1. Igneous Rocks

2. Goals To understand what your “granite” counter top is really made out of and to use that knowledge to make plate tectonic interpretations.

3. Igneous rocks Rocks cooled and crystallized from molten rock Made up of an interlocking network of mineral crystals called grains Grains

4. Igneous rocks Molten rock is called magma—can be either all melt or a “slush” of melt and crystals Magma erupted onto the Earth’s surface called lava

5. Igneous rock compositions Minerals in igneous rocks tell us a lot about the tectonic environment in which rocks formed Calcium plagioclase feldspar Pyroxene Potassium feldspar Quartz

6. Ultramafic igneous rocks Lots of iron and magnesium Very low silica (<45% SiO 2 ) Contain olivine and pyroxene—Often green Make up the Earth’s mantle Peridotite Green is olivine Black is pyroxene This is what the mantle looks like

7. Mafic igneous rocks Lots of iron, magnesium, and calcium Low silica (45–55% SiO 2 ) Contain pyroxene, amphibole, and calcium (black lab) plagioclase—Dark gray to black in color Make up oceanic crust

8. Intermediate igneous rocks Contain iron, calcium, and sodium Medium silica (55–65% SiO 2 ) Contain amphibole, and calcium-sodium (chocolate lab) plagioclase—Salt-and-pepper or medium-gray color Found in the continents

9. Felsic igneous rocks a.k.a. silicic Contain sodium, aluminum, and potassium High silica (>65% SiO 2 ) Contain potassium feldspar, quartz, and sodium (yellow lab) plagioclase—Makes them light colored Make up most of the continents

10. Mafic to Felsic comparison A B C

11. Mafic Felsic (silicic) Intermediate B A C

12. Igneous composition and temperature Bowen’s reaction series: Progression of minerals that can crystallize as temperature decreases

13. Discontinuous: discrete steps between mineral phases. Each one is entirely different Continuous: Plagioclase feldspars can have any amounts of calcium and sodium

14. Fractionation Mafic minerals crystallize first—use up iron and magnesium and increase concentration of aluminum, potassium, and sodium. Both mafic and felsic rocks crystallized from a single melt containing all the major elements. Rock with concentrated pyroxene produced by fractionation

15. Fractional Crystallization

16. Igneous rock textures Texture: Size, shape, and alignment of mineral grains in a rock –Provides information about local environment in which rocks form –Slower cooling = bigger crystal grains

17. Intrusive igneous rocks Cool and crystallize inside the earth –Earth is a great insulator. Intrusive rocks cool slowly –Grow large crystals.

18. Extrusive igneous rocks a.k.a. volcanic Cool and crystallize outside the earth –Water and air remove heat quickly. Extrusive rocks cool rapidly –Grow small crystals or form glass.

19. Intrusive igneous textures Phaneritic texture: coarse-grained –All mineral grains >0.5mm in diameter –Minerals visible to naked eye –Indicates slow cooling

20. Intrusive igneous textures Pegmatitic texture: very coarse-grained –Largest mineral grains >3cm in diameter –Indicates very slow cooling

21. Extrusive igneous textures Aphanitic texture: fine-grained –All mineral grains <0.5mm in diameter –Minerals invisible to naked eye –Indicates rapid cooling

22. Extrusive igneous textures Porphyritic texture: fine and coarse grains –Isolated grains >0.5mm surrounded by grains invisible to the naked eye –Indicates a mixed cooling history

23. Extrusive igneous textures Glassy texture: no mineral grains –Extremely rapid (almost instantaneous) cooling –Looks like glass Arrow head made from volcanic glass

24. Extrusive igneous textures Vesicular texture: bubbly –Gas bubbles trapped in volcanic rock –Like frozen foam from a coke or beer –Indicates explosive eruption and very fast cooling

25. Names of igneous rocks based on composition and texture Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

26. Granite: Felsic, phaneritic (intrusive) Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

27. Names of igneous rocks based on composition and texture Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

28. Diorite: Intermediate phaneritic (intrusive) Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

29. Names of igneous rocks based on composition and texture Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

30. Basalt: Mafic, aphanitic (extrusive) Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

31. Names of igneous rocks based on composition and texture Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic or porphyritic

32. Andesite: Intermediate, aphanitic (extrusive) Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic or porphyritic

33. Names of igneous rocks based on composition and texture Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

34. Rhyolite: Felsic, aphanitic (extrusive) Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

35. Names of igneous rocks based on composition and texture Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

36. Gabbro: Mafic, phaneritic (intrusive) Are these rocks? A.Mafic B.Intermediate C.Felsic Are these rocks? A.Phaneritic B.Aphanitic

37. Peridotite: Ultramafic, phaneritic rock Makes up the Earth’s mantle Crystallized from a melt when the Earth formed Technically metamorphic rocks

38. http://www.monanneeaucollege.com/minerauxphoto/rhyolite-rouge.1.net2.jpg http://academic.brooklyn.cuny.edu/geology/grocha/monument/images/granite.gif http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousCompositions/5Andesite/AndesiteHblPorphDrty.jp http://www.pitt.edu/~cejones/geoimages/2IgneousRocks/IgneousCompositions/4Diorite/DioriteGoreMtnCUp.jpg http://z.about.com/d/geology/1/0/R/K/basalt.jpg http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousCompositions/2Gabbro/GabbroPyroxPlag.jpg http://www.krc.karelia.ru/results/projects/AK.project/s11.jpg http://www.gc.maricopa.edu/earthsci/imagearchive/peridotite.htm Felsic Intermediate Mafic Ultramafic Granite Diorite Gabbro Peridotite Rhyolite Andesite Basalt Aphanitic Phaneritic

39. Intrusive rock bodies Dike: Sheet-like intrusive rock body that cuts across layering in surrounding rocks Mafic dike in the Grand Canyon

40. Intrusive rock bodies Sill: Sheet-like intrusive rock body that follows layering in surrounding rocks

41. Intrusive rock bodies Lacolith: Large mushroom- or dome-shaped intrusive body consisting of a dome-shaped sill and a smaller feeder dike

42. Intrusive rock bodies Batholith: Very large intrusive body with an area of >100 km 2 Sierra Nevada Batholith

43. Igneous rocks and plate tectonics Continental rifts have bimodal volcanism, or, dominated by felsic and mafic igneous rocks

44. Igneous rocks and plate tectonics Oceanic divergent boundaries dominated by mafic igneous rocks

45. Igneous rocks and plate tectonics Oceanic-oceanic convergent boundaries dominated by mafic and intermediate igneous rocks Volcanoes Trench

46. Igneous rocks and plate tectonics Continental-oceanic convergent boundaries dominated by intermediate igneous rocks Mt. Shasta, CA Mt. Rainier, WA