Essentials of Geology: Igneous Rocks & Intrusive Activity
Concept Checks 4.6
1. Bowen's Reaction Series
- Describes the order of mineral crystallization in cooling magma.
- Rocks with granitic composition have minerals with lower crystallization temperatures (e.g., quartz, potassium feldspar).
- Olivine has the highest melting point.
2. Crystallization and Magma Composition
- Early-formed minerals settle, altering remaining magma composition.
- Results in differentiation of magma composition.
3. Assimilation vs. Magma Mixing
- Assimilation: Magma incorporates surrounding host rocks, changing its composition.
- Magma Mixing: Two distinct magma bodies mix via convective flow, creating an intermediate composition.
4.7 Partial Melting & Magma Composition
Partial Melting
- Minerals with the lowest melting temperatures melt first.
- Partial melting yields a silica-rich melt and a mafic residue.
- Unmelted portion remains more mafic.
Formation of Different Magmas
- Basaltic Magma: Derived from partial melting of mantle peridotite.
- Forms at mid-ocean ridges or subduction zones.
- Considered primary magmas.
- Andesitic Magma: Derived from differentiation or assimilation in continental crust.
- Silica-rich crust plays a role in formation.
- Granitic Magma: Forms from melting of continental crust by underlying basaltic magma.
Nature of Intrusive Bodies
Intrusions (Plutons)
- Formed by emplacement of magma into existing rocks.
- Studied after exposure by erosion and uplift.
- Types include dikes, sills, batholiths, stocks, and laccoliths.
Comparison of Intrusive Structures
- Dikes: Tabular bodies that cut across existing structures.
- Sills: Tabular bodies parallel to existing structures.
- Batholiths: Large, irregular bodies.
- Stocks: Smaller, irregular bodies.
- Laccoliths: Mushroom-shaped intrusions.
Conclusion
Understanding the processes of magma formation and the resulting igneous structures is crucial for geologists. Both surface volcanic phenomena and deep intrusive activities are important aspects of geological studies.