Generation of Magma

1
Generation of Magma

• And Magma Properties

2
(No Transcript)
3
What is magma?

• Magma is completely or partially molten material,
  which cools to form an igneous rock.
• The liquid portion is called the melt (usually
  ions of silica and oxygen, with lesser amounts of
  magnesium, aluminum, potassium, calcium, sodium,
  and iron).
• Solid components are silicate minerals that have
  already crystallized.
• As the magma body cools, the number of solids
  increases.

4
The Role of Heat

• Geothermal gradient

5
The Role of Pressure

• Pressure increases with depth in the Earth
• Melting occurs at higher temperatures with depth
  because of confining pressure ? the greater the
  pressure, the higher the melting point
• When confining pressure drops, decompression
  melting occurs
• Occurs when rock ascends as a result of
  convective upwelling (recall that the mantle is
  solid, but flows at very slow rates.
  

6
Role of Volatiles

• Magma also contains dissolved gasses
• Water Vapor (H2O), carbon dioxide (CO2) Sulfur
  Dioxide (SO2)
• Water actually lowers the melting point of rock
• Consequently, wet rock at depth has a lower
  melting point than dry rock

7
Crystallization and Magma Evolution

• Process of magma cooling is called
  crystallization. (Bowens reaction series)

8
Magmatic differentiation
9
The Nature of Volcanic Activity

• Factors that affect magma viscosity
• A magmas viscosity is directly related to its
  silica content

Vs.
Which volcano do you think was formed from a more
silica rich magma?
10
Magmas and Rock types
The key to volcanic rocks
11
Magma and Partial Melting

• Most magma produced from a process called partial
  melting.
• As rock begins to melt, minerals with the lowest
  melting point melt first
• Vice verse with crystallization

12
Where do different rocks form?
13
Formation of Basaltic Magma

• Originates from partial melting of an ultramafic
  rock called peridotite.
• Peridotite is composed of iron-rich minerals such
  as those that are major constituents of the
  mantle.
• Basltic magma upwells at hotspots or spreading
  centers (midicean ridges).
• Occurs in subduction zone settings, but does not
  make it to the the surface.
• Little to no silica incorporated into the mix.
• Ascend rapidly to the Earths surface causing
  large outflows of lava (i.e. Hawaii)

14
Formation of Andesitic and Granitic Magmas

• Interaction between basaltic magma and the
  continental crust.
• As basaltic magma migrates upward, it can melt
  the crust.
• The crust provides incorporated silica into the
  magma making it chemically different from the
  basaltic magma.

15
Andesitic Magma

• Andesitic magmas may evolve from a process called
  magmatic differentiation.
• If you look at Bowens reaction series, as
  basaltic magma crystallizes, silica-poor
  ferromagnesian minerals crystallize first.
• Remaining melt is now silica-rich ? causing the
  andesitic rocks
• These magmas are called secondary magmas

16
Granitic Magma

• Derived from partial melting of the continental
  crust and/or the end product of crystallization
  of andesitic magma.
• Higher in silica
• More viscous (thicker)
• Frequently lose their mobility before making it
  to the surface and produce produces large
  plutonic structures (cool underground).
• When they do make it to the surface, cause
  explosive eruptions