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Lecture 6 Igneous Rocks: Solids from Melts

This lecture explores the formation, texture, and differentiation of igneous rocks. Topics include the different ways igneous rocks form, how they differ from one another in terms of texture and cooling rate, the types of pyroclasts and extrusive rocks, the characteristics of intrusive rocks, the process of magmatic differentiation, and the forms of igneous intrusions.

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Lecture 6 Igneous Rocks: Solids from Melts

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  1. Lecture 6 Igneous Rocks: Solids from Melts

  2. About Igneous Rocks • Igneous rocks form from liquid rock (magma) in several different ways. • Igneous processes within Earth produce intrusive igneous rocks. • Igneous processes on or near Earth’s surface produce extrusive igneous rocks.

  3. How Do Igneous Rocks Differ • from One Another? Texture – size of crystals ● Coarse-grained rocks ● Fine-grained rocks ● Mixed texture rocks

  4. How Do Igneous Rocks Differ • from One Another?

  5. How Do Igneous Rocks Differ • from One Another? Texture is related to rate of cooling. ● Intrusive igneous rocks ● Extrusive igneous rocks

  6. Pyroclasts Igneous Textures

  7. Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air.

  8. Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Extrusive igneous rocks cool rapidly and are fine-grained.

  9. Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Extrusive igneous rocks cool rapidly and are fine-grained. Gabbro Granite Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Intrusive rocks

  10. Pyroclasts Volcanic ash Bomb Pumice Extrusive pyroclasts form in violent eruptions from lava in the air. Extrusive rocks Mafic Felsic Basalt Rhyolite Extrusive igneous rocks cool rapidly and are fine-grained. Porphyry Gabbro Granite Intrusive igneous rocks cool slowly, allowing large, coarse crystals to form. Phenocrysts Intrusive rocks Some phenocrysts grow large, but the remaining melt cools faster, forming smaller crystals during an eruption. Porphyry

  11. Slow cooling or fast cooling?

  12. 2. How Do Magmas Form? Why do rocks melt?

  13. 2. How Do Magmas Form? What is a magma chamber? A rising mass of magma that pushes aside crustal rocks as it rises through the crust.

  14. 2. How Do Magmas Form? A temperature of about 1000C is required for partial melting of crustal rocks. A depth of at least 40 km is required for temperatures of 1000C to occur.

  15. 3. Magmatic Differentiation A process by which rocks of varying composition can arise from a uniform parent magma. The first minerals to crystallize from a cooling magma are the ones that are the last to melt.

  16. 3. Magmatic Differentiation Fractional crystallization is the process by which the crystals are formed in a cooling magma and are segregated from the remaining liquid. Fine grained Coarse grained

  17. 3. Magmatic Differentiation GRANITE AND BASALT Magma chamber A Crystallizing minerals Magma chamber B Partial melting of country rock Basaltic magma

  18. Partial melting creates a magma of a particular composition. Magma chamber A Crystallizing minerals Magma chamber B Partial melting of country rock Basaltic magma

  19. Partial melting creates a magma of a particular composition. Cooling causes minerals to crystallize and settle. Magma chamber A Crystallizing minerals Magma chamber B Partial melting of country rock Basaltic magma

  20. Partial melting creates a magma of a particular composition. Cooling causes minerals to crystallize and settle. Magma chamber A Magma chamber A Crystallizing minerals Magma chamber B Magma chamber B Partial melting of country rock Basaltic magma

  21. Partial melting creates a magma of a particular composition. A basaltic magma chamber breaks through. Cooling causes minerals to crystallize and settle. Magma chamber A Magma chamber A Crystallizing minerals Magma chamber B Magma chamber B Partial melting of country rock Basaltic magma

  22. Partial melting creates a magma of a particular composition. A basaltic magma chamber breaks through. Cooling causes minerals to crystallize and settle. Mixing results in andesitic magma. Magma chamber A Magma chamber A Crystallizing minerals Magma chamber B Magma chamber B Partial melting of country rock Basaltic magma

  23. Partial melting creates a magma of a particular composition. A basaltic magma chamber breaks through. Cooling causes minerals to crystallize and settle. Mixing results in andesitic magma. Magma chamber A Magma chamber A Crystallizing minerals Crystals may accumulate on the sides and roof of the chamber due to turbulence. Magma chamber B Magma chamber B Partial melting of country rock Basaltic magma

  24. 4. Forms of Igneous Intrusions

  25. Lava flow Ash falls and pyroclasts Country rock Volcano Volcanic neck with radiating dikes Stock Dike Sill Dike Sill Dike Sill Pluton Batholith

  26. Lava flow Ash falls and pyroclasts Country rock Volcano Volcanic neck with radiating dikes Stock Dike Sill Dikes cut across layers of country rock … Dike Sill Dike Sill Pluton Batholith

  27. Lava flow Ash falls and pyroclasts Country rock Volcano Volcanic neck with radiating dikes Stock Dike Sill Dikes cut across layers of country rock … Dike Sill Dike Sill Pluton … but sills run parallel to them. Batholith

  28. Lava flow Ash falls and pyroclasts Country rock Volcano Volcanic neck with radiating dikes Stock Dike Sill Dikes cut across layers of country rock … Dike Sill Dike Sill Pluton … but sills run parallel to them. Batholith Batholiths are the largest forms of plutons, covering at least 100 km2.

  29. Dike or Sill?

  30. 5. Igneous Processes and Plate Tectonics Magma factories: Spreading centers Subduction zones Mantle plumes

  31. 5. Igneous Processes and Plate Tectonics

  32. 5. Igneous Processes and Plate Tectonics Origin of magma in magma factories: ● Decompression melting in spreading centers ● Fluid-induced melting in subduction zones

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