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23.3.a igneous rocks ppt.ppt
- 2. General characteristics of magma • Igneous rocks are formed by cooling and solidification of magma. • Characteristics of magma: • Parent material of igneous rocks • Forms from partial melting of rocks • Magma at surface is called lava
- 3. • General characteristic of magma • Rocks formed from lava = extrusive, or volcanic rocks • Rocks formed from magma at depth = intrusive, or plutonic rocks
- 4. • The nature of magma • Consists of three components: – Liquid portion = melt – Solids, if any, are silicate minerals – Volatiles = dissolved gases in the melt, including water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2)
- 5. • Crystallization of magma • Cooling of magma results in the systematic arrangement of ions into orderly patterns • The silicate minerals resulting from crystallization form in a predictable order • Texture - size and arrangement of mineral grains
- 6. Igneous textures • Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals • Factors affecting crystal size • Rate of cooling – Slow rate = fewer but larger crystals – Fast rate = many small crystals – Very fast rate forms glass
- 7. Igneous textures • Factors affecting crystal size • % of silica (SiO2) present • Dissolved gases
- 8. Igneous textures • Types of igneous textures • Aphanitic (fine-grained) texture – Rapid rate of cooling – Microscopic crystals – May contain vesicles (holes from gas bubbles) • Phaneritic (coarse-grained) texture – Slow cooling – Large, visible crystals
- 11. Igneous textures • Types of igneous textures • Porphyritic texture – Minerals form at different temperatures – Large crystals (phenocrysts) are embedded in a matrix of smaller crystals (groundmass) • Glassy texture – Very rapid cooling of lava – Resulting rock is called obsidian
- 12. Igneous textures • Types of igneous textures • Pyroclastic texture – Fragmental appearance produced by violent volcanic eruptions – Often appear more similar to sedimentary rocks • Pegmatitic texture – Exceptionally coarse grained – Form in late stages of crystallization of granitic magmas
- 14. Glassy texture
- 15. Igneous compositions • Igneous rocks are composed primarily of silicate minerals • Dark (or ferromagnesian) silicates – Olivine, pyroxene, amphibole, and biotite mica • Light (or nonferromagnesian) silicates – Quartz, muscovite mica, and feldspars
- 17. Igneous compositions • Granitic versus basaltic compositions • Granitic composition – Light-colored silicates – Termed felsic (feldspar and silica) in composition – High amounts of silica (SiO2) – Major constituent of continental crust
- 18. Igneous compositions • Granitic versus basaltic compositions • Basaltic composition – Dark silicates and calcium-rich feldspar – Termed mafic (magnesium and ferrum, for iron) in composition – Higher dense than granitic rocks – Comprise the ocean floor and many volcanic islands
- 19. Basaltic lava dropping into the ocean along Kilauea Volcano along the southeastern coast of the big island of Hawaii
- 20. Igneous compositions • Other compositional groups • Intermediate (or andesitic) composition – Contain 25% or more dark silicate minerals – Associated with explosive volcanic activity • Ultramafic composition – Rare composition that is high in magnesium and iron – Composed entirely of ferromagnesian silicates
- 21. Igneous compositions • Silica content as an indicator of composition • Exhibits a considerable range in the crust – 45% to 70% • Silica content influences magma behavior • Granitic magmas = high silica content and viscous • Basaltic magmas = much lower silica content and more fluid-like behavior
- 23. Igneous compositions • Naming igneous rocks – granitic rocks • Granite – Phaneritic – Over 25% quartz, about 65% or more feldspar – Very abundant - often associated with mountain building – The term granite includes a wide range of mineral compositions
- 24. Granite
- 25. Igneous compositions • Naming igneous rocks – granitic rocks • Rhyolite – Extrusive equivalent of granite – May contain glass fragments and vesicles – Aphanitic texture – Less common and less voluminous than granite
- 26. Rhyolite
- 27. Igneous compositions • Naming igneous rocks – granitic rocks • Obsidian – Dark colored – Glassy texture • Pumice – Volcanic – Glassy texture – Frothy appearance with numerous voids
- 28. Pumice is very glassy and sharp, with countless vesicles.
- 29. Igneous compositions • Naming igneous rocks – intermediate rocks • Andesite – Volcanic origin – Aphanitic texture • Diorite – Plutonic equivalent of andesite – Coarse grained
- 30. Andesite
- 31. Diorite
- 32. Igneous compositions • Naming igneous rocks – basaltic rocks • Basalt – Volcanic origin – Aphanitic texture – Composed mainly of pyroxene and calcium-rich plagioclase feldspar – Most common extrusive igneous rock
- 33. Basalt
- 34. Igneous compositions • Naming igneous rocks – mafic rocks • Gabbro – Intrusive equivalent of basalt – Phaneritic texture consisting of pyroxene and calcium-rich plagioclase – Significant % of the oceanic crust
- 35. Gabbro
- 36. Igneous compositions • Naming igneous rocks – pyroclastic rocks • Composed of fragments ejected during a volcanic eruption • Varieties – Tuff = ash-sized fragments – Volcanic breccia = particles larger than ash
- 38. Origin of magma • Highly debated topic • Generating magma from solid rock • Role of heat – Temperature increases in the upper crust (geothermal gradient) average between 20oC to 30oC per kilometer of depth – Rocks in the lower crust and upper mantle are near their melting points – Any additional heat may induce melting
- 39. Origin of magma • Role of pressure – Increases in confining pressure cause an increase in a rock’s melting temperature – When confining pressures drop, decompression melting occurs • Role of volatiles – Volatiles (primarily water) cause rocks to melt at lower temperatures – Important factor where oceanic lithosphere descends into the mantle
- 41. Magma evolves as the hotter minerals crystallize and settle to the bottom of the magma chamber