Making Magma

Magma is molten rock, and contains:

• Melt - Liquid portion of the magma, composed of mobile ions
• Solids - Silicate minerals that have already crystallized from the melt
• Volatiles - gases dissolved in the melt, including water vapor (H₂O), carbon dioxide (CO₂), and sulfur dioxide (SO₂)

 

How is magma made?

• Heat - The heat needed to melt the rocks can originate in one of three ways:
  • Friction in subduction zones
  • Partial melting in crust / upper mantle
  • Geothermal Gradient
  • Heat Transfer
  
  
• Pressure
  • An increase in confining pressure increases the temperature required to melt rock.
  • When pressure decreases, the temperature required to melt the rock decreases, too
  • Therefore, the rock can melt at a lower T
  
  
• Volatiles - gases found within the magma.
  • Lowers the temperature required to melt rock
  • Primarily water
  • Where would you find this?

 

Magma Variation

Magmas vary chemically due to…

• Initial source rock compositions.
  • Mantle source: Ultramafic and mafic magmas.
  • Crustal source: Mafic, intermediate, & felsic magmas.
• Partial melting.
  • Removing a partial melt from its source creates a felsic (silica rich) magma with a mafic residue. Silica rich minerals melt first, while the darker, silica poor minerals melt last. This results in a silica-rich magma.
• Assimilation.

  • 	As magma rises, it intrudes into the country rock, uplifting the layers above it and squeezing into cracks.
    	The magma also melts the country rock, which changes the composition of the magma

    
    
• Magma Mixing.
  • Different magmas may blend in a magma chamber. The result combines the characteristics of the two
    
    
    
• Fractional crystallization
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How does magma get to the surface?

• Rocks at great depth - Magma is less dense than the surrounding (host) rock. The host rocks warm up and, deform plastically. Magma can easily push aside softened host rock
• Rocks near the surface - The host rocks are cold & brittle. Magma ascends through fractures or stoping (blocks of wall rock break off and then sink into the magma)
• Visit https://courses.nus.edu.sg/course/geosnk/internet/animations/resources/anim/MagmaRisingMantle_GG.html to see an animation of this process