IGNEOUS ROCKS

1. IGNEOUS ROCKS rock = mixture of minerals, mineraloids, glass, or organic matter bound together in some way 1. magma is parent material for all rocks 2. three major types: igneous, sedimentary, & metamorphic igneous rock = forms when magma cools and hardens

2. Two Categories: classified according to where magma cools and hardens 1. intrusive igneous rock = cooling & solidification of magma deep below crust within Earth a. can intrude and cut across into other rock masses below surface b. also called plutonic rocks c. pluton = intrusive igneous rock mass d. cools slowly → large crystals → called mineral grains→ coarse grained e. most common type [granite most common]

3. 2. extrusive igneous rock = formed when lava cools on Earth’s surface a. also called volcanic rocks b. cool quickly → fine-grained c. often grains are too small to see with naked eye d. if cools within a few hours → no grains → volcanic glass Ex: obsidian, pumice, scoria

4. magma = mixture of molten rock, gases, and mineral crystals 1. composition = oxygen, silicon, aluminum, iron, magnesium, calcium, potassium, & sodium 2. sources of Earth’s thermal energy = remaining energy from Earth’s molten formation & heat generated from decay of radioactive elements

5. 3. factors affecting formation: a. temperature: generally increases with depth in Earth’s crust = geothermal gradient b. pressure: increases with depth in crust 1) due to weight of overlying rock 2) as pressure on rock increases, its melting point also increases c. water content: as amount of water content increases, melting point decreases d. mineral content: different minerals have different melting points

6. Rock Melting/Formation: 1. partial melting = process where some minerals melt at low temperatures while other minerals remain solid a. as each group of minerals melts it is added to the magma b. if temperatures are not high enough to melt all minerals in rock then magma will have different composition from that of the rock

7. 2. fractional crystallization = process where different minerals form at different temperatures a. can change composition of magma b. change due to minerals that crystallize being removed from magma

8. 3. Bowen’s reaction series = shows relationship between cooling magma and crystal formation a. developed by N. L. Bowen (Canadian) [Figure 5-6 p.104 text] b. describes how when magma cools, minerals from in predictable patterns

9. c. first pattern: described by a continuous, gradual change of mineral compositions in feldspar group 1) right branch of reaction series 2) undergo continuous change in composition 3) first to form are rich in calcium → react with magma to form sodium rich 4) if cools too quickly can have zones of calcium and sodium

10. d. second pattern: described by abrupt change of mineral type in iron-magnesium groups 1) left branch of reaction series 2) olivine first to form next to form is pyroxene, etc. e. as series proceeds elements are removed from the magma until silicon and oxygen are left

11. 4. minerals from into distinct bands in order shown in reactions series in some magma bodies a. forms a layered intrusion (Figure 5-9. p.106) b. settling of crystals, flowing currents in magma, temperature gradients within magma chamber all role in formation c. some may have very high concentrations of specific elements [Ex: platinum]

12. Classification: three main groups = felsic, mafic, intermediate composition = determine by chemical composition of magma 1. felsic rocks = light-colored, high silica contents, contain quartz, feldspars orthoclase, & plagioclase a. light coloring 1) main mineral components = quartz, orthoclase, feldspar 2) low density

13. b. rocks include: [granite family] 1) granite = coarse grains 2) rhyolite = fine grains, light gray to pink 3) felsite = fine grains, light-colored 4) obsidian = no grains, dark brown or black, 5) pumice = full of holes, resembles a sponge, light enough to float

14. 2. mafic rocks = dark-colored, lower silica contents, rich in Fe & Mg a. dark coloring 1) main mineral components = plagioclase, biotite, amphibole, pyroxene, & olivine 2) also include ferromagnesian minerals 3) heavy, higher density

15. b. rocks include: [gabbro family] 1) basalt = fine grains, most common, 2) gabbro = coarse grains 3) daibase = grain size between fine & coarse, 4) basalt glass = no grains 5) scoria = full of holes, dense, not float

16. 3. intermediate rock = characteristics of both felsic & mafic a. main mineral components = moderate amounts of plagioclase, hornblende, augite, biotite b. have little or no quartz c. color commonly medium grays or greens d. rocks include: [diorite family] 1) diorite = coarse grains 2) andesite = fine grains

17. 4. ultramafic rocks = low silica content, very high levels of Fe & Mg a. include peridotite & dunite b. usually coarse grained

18. texture = refers to size, shape, and arrangement of mineral grains or crystals 1. intrusive: a. large, well formed crystalline grains b. medium to coarse grain c. can see individual grains of different colors d. core of continental crust = granite

19. 2. extrusive: a. no grains to very fine grains b. grains cannot seen with unaided eye c. make up oceanic crust = basalt d. obsidian (volcanic glass ) =extremely rapid cooling allows no grain to form e. pumice = gases trapped in material form small bubbles leaving holes

20. 3. porphyritic = mixture of large and small crystals a. due to magma cooling slowly then quickly producing both large and small mineral grains b. large crystals appear embedded into smaller grains

21. Ore Deposits: 1. veins = hot mineral solutions spread through many small fractures depositing minerals in bands a. lode = large number of thick mineral veins b. many can be metal-rich quartz veins

22. 2. pegmatites = veins of extremely large-grained minerals a. find ores of lithium & beryllium b. can produce beautiful crystals since minerals grow into voids and retain their shape 3. kimberlites = ultramafic rock that contains diamonds a. form deep in the crust 150 – 300 km below or in the mantle b. form only under very high pressure