G-100 Lectures 6 & 7 Introduction to Rocks Focus on: Igneous Rocks 1 What are rocks? 2 Three Rock Types 3 4 Igneous rocks 5 Sedimentary rocks 6 Metamorphic rocks 7 The Rock Cycle rifting rifting continues subsidence divergent margin ocean spreading sedimentation sedimentation magmatic activity erosion, streams ocean basin closes subduction of ocean crust sedimentation continental collision magma forms sedimentary basins mountains → metamorphism volcanoes 8 • Classification of Igneous Rocks –Texture – Mineral & Chemical Composition 9 MAGMA • Molten Rock – with dissolved gas: H2O + CO2 • Generated at depth • Eruption if magma reaches surface • If doesn’t reach surface, solidifies underground –Intrudes country rock –Intrusive contact 10 11 12 How does magma form? • Partial melting of rock at depth • Source of heat? – Geothermal gradient • Rate of Temperature increase with depth • Average 30 oC/km depth • Not same everywhere • Tells us that heat is being conducted to the surface of the earth from the interior 13 How does magma form? • Factors that control melting temperatures –Pressure • melting points of minerals increase with pressure – This is why increasing temperature along the geotherm alone fails to melt crustal rocks • reduction in pressure can therefore induce melting –Water added under pressure • lowers melting point of minerals 14 Where does all that basalt come from, dad? ?? 15 How does magma evolve? • Magmatic differentiation –Mirror image of partial melting • Crystal formation over a range in temperature – First to melt are also first to crystallize • Fractional crystallization – As crystallization proceeds, crystals are separated from the melt, thus the melt evolves away from the composition of the crystals removed • Bowen’s reaction series, a model of crystallization: 16 How does magma evolve? 17 Palisades Sill, New Jersey nature, matched by experiments From: Paul Olsen web pages LDEO Sandstone Basalt Mostly sodium-rich plagioclase feldspar; no olivine Calcium-rich plagioclase feldspar and pyroxene; no olivine Olivine Basaltic intrusion ft) (800–900 m 245–275 Basalt Sandstone 18 Palisades Sill, New Jersey nature, matched by experiments Palisades intrusion ~1200°C ~600°C Plagioclase Plagioclase Olivine Plagioclase feldspar crystals feldspar feldspar Pyroxene Pyroxene Magma with Olivine Olivine composition C Magma with Magma with Magma with composition A composition B composition C Olivine Pyroxene and A gradient of Plagioclase crystallizes plagioclase pyroxene and feldspar first. feldspar feldspar is continues to crystallize. established. crystallize. 19 How does magma evolve? Magma chamber A rhyolite Mixing of the basaltic and rhyolitic magmas results Magma in andesitic magma. chamber B basalt Basaltic inclusions in granite, Adamello batholith, Italy Basaltic Qcf 3 host inclusions dacite (grey) in glassy dacite lava flow (black), Quenched San Pedro inclusions Volcano, Chilean Andes 20 Lava flow Ash falls and pyroclasts Country Volcano Volcanic neck with rock radiating dikes Stock ee kk Sill Di Di ee kk ii Dikes cut DD Sill across layers ee k of country k ii Sill DD rock… ton Pllu h lliith tho …but sills are Bat B parallel to them. Batholiths are the largest forms of plutons, covering at least 100 km2. 21 A sill parallels country rock layers. Sill A dike cuts across layers. Dike 22 Chalten (Cerro Fitzroy) pluton Patagonian Batholith Southern Andes, Argentina A dike cuts across layers. 23 Granite Cordilleran Batholiths of the North and Batholiths South American North and Continental Margins South America El Chalten from Winter (2001) 24 Tectonic/Dynamic setting of magmatism 25 Island arc subduction zone ISLAND ARC PLATE SUBDUCTION Mafic to intermediate intrusives (plutonism) Mafic to intermediate Island arc extrusives (volcanism) volcano Oceanic lithosphere 26 Divergent plate boundary-spreading center PLATE DIVERGENCE Basaltic extrusives Mid-ocean ridge Basaltic intrusives Partial melting Of upper mantle Rising magma 27 Hot-spot volcanism-upwelling mantle plume HOT-SPOT VOLCANISM Basaltic extrusives Hot-spot volcano Basaltic intrusives Mantle plume (hot spot) Mantle 28 Decompression melting at mid ocean ridge A thin dike intrudes Forming sheeted dikes, Occasionally erupts, spilling lava in “pillows.” Pillow lava Newer, thinner sediments Older, thicker sediments Sediments are deposited on the spreading Sheeted dikes Sheeted dikes seafloor. Oceanic inin basaltbasalt crust Gabbro Moho Peridotite layer Mantle Spreading Hot mantle rises, center decompresses, and melts. 29 Fluid induced melting in subduction zone Andesitic magma erupts, forms arc volcanoes Trench Magma ascends, interacts with Oceanic sediments lithosphere, Magma chamber possibly Oceanic crust basalt mixing with Oceanic mantle existing lithosphere magma The water Asthenosphere and molten sediments melt parts of the Subducting oceanic crust overlying plate. carries sediments with it. H O H O 2 H O 2 …causing the 2 sedimentary Water remains trapped rocks to melt as the pressure and at lower temperature increase. temperatures. Sediment grains The trapped water is Water released as the temperature increases,… 30.