodium, and potassium, eventually leading to the formation of more evolved rocks like granite.
The Relationship:
* Opposite but not Inverse: While they involve the opposite processes of melting and crystallization, they are not complete inverses. Partial melting generates a melt with a different composition from the original rock, while fractional crystallization changes the composition of the original melt.
* Complementary: In nature, they often work together to create a wide range of rock types. Partial melting generates magmas that can then undergo fractional crystallization, further modifying their compositions.
* Continuous Processes: Both partial melting and fractional crystallization are continuous processes that can occur over a range of temperatures and pressures.
Key Differences:
| Feature | Partial Melting | Fractional Crystallization |
|---------------------|-----------------------------------|----------------------------------|
| Starting Point | Solid rock | Magma |
| Outcome | Creation of a melt | Modification of existing melt |
| Composition Change| Enriched in low-melting point minerals| Enriched in high-melting point minerals |
| Role of Temperature | Increasing temperature | Decreasing temperature |
Understanding these processes is crucial for understanding how magmas are generated and how they evolve to form the diverse array of igneous rocks we see on Earth.
