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Geodynamics Kinematics of plate tectonics Lecture 2.1 - Divergent plate boundaries Lecturer: David Whipp [email protected] Geodynamics www.helsinki.fi/yliopisto 1 Goal of this mini-lecture • Present divergent (accreting) plate margins 2 Plate boundaries • Three different principal types of plate boundaries • • • Divergent (accreting) Convergent (subduction) Transform 3 Plate boundaries • Three different principal types of plate boundaries • • • Divergent (accreting) Convergent (subduction) Transform 4 Plate boundaries - Divergent (accreting) • Fig. 1.4, Turcotte and Schubert, 2014 Plate boundary where new plates form and with divergent motion 5 Plate boundaries - Divergent (accreting) • • Fig. 1.4, Turcotte and Schubert, 2014 Plate boundary where new plates form and with divergent motion Thermal buoyancy produces topographic highs near spreading ridges, which decrease away from the ridge 5 Plate boundaries - Divergent (accreting) Fig. 1.4, Turcotte and Schubert, 2014 • • Plate boundary where new plates form and with divergent motion • Typical spreading rates: 4 cm per year Thermal buoyancy produces topographic highs near spreading ridges, which decrease away from the ridge 5 Plate boundaries - Divergent (accreting) • Plate accretion occurs via decompression melting, where the nearly isothermal asthenosphere ascends beneath a spreading center and crosses the solidus for the basaltic component • Assume a simple linear solidus T (K) = 1500 + 0.12p (in MPa) Fig. 1.5, Turcotte and Schubert, 2014 • where 𝑇(𝐾) is the temperature in Kelvins and 𝑝 is pressure in MPa If we assume the density of the rising asthenosphere is 𝜌 = 3300 kg m-3, 𝑔 = 10 m s-2 and constant temperature, at what depth 𝑦 does the asthenosphere begin to melt? Note: Pressure 𝑝 = 𝜌𝑔𝑦. 6 Plate boundaries - Divergent (accreting) • Plate accretion occurs via decompression melting, where the nearly isothermal asthenosphere ascends beneath a spreading center and crosses the solidus for the basaltic component • Assume a simple linear solidus T (K) = 1500 + 0.12p Fig. 1.5, Turcotte and Schubert, 2014 • where 𝑇(𝐾) is the temperature in Kelvins and 𝑝 is pressure in MPa If we assume the density of the rising asthenosphere is 𝜌 = 3300 kg m-3, 𝑔 = 10 m s-2 and constant temperature, at what depth 𝑦 does the asthenosphere begin to melt? Note: Pressure 𝑝 = 𝜌𝑔𝑦. 7 Plate boundaries - Divergent (accreting) • Plate accretion occurs via decompression melting, where the nearly isothermal asthenosphere ascends beneath a spreading center and crosses the solidus for the basaltic component • Assume a simple linear solidus T (K) = 1500 + 0.12p Fig. 1.5, Turcotte and Schubert, 2014 • • where 𝑇(𝐾) is the temperature in Kelvins and 𝑝 is pressure in MPa If we assume the density of the rising asthenosphere is 𝜌 = 3300 kg m-3, If𝑔 we assume the constant density oftemperature the rising asthenosphere 𝜌 = depth 3300 kg m-3, the = 10 m s-2 and of 1600 K, at is what 𝑦 does 𝑔 = 10 m s-2 and constant temperature, at what asthenosphere begin to melt? Note: Pressure 𝑝 =depth 𝜌𝑔𝑦. 𝑦 does the asthenosphere begin to melt? Note: Pressure 𝑝 = 𝜌𝑔𝑦. 8 Plate boundaries - Divergent (accreting) • Plate accretion occurs via decompression melting, where the nearly isothermal asthenosphere ascends beneath a spreading center and crosses the solidus for the basaltic component • Assume a simple linear solidus T (K) = 1500 + 0.12p Fig. 1.5, Turcotte and Schubert, 2014 • • where 𝑇(𝐾) is the temperature in Kelvins and 𝑝 is pressure in MPa If we assume the density of the rising asthenosphere is 𝜌 = 3300 kg m-3, If𝑔 we assume the constant density oftemperature the rising asthenosphere 𝜌 = depth 3300 kg m-3, the = 10 m s-2 and of 1600 K, at is what 𝑦 does 𝑔 = 10 m s-2 and constant temperature, at what asthenosphere begin to melt? Note: Pressure 𝑝 =depth 𝜌𝑔𝑦. 𝑦 does the asthenosphere begin to melt? Note: Pressure 𝑝 = 𝜌𝑔𝑦. ~25 km depth 9 Plate boundaries - Divergent (accreting) FRP FRP Fig. 1.4, Turcotte and Schubert, 2014 • Divergent plate movement here is a form of gravitational sliding driven in part by the ridge push force 𝐹RP owing to the high elevations of the ridge relative to the majority of the oceanic plate 10 Age of the oceanic crust 11 Test your might • You can find a short quiz about this lecture at https://elomake.helsinki.fi/lomakkeet/62991/lomake.html • • Please take the quiz to help me know what you have learned Your answers are anonymous and will not count in your course grade 12