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The Role of Plate Tectonics in Earth Sciences Dirk Spengler Department of Geology and Mineralogy Kyoto University 1 The Role of Plate Tectonics in Earth Sciences Outline 2 1 What are Earth Sciences? 2 What is Plate Tectonics? 3 Ocean Floor and Continents Under Pressure 4 ’Listen’ to the Rocks The Role of Plate Tectonics in Earth Sciences What are Earth Sciences? Earth Sciences ... ... are all type of science related to the planet Earth ... investigate the Earth’s: Structure Composition Deformation processes Climate Life Environmental preservation studies 3 The Role of Plate Tectonics in Earth Sciences What are Earth Sciences? Earth Sciences ... Disciplines Geography (surface processes, human interactions) Geology (rocks) Geophysics (Earth’s interior) Geodesy (size, shape, position) Usage of Physics Chemistry Biology Mathematics Chronology (dating) To understand the Earth’s Past Present Future 4 The Role of Plate Tectonics in Earth Sciences What are Earth Sciences? Earth’s Evolution Sun ? Solar nebular Proto planets Future (0.5 Ga?) 4.54 Ga ago (4.540.000.000) Today Solar nebular→planets Early Earth = hot, homogeneous No tectonic plates Earth = cooled surface, chemically & physically layered Active plate tectonics Takahashi, 1990 Earth = frozen End of plate tectonics Continuous heat loss 5 The Role of Plate Tectonics in Earth Sciences What is Plate Tectonics? Historical View of the Earth (Fixism) Egyption idea (∼5000 years ago) Greek idea (∼2200 years ago) Modern proof (1519-1522) Plate shape Sphere shape Ferdinand Magellan Continents are fixed 6 The Role of Plate Tectonics in Earth Sciences What is Plate Tectonics? Continental Drift Theory (Mobilism) Continental shape Antonio Snider-Pellegrini (1858) 7 The Role of Plate Tectonics in Earth Sciences What is Plate Tectonics? Continental Drift Theory (Mobilism) Flora & Fauna distribution Alfred Wegener (1915) Pangaea (’Old Land’) Distribution of fossils across southern Pangaea Driving force for continental drift? 8 The Role of Plate Tectonics in Earth Sciences What is Plate Tectonics? Ocean Floor Mapping (1950’s and 1960’s) World ocean floor New findings Mid ocean ridges Magmatism Magnetic anomalies Sediment thickness varies systematically Deep trenches along some continents Renewal of the improbable concept of continental drift 9 The Role of Plate Tectonics in Earth Sciences What is Plate Tectonics? Seafloor Spreading Formation of Ocean Floor and Passive Continental Margins Hess and Dietz (1960/62) Theory Vine and Matthews (1963) Proof of Theory Pattern of magnetic stripes at midAtlantic ridge Rising magma forms new ocean floor = Proof of continental drift → Global volume problem: where does the ocean floor go? 10 The Role of Plate Tectonics in Earth Sciences What is Plate Tectonics? Inclined Plane of Seismicity Kiyoo Wadati (1935) Wadati-Benioff-Zone Plunging of Ocean Floor = Subduction Zone (Active Plate Margin) 11 The Role of Plate Tectonics in Earth Sciences What is Plate Tectonics? Plate Tectonic Model = Continental Drift + Seafloor Spreading + Subduction Zones Global seismic activity Tectonic plates The Earth outer shell is broken into large plates. Size and position of these plates change over time. Earthquakes and volcanoes help locating plate boundaries. Divergent plate margins form plates, convergent plate margins destroy plates (constant surface area of the Earth). 12 The Role of Plate Tectonics in Earth Sciences What is Plate Tectonics? Plate Tectonic Model Mantle convection Synthesis (End of 1960’s) Driving mechanisms: 3 major types of plate boundaries (transform, divergent, convergent) Widely accepted since 1970’s Does not explain driving mechanism → Mantle convection model 13 High density of cooled ocean floor (slab pull) Low density of hot soft mantle (asthenosphere) The Role of Plate Tectonics in Earth Sciences What is Plate Tectonics? Consequences of Plate Tectonics Ocean floor age Supercontinents ≤180 million years Pangaea 300 – 180 Ma Rodinia 1100 – 750 Ma Columbia 1800 – 1500 Ma Periodical opening and closing of ocean basins (Wilson Cycle) Ocean floor is recycled, continents are stable 14 The Role of Plate Tectonics in Earth Sciences Ocean Floor and Continents Under Pressure Convergent Plate Boundaries 15 Ocean–Ocean Ocean–Continent Continent–Continent (Japan) (Andes) (Himalaya) Subduction of (heavy) oceanic plate Subduction of (light) continental plate What happens with ocean floor under pressure? What’s about continents under pressure? The Role of Plate Tectonics in Earth Sciences Ocean Floor and Continents Under Pressure Subduction of Oceanic Crust Seafloor composition 0.5 km Sediments 1.5 km Pillow lava (Basalt) ➀ 5.0 km Gabbro ➁ Seafloor magmatic rocks Crystallization from a melt Lava at seafloor Lava at land surface 3 minerals: feldspar (white) pyroxene (black) olivine (green) Regular distributed Gabbro Every rock consists of minerals. 16 The Role of Plate Tectonics in Earth Sciences Ocean Floor and Continents Under Pressure Subduction of Oceanic Crust Metamorphic rocks Metamorphic facies Change in minerals and/or texture ➀➁ (Photo: B. Hacker) Blueschist ➂ glaucophane (blue) feldspar (white) garnet (red) garnet (red) omphacite (green) zoisite, quartz (white) Subduction transforms magmatic into metamorphic rocks Stability of minerals depends on P & T → Mineral assemblage tells the history of a rock 17 ➂ ➃ Eclogite ➃ Groups of minerals that occur together ➀➁ ➂ ➃ The Role of Plate Tectonics in Earth Sciences Ocean Floor and Continents Under Pressure Subduction of Continental Crust Continent–Continent Known Sampling of mantle rocks Continental and oceanic lithosphere break apart Subduction of light continental crust is temporarily → exhumation 0 km ➅ ➄ 400 Less known Max depth of continental subduction Timing 3 possible scenarios for subduction / exhumation Suitable study objects High grade metamorphic gneiss ➄ Enclosed mantle fragments ➅ 18 The Role of Plate Tectonics in Earth Sciences Ocean Floor and Continents Under Pressure Subduction of Continental Crust High grade metamorphic rocks Index minerals 20 µm Mineral microstructures Since 1984: Since 1998: Coesite (>70 km) Majoritic garnet (>180 km, f (T)) Massonne (2001) Since 1990: Diamond (>120 km) 10 µm Korzakov & Hermannn (2006) Today: subduction / exhumation probably max. 200 km depth 19 The Role of Plate Tectonics in Earth Sciences ’Listen’ to the Rocks How do we get to know all this? As a Geologist Motivation: without interest nothing gets discovered Observation In the field (excursions) In the laboratory (analysis of natural / artificial rocks) Comparing data of natural and experimental rocks Discussions: different people have different ideas Testing ideas 20 The Role of Plate Tectonics in Earth Sciences ’Listen’ to the Rocks How does it look in practise? Geological Field Work British GS mapping in Scotland (1902) 21 MSc Student maps eclogite, Norway (2002) The Role of Plate Tectonics in Earth Sciences ’Listen’ to the Rocks How does it look in practise? Geological Field Work Taking samples Taking directions 22 Taking scale Taking notes The Role of Plate Tectonics in Earth Sciences ’Listen’ to the Rocks How does it look in practise? Analyses Light microscopy Electron microscopy Laser ablation ICP-MS Relationship between minerals Crystal orientations Mineral trace elements 23 The Role of Plate Tectonics in Earth Sciences ’Listen’ to the Rocks Thanks for listening! 24 The Role of Plate Tectonics in Earth Sciences