GS1004 Spotila9 September, 2016Lecture 6: Plate Tectonics II - BoundariesMotivating questions:So the mantle convects, and it drags lithospheric plates around; what happenswhere the plates touch and move relative to each other?What imprint do plate boundaries leave on the Earth?If I want to stand on an active plate boundary, where do I have to go?1) MID-OCEAN RIDGESCreate new oceanic crust/lithosphere. Made by partial melting of mantleasthenosphere as it rises due to convection.Characteristics and important features:1) basaltic magmatism, sea-floor spreading2)ridge in sea-floor (buoyant)3) age of oceanic crust4)Extension: divergent motion on faults, like fissures.5) Cold seawater interacts with magma; “hydrothermal circulation”. Entire ocean passes through ridges ~10 m.y.; 25% of Earth cooling.LOOK-UP ITEMS. Define "hydrothermal circulation". Where can you stand on a mid-ocean ridge?2. SUBDUCTION ZONES: convergent (coming together)Rules:a) Oceanic lithosphere subducts (sinks) once cold (dense); recycled.