Chapter 3 Igneous Rocks

1. Chapter 3 � Igneous Rocks Igneous rocks form as molten rock within or below the crust and then cool. Magma � present below the Earth�s surface. When these cool, they become intrusive (plutonic) igneous rocks. When intrusive igneous rocks are exposed, it is usually either because of erosion or faulting. Lava � erupted on the Earth�s surface. When these cool, they become extrusive or volcanic rocks.

2. Magma can be either completely or partly molten, depending on temperature, pressure, and composition. Most magmas consist of a liquid phase (melt), a solid phase (crystallized silicate minerals), and volatiles (pressurized gases). The liquid phase of a magma consists of randomly moving ions. As temperature decreases, the ionic movement slows, allowing the formation of chemical bonds (crystallization of minerals).

3. The simple silica tetrahedra of olivine form first, followed by the single chain of tetrahedra of pyroxenes.

6. Igneous Rocks are classified by texture and mineral composition. Texture is the size, shape, and arrange-ment of randomly-oriented crystals. Texture is affected by cooling history, silica %, & gases (as fluids because of high pressure. Cooling history (and crystal size) is affected by crustal depth, pressure conditions, & fluid content. Gas content is related to origin of magma.

7. Magma composition is related to plate tec-tonic setting and history of pluton as it rised through the crust. Mafic igneous (Gabbro/Basalt) magmas occur within ocean basins along rift (divergent) zones, hot spots (Hawaii), and where continental crust is thinned. Felsic igneous (Granite/Rhyolite) magmas occur where subsurface igneous activity has partially-melted continental crust. Intermediate igneous (Diorite/Andesite) occur in areas of magmatic mixing.

8. Igneous Associations Phaneritic - Aphanitic Gabbro � basalt Dark silicate minerals & Ca plagioclase feldspars, little or no quartz. Higher temperatures. Diorite � andesite 25+% dark silicates, Na plagioclase feldspars, small % of quartz Granite � rhyolite Quartz and orthoclase (K) feldspar & 10% dark silicates (biotite & amphibolite). Lower temperatures. Silica (quartz) content influences igneous eruptive style. Mafic (dark) magmas/lavas less viscous � flow easily. Felsic (light) magmas/lavas more viscous � �freeze up� � are more explosive.

9. Where new oceanic crust is formed, study is difficult, so we study old ocea- nic crust, called �ophiolites�, preserved as fault slices in areas of Conti- nental collision.

12. Intermediate igneous rocks usually form in association with convergent (sub-duction) zones. Descending oceanic crust & oceanic sediments are partially melted, producing intermediate magmas.

27. More classification details: Granites best known igneous rocks because of abundance, exposure, and natural beauty, when polished. Quarry areas include: Elberton, GA, Barre, VT, Mt. Airy, NC, and St. Cloud, MN.

29. Where Does Molten Rock Originate? Normal temperature increases with depth � Geothermal Gradient. Local variations due to thickness of crust or presence of igneous plutons. Example: Rio Grande Rift - crustal thinning, higher heatflow. Role of Pressure � decrease in confining pressure = lowering of melting temperature, as in Rio Grande Rift example above. Role of Volatiles � Water and other fluids/gases under pressure mobilize ions = lowering of melting temperature. Example: In subduction zone � oceanic crust + oceanic sediments + seawater = more volatiles, lower melting point. Also, lighter minerals (bottom of Bowen�s Reaction Series) melt first = Partial Melting. Causes � heat added; pressure decreases; volatiles increase.

30. How magmas evolve (change). Lab experiments by N.L. Bowen, early 20th century, demonstrated systematic crystallization of minerals in a basaltic melt. Different presentation of Bowen�s Reaction Series on p. 83, Fig. 3.23. Magmatic differentiation � crystal settling. If melt moves to another location, chemistry will be different from parent magma.