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Petrologi 2-genesis magma
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Petrologi 2-genesis magma


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  • 1. Production of Igneous Rocks: Convergent Plate Boundaries  Subduction at convergent plate boundaries causes partial melting to form magma and resulting igneous rocks.
  • 2. Production of Igneous Rocks: Convergent Plate Boundaries
  • 3. Production of Igneous Rocks: Divergent Plate Boundaries  Magma rises from the mantle at divergent plate boundaries.
  • 4. Origin of Magma  Where does the heat come from that melts rocks?  Formation of Earth  Heat from the decay of radioactive elements
  • 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. 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. SiO2 Content Type of Magma Rhyolitic 70% Andesitic 60% Basaltic 50%
  • 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. 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. Scheme For Igneous Rocks
  • 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. 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. 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. 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. 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. Estimated temperatures in the crust and mantle
  • 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. 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. Decompression melting
  • 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. 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. 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. 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. Assimilation and magmatic differentiation
  • 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. 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. 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. 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. 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