Understanding Igneous Rocks: Formation, Classification, and Key Characteristics

1. Igneous Rocks • Magma – molten rock below the Earth’s surface • Granite • Diorite • Gabbro • Lava – molten rock extruded on/at the Earth’s surface • Rhyolite • Andesite • Basalt

2. Igneous Rocks • Make up some of the continents oldest rocks • Zoroaster Granite in the Grand Canyon’s Inner Gorge • Compose or underlie all of the Ocean floors • Mid-Ocean ridges • Origin of continents • First continents must have appeared as some extrusive/intrusive complex

3. Melting Rocks and Crystallizing Magma • Melting preexisting rocks • Various minerals have different melting temperatures • Composition of melt/liquid changes as new elements are introduced through melting process • Crystallizing magmas • Reverse the melting process • High temp minerals begin to crystallize • Composition of melt changes as elements are extracted through crystallization • Mineral crystals grow as cooling continues and eventually form interlocking crystal structure • Size of crystals is directly related to pace of cooling • Fast cooling small crystals • Slow cooling large crystals

4. Creating Magma • Partial melting • Ice – single mineral melts all at once • Rocks – several minerals that melt at different temps • Heat • Geothermal Gradient • Temperature increases with depth • 50-250km are the depths at which rocks begin to melt • These temperatures are found in the lower crust upper mantle • Radioactive decay • Residual heat from formation of planet • Friction at plate boundaries • Pressure • > depth > pressure • Increases melting temperature • Fluids • Mostly water • Decreases melting temperature • Important at plate boundaries

5. Bowens Reaction Series • The sequence at which silicate minerals crystallizes as a magma cools • High temp – olivine and Ca-plagioclase • Low temp – quartz and K-feldspar • Continuous and discontinuous sides • Cont = plagioclase series • Ca-rich plag - hi temp • Na-rich plag – low temp • Discont = discrete minerals assemblages

6. Bowens Reaction Series • Continuous vs discontinuous sides • Simple compositional diagram • To 1st order : where magmas evolve

7. Igneous Textures • Intrusive – Plutonic • Phaneritic – large crystals, easily seen and identified • Cooled slowly underground • Pegmatities – very large crystals, low temp, high water content, often associated with economic deposits • Extrusive – Volcanic • Aphanitic – small crystals, hard to identify without magnification • Cooled quickly, usually above ground • Volcanic glass – obsidian, pumice

8. Igneous Compositions • Felsic – light in color • >65% silica • High viscosity • Al, K, Na • Rhyolites (Granites) • Intermediate – usually light in color • 55-65% silica • Medium viscosity • Al, Ca, Na, Fe, Mg • Andesite (Diorites)

9. Igneous Compositions • Mafic – dark in color • 45-55% silica • Low viscosity • Al, Ca, Fe, Mg • Basalt (Gabbro) • Ultramafic – dark in color • <40% silica • Very low viscosity • Komatiite (Peridotite)

10. Viscosity of Magmas • Viscosity – measurement of a fluids resistance to flow • Increasing silica content = increasing viscosity (direct correlation) • Decreasing temperature = increasing viscosity (inverse correlation) • Magma rises because • Less dense / hotter than surrounding rock • Assimilation & stoping • Gas content expands as it rises fracturing country rock • Surrounding pressures squeeze it upwards

11. Basalt (Gabbro) • Most abundant rock in Earth’s crust • Mafic (dark) • Low silica content (45-55%) • Low viscosity • High temp / high pressure • Pyroxene, Ca-feldspar, sometimes olivine (no quartz) • Most abundant rock in the Earth’s crust, dense • Oceanic plates, Hawaiian Islands, Northwest U.S. (Columbia Plateau), San Francisco Peaks and surrounding volcanic field (cinder cones & flows – Sunset Crater) • Effusive (quiet) eruptions

12. Andesite (Diorite) • Intermediate (salt & pepper colored) • Moderate silica content (55-65%) • Sometimes difficult to distinguish between basalt • Intermediate viscosity • Moderate temp / moderate pressure • Pyroxene, amphibole, intermediate-plagioclase, mica • Second most abundant rock at Earth’s surface • “Typical” volcanic cone (composite cone) • Andes Mtns., Mt. Fuji, Lassen Peak, Mt. Rainier, Mt. St. Helens, Mt. Kilimanjaro, Mt. Etna • San Francisco Peaks • Flows and explosive events

13. Rhyolite (Granite) • Felsic (light colored) • High silica content (65% or more) • High viscosity • Low temp / low pressure • Intrusive much more common than extrusive • K-feldspars, quartz, Na-feldspars, sometimes micas • Mt. Elden (intrusive dacite, endogenous/exogenous dome) • Super continental pyroclastic eruptions • Yellowstone • Long Valley Caldera • Mt. Mazama/Crater Lake • Mt. Toba • Sierra Nevada batholiths • Prescott – Granite Mtn. and Dells • Inner Gorge Grand Canyon – Zorosater Granite • Explosive eruptions or thick, quiet, slow eruptions

14. Plate TectonicsOrigin of Basalts/Gabbros • Mid-ocean ridges • divergent plate boundaries • creation of new ocean crust • Gabbros overlain by basalts overlain by sediments • Ocean island hot-spots • Hawaii, Iceland • Some continental hot-spots • Varied composition due to assimilation of surrounding country rock • SF Volcanic Field, Cima Volcanic Field, Columbia Plateau

15. Plate Tectonics Origin of Andesites/Diorites • Subduction zones • Plate collision boundary where one plate overrides another • Pacific Ring of Fire • Borders Pacific Ocean • Mostly subduction zones • High rate of volcanism and seismic activity • Produced by partial melting of the upper mantle • Inclusion of water and felsic material from the subducting plate • Assimilation of felsic material from surrounding country rock and magma rises

16. Plate TectonicsOrigin of Rhyolites/Granites • Nearly all occur on continents • Originate from partial melting of crust • Most appear at or near subduction zones • Very viscous • Rise slowly • Generally cool before eruption • Hence larger population of intrusive than extrusive • Produce many economic deposits • Hydrothermal deposits near plutons

17. Shield VolcanoMauna Loa

18. Basalt FlowMafic/low silica/low viscosity

19. Lava FountainMafic/low silica/low viscosity

20. Aa LavaMafic/low silica/low viscosity

21. Pahoehoe LavaMafic/low silica/low viscosity

22. Spatter coneMafic/low silica/low viscosity

23. Pillow LavaMafic/low silica/low viscosity

24. Andesite Mt. St. HelensMafic/mod silica/mod viscosity

25. Mt. St. Helens BulgeMafic/mod silica/mod viscosity

26. Mt. St. Helens PyroclasticMafic/mod silica/mod viscosity

27. Mt. St. Helens afterMafic/mod silica/mod viscosity

28. Mt. St. Helens domeFelsic/hi silica/hi viscosity

29. Alaskan DomeFelsic/hi silica/hi viscosity

30. Mt. St. Helens pumice flowFelsic/hi silica/hi viscosity

31. Caldera Eruption, felsic, Mt. Toba, Indonesia • Eruption occurred ~67,500-75,500ya • Latest of 3 caldera-forming eruptions (700,000 and 840,000ya) • Volcanic Explosivity Index of 8 (mega-colossal) • possibly the largest explosive volcanic eruption in the last 25 my • Material erupted : • ~ 2,800 km3 (670 cu mi) • 2,000 km3 (480 cu mi) of ignimbrite that flowed over the ground • 800 km3 (190 cu mi) that fell as ash • The pyroclastic flows of the eruption destroyed an area of 20,000 square km (7,722 sq mi), with ash deposits as thick as 600m (1,969 ft) by the main vent • Deposited an ash layer ~15 cm (5.9 in) thick over South Asia • in central India, Toba ash layer up to 6 m (20 ft) thick • parts of Malaysia were covered with 9 m (30 ft) of ash • 10,000 million metric tons of sulphuric acid or 6,000 million tons of sulphur dioxide were ejected into the atmosphere by the event

32. TephraFelsic/hi silica/hi viscosity

33. Volcanic necks(Ship Rock/Kayenta)