How Large Mafic and Felsic Magma Zones Form and Behave
1. Where Mafic and Felsic Materials Come From (Before Melting)
- Mafic material (basaltic) comes from partial melting of the mantle.
➔ Temperature: ~1200–1400°C.
➔ Silica content: ~45–52% SiO₂.
➔ Dense: ~2900–3100 kg/m³. - Felsic material (granitic/rhyolitic) forms from partial melting of continental crust.
➔ Temperature: ~650–800°C.
➔ Silica content: ~65–75% SiO₂.
➔ Lighter: ~2500–2700 kg/m³.
2. How They Become or Stay Separate
- Physical separation is controlled by:
➔ Temperature difference
➔ Density difference
➔ Viscosity difference
➔ Motion inside the magma reservoir
Main ways they stay separate:
- Density stratification: heavier mafic magma sinks, lighter felsic magma floats.
- Crystallization: minerals separate out as the magma cools.
- Temperature gradients: hotter at depth, cooler at the surface preserves layers.
3. How They Mix (or Fail to Mix)
- New mafic magma injected into cooler felsic magma can:
➔ Partially melt felsic zones.
➔ Form blobs and enclaves without full mixing if viscosities are very different.
➔ Cause turbulent mixing only if injection is extremely violent.
4. Large Pockets or Zones
- Size: Kilometer-scale zones possible (hundreds of millions of cubic meters).
- Motion: Extremely slow (centimeters to meters per year).
- Temperature range: 650–1200°C inside the chamber.
- Density-driven layering: Felsic layers stay on top unless disturbed by injections or earthquakes.
Summary in Simple Physical Terms:
| Factor | Mafic | Felsic |
|---|---|---|
| Temperature (°C) | 1200–1400 | 650–800 |
| Density (kg/m³) | 2900–3100 | 2500–2700 |
| Viscosity | Low (flows easily) | High (sticky) |
| Movement | Sinks | Floats |
| Mixing | Poor unless very turbulent | Poor unless very turbulent |
| Volume scale | ~10⁸–10⁹ m³ possible | ~10⁸–10⁹ m³ possible |
