Petrologi 2-genesis magma


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Petrologi 2-genesis magma

  1. 1. Production of Igneous Rocks: Convergent Plate Boundaries  Subduction at convergent plate boundaries causes partial melting to form magma and resulting igneous rocks.
  2. 2. Production of Igneous Rocks: Convergent Plate Boundaries
  3. 3. Production of Igneous Rocks: Divergent Plate Boundaries  Magma rises from the mantle at divergent plate boundaries.
  4. 4. Origin of Magma  Where does the heat come from that melts rocks?  Formation of Earth  Heat from the decay of radioactive elements
  5. 5. Factors that Affect Magma Formation  Temperature  Increases with depth  Pressure  Increases with depth  Water Content  Decreases melting point  Mineral Composition  Different minerals, different melting points
  6. 6. Characteristics of Magma  Slushy Mix of molten rock, gases, and mineral crystals  Common Elements         Oxygen (O) Silicon (Si) Aluminum (Al) Iron (Fe) Magnesium (Mg) Calcium (Ca) Potassium (K) Sodium (Na)  Compounds in Magma  Silica ( Si02)  Most abundant  Greatest effect on Magma Characteristics  Effects melting temp  Viscosity of Magma  Types of Magma  Based on amount of Silica  Basaltic  Andesitic  Rhyolitic
  7. 7. SiO2 Content Type of Magma Rhyolitic 70% Andesitic 60% Basaltic 50%
  8. 8. FLUIDITY OF MAGMA Fluidity or Viscosity of magma depends on content (%) of Silica Silica Rich -known as Acidic magma -More viscous, so do not spreads and piles up at one place Silica poor -Known as Basic magma -Less viscous, moves faster and occupies larger area However, the viscosity of magma is considerably influenced by temperature too. When temperature is less-more viscous and when High temperature- less viscous.
  9. 9. Four main groups of igneous rocks based on magma type/mineral composition  Felsic: high silica content, light colored, from thick & slow moving magma, contains low amounts of Ca, Fe, and Mg dominant minerals quartz, potassium rich feldspar  Ex: granite, pumice, rhyolite  Magma Type:  Intermediate: moderate amount of silica, mixture of colors dominant minerals: sodium and calcium rich feldspar  Ex: andesite, diorite  Magma Type:  Mafic: low silica content, dark colored, high levels of Fe & Mg formed from thinner, more fluid, & hotter magma than Felsic rocks dominant minerals hornblende, calcium rich feldspar  Ex: basalt, gabbro  Magma Type:  Ultramafic: very low silica content, dark colors, high levels of Fe & Mg dominant minerals: olivine, pyroxene  Ex: peridotite, dunite  Magma Type:
  10. 10. Scheme For Igneous Rocks
  11. 11. Characteristics of magma  Igneous rocks form as molten rock cools and solidifies  General Characteristic of magma  Parent material of igneous rocks  Forms from partial melting of rocks inside the Earth  Magma that reaches the surface is called lava
  12. 12. Characteristics of magma  General Characteristic of magma  Rocks formed from lava at the surface are classified as extrusive, or volcanic rocks  Rocks formed from magma that crystallizes at depth are termed intrusive, or plutonic rocks
  13. 13. Characteristics of magma  The nature of magma  Consists of three components:  A liquid portion, called melt, that is composed of mobile ions  Solids, if any, are silicate minerals that have already crystallized from the melt  Volatiles, which are gases dissolved in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)
  14. 14. Characteristics of magma  Crystallization of magma  Texture in igneous rocks is determined by the size and arrangement of mineral grains  Igneous rocks are typically classified by  Texture  Mineral composition
  15. 15. Origin of Magma  Highly debated topic  Generating magma from solid rock  Produced from partial melting of rocks in the crust and upper mantle  Role of heat  Temperature increases within Earth’s upper crust (called the geothermal gradient) average between 20oC to 30oC per kilometer
  16. 16. Estimated temperatures in the crust and mantle
  17. 17. Origin of Magma  Role of heat  Rocks in the lower crust and upper mantle are near their melting points  Any additional heat (from rocks descending into the mantle or rising heat from the mantle) may induce melting
  18. 18. Origin of Magma  Role of pressure  An increase in confining pressure causes an increase in a rock’s melting temperature or conversely, reducing the pressure lowers the melting temperature  When confining pressures drop, decompression melting occurs
  19. 19. Decompression melting
  20. 20. Origin of Magma  Role of volatiles  Volatiles (primarily water) cause rocks to melt at lower temperatures  This is particularly important where oceanic lithosphere descends into the mantle
  21. 21. Evolution of magmas  A single volcano may extrude lavas exhibiting very different compositions  Bowen’s reaction series and the composition of igneous rocks  N.L. Bowen demonstrated that as a magma cools, minerals crystallize in a systematic fashion based on their melting points
  22. 22. Evolution of magmas  Bowen’s reaction series  During crystallization, the composition of the liquid portion of the magma continually changes  Composition changes due to removal of elements by earlier-forming minerals  The silica component of the melt becomes enriched as crystallization proceeds  Minerals in the melt can chemically react and change
  23. 23. Evolution of magmas  Processes responsible for changing a magma’s composition  Magmatic differentiation  Separation of a melt from earlier formed crystals to form a different composition of magma  Assimilation  Changing a magma’s composition by the incorporation of foreign matter (surrounding rock bodies) into a magma
  24. 24. Assimilation and magmatic differentiation
  25. 25. Evolution of magmas  Processes responsible for changing a magma’s composition  Magma mixing  Involves two bodies of magma intruding one another  Two chemically distinct magmas may produce a composition quite different from either original magma
  26. 26. Evolution of magmas  Partial melting and magma formation  Incomplete melting of rocks is known as partial melting  Formation of basaltic magmas  Most originate from partial melting of ultramafic rock in the mantle  Basaltic magmas form at mid-ocean ridges by decompression melting or at subduction zones
  27. 27. Evolution of magmas  Partial melting and magma formation  Formation of basaltic magmas  As basaltic magmas migrate upward, confining pressure decreases which reduces the melting temperature  Large outpourings of basaltic magma are common at Earth’s surface
  28. 28. Evolution of magmas  Partial melting and magma formation  Formation of andesitic magmas  Interactions between mantle-derived basaltic magmas and more silica-rich rocks in the crust generate magma of andesitic composition  Andesitic magma may also evolve by magmatic differentiation
  29. 29. Evolution of magmas  Partial melting and magma formation  Formation of granitic magmas  Most likely form as the end product of crystallization of andesitic magma  Granitic magmas are higher in silica and therefore more viscous than other magmas  Because of their viscosity, they lose their mobility before reaching the surface  Tend to produce large plutonic structures