EASC 116 Ch. 3 Igneous Rx
- 1. CH. 3 IGNEOUS ROCKS
- 2. Igneous Rocks Ignis = fire Form from cooling & crystallization of magma (molten rock) Lava = molten rock at surface
- 3. Igneous Rocks Classified by texture & composition Texture = crystal size of minerals Composition = minerals present
- 4. Changes of State of Matter Add heat (melting): Rx Magma (atoms faster) Remove heat (crystallize): Rx (atoms slower) Magma
- 5. Texture of Igneous Rocks Depends on magmas cooling rate 1) Cools slowly Atoms can move through liquid - crystals grow (large, small)
- 6. Magmas Cooling Rate 2) Cools quickly Forms a solid faster than atoms can move around - crystals are (large, small)
- 7. Texture (Crystal Size) Two environments where magma cools: 1) At surface or shallow burial 2) Deep underground
- 8. Types of Texture 1) Aphanitic - crystals invisible to naked eye - magma cooled ___________ - extrusive/volcanic
- 9. Types of Texture 2) Vesicular - aphanitic rock w/many holes (vesicles) in it - gas trapped as magma cools very quick
- 10. Types of Texture 3) Phaneritic - crystals easily visible - magma cooled ___________ - intrusive/plutonic
- 11. Types of Texture 4) Porphyritic - two size crystals in same rock Large crystals = phenocrysts Small crystals = groundmass
- 12. Porphyritic Texture 2-stage cooling process: 1st stage = slow cooling at depth _______________ are formed 2nd stage = rapid cooling of remaining magma _______________ are formed
- 13. Types of Texture 5) Glassy - no crystalline structure Ex: Obsidian; pumice
- 14. Types of Texture 6) Pegmatic All minerals larger than 1 cm Forms during later stage of crystallization - large percentage of liquid allows ions to migrate easier
- 15. Types of Texture 7) Pyroclastic pyro = fire; clastic = pieces Consolidated rock fragments ejected during violent volcanic eruptions
- 16. Pyroclastic texture Fragments can be volcanic ash, blobs of magma, or angular blocks - not composed of interlocking crystals!
- 17. Igneous Rock Composition The most abundant elements of Earths crust are _____ & ______ Magma is mostly silica with varying amounts of other six elements
- 18. Composition Other elements collect in water at top of magma chamber Creates hydrothermal solution - cools in fractures as highly concentrated ore deposits
- 19. Igneous Compositions Mafic - 45-50% silica (silica-poor) - rich in magnesium, iron (Fe), & calcium - dark-colored: black, dark gray, brownish-red
- 20. Igneous Compositions Felsic (feldspar & silica) - 60-75% silica (silica rich) - rich in aluminum, potassium, & sodium - light-colored: light gray, pinkish
- 21. Igneous Compositions Intermediate Aphanitic: medium gray Phaneritic: black & white crystals
- 22. Importance of Igneous Rocks Compose 90% of Earths crust Two types of crust: oceanic crust = basalt continental crust = granite
- 23. Question: Why different igneous rock compositions if only 8 elements? N.L. Bowen experimented with mafic magmas
- 24. Results: ** Same sequence of minerals crystallized from magma Referred to as Bowens Reaction Series (Fig. 3.16)
- 25. Bowens Reaction Series Discontinuous series (left) vs. continuous series (right) Discontinuous series: - each mineral has a unique silicate structure
- 26. Continuous series Different compositions of plagioclase feldspars - ions of Ca2+ and Na+ can interchange within crystalline structure
- 27. Bowens Reaction Series As magma crystallizes: First mineral to form = mafic Composition of remaining magma changes
- 28. Bowens Reaction Series Biotite uses last mafic elements in magma - remaining magma becomes more silica rich
- 29. Bowens Reaction Series Minerals that form in same temp. conditions are found together in same igneous rock QUESTION: Which mineral would you expect to form phenocrysts in a porphyritic basalt?
- 30. Phenocryst Mineral: a) Quartz b) Potassium feldspar c) Biotite d) Calcium-rich plagioclase f-spar e) Sodium-rich plagioclase f-spar
- 31. Felsic magma Felsic minerals melt at cooler temps - magma not as hot as mafic magma - felsic magma not as buoyant
- 32. Felsic magma Magma doesnt reach surface Ex: Continental crust = granite
- 33. Important Concepts A mineral is stable if the environmental conditions are similar to where it formed Ex: temperature, pressure, water- content
- 34. Example: Mafic minerals are stable in ____ temperatures Felsic minerals are stable in ____ temperatures
- 35. Important Concepts If a mineral is put under different environmental conditions, it becomes unstable - mineral has to react to new environment by changing
- 36. New Environmental Conditions: If at the surface, weathering occurs If within the crust, metamorphism occurs