Model Answer
0 min readIntroduction
The Earth’s composition is broadly divided into the mantle and the crust. While the mantle, constituting about 84% of Earth’s volume, is predominantly composed of silicate minerals like olivine and pyroxene, the continental crust is remarkably rich in quartz. This stark contrast raises a fundamental question: why is quartz, completely absent in the mantle, the most common mineral on the continents? This disparity isn’t accidental; it’s a direct consequence of the Earth’s differentiation processes, the principles of Bowen’s Reaction Series, and the unique geochemical environment of continental crust formation.
Geochemical Constraints in the Mantle
The mantle’s high-pressure and high-temperature conditions (estimated to be between 400-3500°C and 140-360 GPa) dictate the stability of minerals. Quartz (SiO2) is a polymorph, meaning it can exist in different crystalline forms depending on pressure and temperature. At mantle conditions, quartz is unstable and transforms into denser, high-pressure polymorphs like stishovite and coesite. These polymorphs, while containing silicon and oxygen, are structurally different and do not exhibit the characteristics of quartz. Furthermore, the mantle is depleted in elements that readily form felsic minerals like quartz.
Bowen’s Reaction Series and Magma Differentiation
Bowen’s Reaction Series describes the order in which minerals crystallize from a cooling magma. The series is divided into two branches: discontinuous and continuous. The discontinuous branch involves minerals like olivine, pyroxene, amphibole, and biotite, which crystallize at distinct temperature ranges. The continuous branch involves plagioclase feldspar, where the calcium content decreases with decreasing temperature. Crucially, quartz is one of the *last* minerals to crystallize from a magma. This means that for quartz to form, the magma must undergo significant differentiation – a process where early-formed minerals are removed, leaving a silica-rich melt.
Partial Melting and Crust Formation
The continental crust isn’t formed by the crystallization of the entire mantle. Instead, it’s formed through partial melting of the mantle. When the mantle melts, it doesn’t melt completely; instead, certain minerals melt preferentially over others. Minerals with lower melting points, which are typically silica-rich, are the first to melt. This initial melt is relatively enriched in silica. As this melt rises and cools, it undergoes fractional crystallization, further concentrating silica.
Role of Subduction and Crustal Recycling
Subduction zones play a critical role in continental crust formation. When oceanic crust subducts beneath continental crust, it carries water-rich sediments and altered oceanic crust. This water lowers the melting point of the overlying mantle wedge, initiating partial melting. The resulting magmas are often silica-rich and contribute to the growth of continental crust. Furthermore, the recycling of crustal material back into the mantle through subduction is limited, meaning silica-rich components tend to accumulate in the crust over geological time.
Incompatibility of Quartz in Mantle Minerals
Quartz is incompatible in the crystal structure of most mantle minerals like olivine and pyroxene. This means that silicon and oxygen, the building blocks of quartz, don’t readily fit into these structures. During partial melting, these incompatible elements are preferentially partitioned into the melt, leading to silica enrichment. The resulting magma, upon cooling, crystallizes quartz-rich minerals like granite and granodiorite, forming the bulk of the continental crust.
| Feature | Mantle | Continental Crust |
|---|---|---|
| Dominant Minerals | Olivine, Pyroxene | Quartz, Feldspar |
| Silica Content | ~45% | ~70% |
| Pressure & Temperature | High (140-360 GPa, 400-3500°C) | Relatively Low |
| Quartz Presence | Absent (Polymorphs exist) | Abundant |
Conclusion
The absence of quartz in the mantle and its prevalence on the continents is a direct consequence of the Earth’s differentiation, the principles of Bowen’s Reaction Series, and the processes of partial melting and crustal recycling. The high-pressure, high-temperature conditions of the mantle preclude the stability of quartz, while the silica-rich nature of magmas generated through partial melting, coupled with the incompatibility of silica in mantle minerals, leads to its concentration in the continental crust. This geochemical disparity highlights the dynamic processes that have shaped our planet over billions of years.
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.