1.6 Igneous Rocks

Form 

• By cooling and crystallization of magma at the depth of lava at the surface 

How magma is generated 

• Add heat 

  • Magam from depth ascends and partially melts crust and/or mantle 

    • Could be caused from hotspots 

  • Overthickened crust (mountain building) 

    • Over-thickening can cause heat to melt the rock creating magma 

• Decrease pressure 

  • Called decompression melting 

  • Lowers melting temperature 

    • Divergent: Making crust thinner (decreasing pressure) allows the rocks to begin melting  

• Add water (flux melting) 

  • Lowers melting temperature 

    • Water has infiltrated the rocks, as the wet plate is drug down, the water enters the upper mantle between the lithosphere creating magma 

      • The water acts as a catalyst (triggers) 

    • This is Subduction 

Magma components 

• Consists of solid, liquid, and gases 

  • Solid: crystallized elements 

  • Liquid (melt): mobile, unordered elements 

  • Gases (volatiles): dissolve gases within melt 

    • CO2 H2O, S, CL, Cl, F 

Magma Crystallization  

• Minerals crystallized in an ordered, predictable way based on melting point of minerals 

• As minerals crystallize, composition of remaining melt changes (becomes more enriched in silica, Na, K, and more depleted in Fe, Mg, and Ca)