Lecture Notes: Magma Formation

Introduction

• Lesson Number: 3 of the Second Quarter of Earth Science
• Topic: Magma Formation
• Previous Lesson: Earth's Internal Heat

Definition of Magma

• Magma: Molten or partially molten rock stored beneath the Earth's surface.
• When magma reaches the surface, it is referred to as lava.

Conditions for Magma Formation

1. Decrease of Pressure
   • Occurs in the mantle and crust.
2. Addition of Volatile Materials
   • Introduced to hot rocks, causing melting.
3. Transfer of Heat
   • From rising magma in the mantle to the crust.

Processes of Magma Formation

1. Decompression Melting

• Occurs when pressure decreases at a constant high temperature.
• Converts solid material into liquid/semi-liquid due to reduced pressure.

2. Flux Melting

• Caused by the introduction of volatile materials (e.g., water, hydrous minerals).
• Leads to chemical reactions with hot rocks that melt the rock.

Locations of Magma Formation

• Rift Valley: Decompression melting occurs as continental crusts diverge.
• Mid-Oceanic Ridge: Similar to rift valleys, where oceanic crusts are diverging.
• Hot Spots: Heat from the mantle directly produces magma; more direct than mid-ocean ridges.
• Subduction Zones: Flux melting occurs when oceanic crust subdues under continental crust, introducing water that causes melting.

Why and How Magma Rises

1. Density Contrast

• Greater density difference between magma and surrounding rock leads to faster rising.
• Magma is less dense than surrounding rocks.
• At deeper levels, magma seeps through mineral grain boundaries and cracks.

2. Buoyancy and Mass

• Magma must have sufficient buoyancy and mass to rise.
• If density differences are minimal, magma may remain trapped beneath the crust.

3. Viscosity

• Viscosity: Fluid's resistance to flow.
• Low Viscosity Magma: Flows easily, rises faster.
• High Viscosity Magma: More difficult to flow.
• Influenced by:
  • Temperature: Higher temperatures decrease viscosity.
  • Silica Content: More silica increases viscosity.
  • Volatile Content: More dissolved water decreases viscosity, allowing for faster flow.

Conclusion

• Understanding magma formation is crucial to grasping geological processes and phenomena.