III. Igneous Rocks Minerals Crystallized from Melts 1. The Rock Cycle 2. Formation of Igneous Rocks 3. Classification of Igneous Rocks 4. Igneous Rocks and Plate Tectonics

The Rock Cycle Igneous Rock Solidification Magma Partial Melting Mantle Rock Fig 3.1 Geological Materials Transformation Processes See Kehew, Fig. 1-6

Partial Melting and Magma E.g., partial melting results in Silica enrichment Partial Melting  Magma Enriched: O, Si, Al, Na, K, Depleted: Ca, Fe, Mg Poor in: O, Si, Al, Na, K, (<50%) Rich in: Ca, Fe, Mg (>50% wt.) See Kehew, Fig 3-6

Formation of Magma How are rocks melted? 1. Heating ■ 2. Depressurization 3. Increase water content 4. Increased silica content Where do rocks melt? Subduction zones (Silicic and Intermediate) Mantle Plumes (“Hot Spots”) not only at Divergent Boundaries Mafic See Kehew, Fig 3-6 Hot and High Pressure Hot and Low Pressure See Kehew Fig. 3-8

Solidification of Melts Magma, Intruded or Extruded, Solidifies (crystallizes) to form Intrusive or Extrusive Igneous Rocks Fig 3.12, 3.13 See Kehew, Fig 3-6

Intrusive vs. Extrusive (Silicic Rocks) Silicic (a.k.a, felsic) Magmas Cool (<700 o C) Viscous (sticky, doesn’t flow easily) Gaseous (steam of H 2 O and C0 2 ) Silicic Rocks Usually intrusive, course-grained, Silicic (Granite) to Intermediate (Diorite) rock forms plutons If extrusive, fine-grained rocks formed by explosive volcanoes Rhyolite or Andesite Volcanoes  Also injects surrounding rocks with silica laden steam See Kehew, Fig 3-40

Intrusive Igneous Rock E.g., Granite (Silicic, Phaneritic): Crystallized (Solidified) Silicic Magma Poor in: Fe, Mg, Ca, (<20%) Rich in: Silica (>70%) Quartz Na Plagioclase Biotite

Composite Volcanoes Silicic magma High-viscosity Gaseous Explosive eruptions Steep sloped, cone shaped volcanoes

Extrusive vs. Intrusive (Mafic Rocks) Mafic Magmas Hot (>1000 o C) Non-Viscous (runny, flows easily) “Dry” (no H 2 O or C0 2 ) Mafic Rocks Usually Extrusive, Fine-grained, Mafic (Basalt) rock forms oceanic crust, Shield Volcanoes and Basalt Floods If Intrusive, course-grained mafic rocks are formed Gabbro. If intrusive, Dikes and Sills more common See Kehew, Fig 3-40

E.g., Mafic Volcanism, Hawaii Mantle Hot Spot Volcano See Kehew, 3-8

Hawaii Midway Current Plate Motion Hawaii A long chain of inactive volcanoes Island ages, millions of years

Shield Volcanoes Mafic Magma Low-viscosity Non-explosive eruptions Gentle slopes Covering large areas

Types of Eruptions (Mafic Volc.) Types of Eruptions Lava floods Lava fountains Fissure eruptions Rock Textures (Table 3-1) Aphanitic Porphyritic Vesicular Glassy (Obsidian)

Surface Textures (Extrusive, Mafic Rocks) Ahah (Rubbley) Mostly solid when flowing Pahoehoe (Ropey) mostly liquid when flowing) See Kehew, Fig 3-9, 3-10

Volcanic Hazards Relative Hazard (see Kehew, Fig. 3-30, 31) based on Type of volcano Active Vents Earthquakes Calderas Historical Flows Rift Zones Slopes

Mafic Sill: Intruded between layers Mafic magma is less viscous and hotter so Does not form plutons but Cuts along layers (Sills) or even across layers (Dikes) Also Baked Zones of adjacent country rock and Chill Zones within the intrusion

Igneous Rock Classification Intrusive (Plutonic) Extrusive (Volcanic) See Kehew, Fig. 3-5 Mineral Percentage Continental Crust Oceanic Mantle Crust

Igneous Rocks and Silicates The Igneous Rock Classification parallels the systematic Silicate Mineral Classification Frame- Double Single Iso. work Sheet Chain Chain Fig. 3-6 Mineral Percentage

Igneous Rock Classification Silicic Intermediate Mafic Granite Diorite Gabbro Rhyolite Andesite Basalt (Porphyritic) Intrusive Extrusive

1200 o C 1000 o Bowen’s Reaction Series Two series of minerals formed during crystallization of magma Intrus. Extrus. Gabbro Basalt Diorite Andesite Granite Rhyolite Low Silica Magma High Silica Magma Frame- Double Single Isolated work Sheet Chain Chain Temperature of Crystallization 750 o

Crystallization of Gabbro and Basalt If melt is extracted Ultramafic Rock will be formed Mafic Melt ~1,450 o C ~1,400 o ~1,350 o The remaining melt is enriched in silica (Int-Felsic) Ca-Feldspar Olivine Olivine converts Forms Forms to Pyroxene ~1,300 o ~1,250 o Gabbro Basalt Remaining silica crystallizes into Ca-Feldspar. Intrusive Extrusive Cooling: Slowly Quickly

Crystallization of Diorite and Andesite If cooled slowly in the magma chamber then extruded, Andesite Porphyry will be formed ~1,400 o C ~1,300 o ~1,200 o Ca Feldspar Olivine converts Ca Feldspar Forms to Pyroxene absorbs Na ~1,100 o ~1,000 o Diorite Andesite Pyroxene converts Remaining silica Intrusive Extrusive to Amphibole forms Na-Ca Fldspr. Slowly Cooled Quickly Cooled Intermediate Melt

Crystallization of Granite and Rhyolite Silicic Melt ~1,200 o C ~1,100 o ~1,000 o Na-Feldspar Pyroxene Pyroxene dissolves forms and grows forms to form Amphibole ~900 o ~750 o Granite Rhyolite Amph. dissolves Remaining silica Intrusive Extrusive to form Biotite forms Quartz Slowly Cooled Quickly Cooled

Crystallization of Granite and Rhyolite