magma formation

• mechanisms through which rocks melt:

  • decompression melting
  • flux melting
• decompression comes from the rate of change of temperature which is the thermal gradient
• decompression occurs at mid ocean ridges where pressure is reduced by thinning of the lithosphere and convection brings the mantle @ mantle plumes. the temperature gradient will be locally raised
• when rocks are heated, they often partially melt to make magma richer in oxygen or silicon than the original rock
• flux melting occurs at convergent plate boundaries, from the combination of h2o and co2 on subjected plate sediment layers melting into the asthenosphere
• flux meeting is water and co2 added to rock lowering the melting temp, this causes the tock to partially melt and generate magma
• Ultramafic magma is low in silica content and has a low viscosity, making it very fluid and capable of flowing long distances.
• magma is classified depending on their silica content ranging from: ultra mafic, low in silica; mafic, then intermediate and silicic with high silica content
• the mores silica in magma the greater the viscosity
• the mantle is ultra-mafic
• for all magma the hotter the temp the less viscous it is
• magma density is controlled by composition, pressure and temp. (comp is the most important)

  • mafic is the most dense (iron rich)
  • average density of the crust is 2.75 - which is higher than mafic magma so mm can still erupt
• as magma rises its pressure reduces, allowing dissolved gases to exolve, making bubbles which increase pressure in magma chambers triggering eruptions.
• the expansion of silicic magma due to gas is resisted because the lava is sticky and thick.
• expansion in mafic magma is much easier because it is less viscous and eruption is more effusive in nature