Classification of igneous rocks (texture, composition) 2) Naming of igneous rocks

1. Today: Chapter 5: Igneous rocks Classification of igneous rocks (texture, composition) 2) Naming of igneous rocks 3) Origin of magma 4) Igneous diversity 5) Magma recipes

2. 1) Igneous textures As magma cools, atoms arrange in an orderly crystal structure • crystallization

3. 1) Igneous textures Note: different crystal sizes

4. 1) Igneous textures Close up of crystals • Note the variety of crystal sizes

5. 1) Igneous textures Texture Texture a. size b. shape c. arrangement of interlocking crystals

6. 1) Igneous textures • Factors affecting crystal size • Rate of cooling • Amount of silica (SiO2) present • Amount of dissolved gases

7. 1) Igneous textures cooling rate crystal size Slow cooling larger crystals Fast cooling small or no crystals

8. 1) Igneous compositions • mainly silicate minerals • determined by composition of magma from which it crystallized • magma mainly 8 elements: • Si, O, Al, Ca, Na, K, Mg, Fe Most abundant constituents

9. 1) Igneous compositions • Magma • cools • solidifies • forms silicate minerals Two major silicate mineral groups: LIGHT silicates DARK silicates

10. 1) Igneous compositions Silicate group Poor in Examples Rich in DARK “ferromagnesian” Si olivine pyroxene amphibole biotite Fe and/or Mg Fe and/or Mg Si, K, Na, Ca LIGHT “non- ferromagnesian” quartz muscovite Feldspars (40% of most igneous rocks) See Table 5.1

11. The classification model of igneous rock. 1) Igneous compositions Fig. 5.4, see Table 5.2

12. granite rhyolite See Fig. 5.3 2) Naming igneous rocks zooming in volcanic f e l s i c plutonic

13. See Fig. 5.3 2) Naming igneous rocks • Obsidian • Dark colored • Glassy texture obsidian volcanic f e l s i c • Pumice • Volcanic • Glassy texture • Frothy appearance with numerous voids pumice volcanic

14. 2) Naming igneous rocks Another type of igneous rock • pyroclastic rock from fragments ejected during eruption most common: “tuff” (ash-sized fragments, cemented together called volcanic breccia if it mostly contains fragments larger than ash (rocks from vent wall,crystals, glass fragments…)

15. pyroclastic rockAsh and pumice layers 2) Naming igneous rocks Common in western US which had high volcanic Activity millions of years Ago

16. See Fig. 5.3 2) Naming igneous rocks zooming in andesite volcanic i n t e r m e d i a t e plutonic diorite

17. See Fig. 5.3 2) Naming igneous rocks zooming in basalt volcanic m a f i c gabbro plutonic

18. 3) Origin of magma Recall igneousrock = cooled & solidifiedmagma or lava magma= from melting solid rock in crust and/or upper mantle …but how do we melt rock???

19. composition • pressure • water content Depends on: 1st, let’s define “melting temperature” = the temperature at which something melts

20. 5 9 oC = (oF - 32) Temperature scale digression… Celsius (centigrade) scale Fahrenheit scale Water boils 100oC 212oF Water freezes 0oC 32oF

21. Melting temperaturedecreases with increasing water content. LIQUID LIQUID Melting temperature curve Melting temperature curve Temp. SOLID SOLID Pressure Melting temperatureincreases at higher pressure.

22. 3 ways to melt rock Inside Earth examples: Rocks in the lower crust and upper mantle are near their melting points. Rising magma heats rocks further Rock ascends to shallower depth in convective flow. When confining pressures drop, decompression melting occurs without additional heat Water percolates up from subducting oceanic lithosphere into overlying mantle rocks • raise the temperature • lower the pressure • add volatiles (water) See Fig. 5.15

23. Hot mantle rock ascends and moves into zones of lower pressure. This drop in confining pressure may trigger melting. Decompression melting Ascend rates of magma are 0.3 to 50 m/yr. Magma chambers (several km3 big) form in the cavities of the lithosphere as magma rises.

24. Effect of water in melting mantle rock: Water vapor mixed with molten sedimentary rocks from subducting plate rise and lower the melting temperature of mantle and lithospheric rock. = fluid-induced melting 5.15

25. New… 4) Igneous diversity Mechanisms to account for the great diversity of igneous rock magmatic differentiation assimilation magma mixing

26. 4) Igneous diversity Mechanisms to account for the great diversity of igneous rock magmatic differentiation In a magma body: • crystals form as magma cools • not all at the same time! = fractional crystallization • heavy crystals sink to bottom =crystal settling • more than 1 type of magma from same parent • variety of igneous rocks from same source See Figure story 5.5

27. 4) Igneous diversity assimilation (see Fig 5.6) As magma migrates upward: • may incorporate surrounding host rock • In deeper layers magma may melt surrounding host rock magma mixing If magma body collides into another: • magmas can mix

28. 4) Igneous diversity Mechanisms to account for the great diversity of igneous rock magmatic differentiation assimilation magma mixing Mixing of two magmas=andesitic magma Fig. 5.6

29. 5. Magma recipes 3 main types Partial melt of: How/ process: “mafic” buoyant rock ascends/reduction in pressure, large outpourings mantle rock: peridotite • basaltic • andesitic • granitic basalt ascends, melts/assimilates crustal rocks Mantle peridotites + felsic crustal rocks Basalt heats & melts crust from below (assoc. w/ subduction) It is more viscous And stays subsurface. Si-rich continental crust rocks “felsic” “felsic”