Plate Boundaries 3 main types of boundaries • Divergent (plates diverge, or move away) • Convergent (plates converge, or come together) • Transform (plates slide past each other) Divergent boundary • Plates move away (or spread) from each other. • New crust is created here. – Magma forces plates apart. • Example: – Mid-Atlantic Ridge – Great Rift Valley in East Africa Geologic phenomena at divergent boundaries: * = unique to divergent boundaries – Iceland* (formed right on top of Mid- Atlantic Ridge) – Volcanoes – Earthquakes – Mountains • Fault-block mountains (one side is dropping lower as crust spreads) – Mid-Ocean Ridges* – Rift Valleys* – Spreading Zones* • Crust is spread out and stretched as plates move apart. Mid-Ocean Ridges • New crust created here. • See “Evidences of Plate Tectonics” Notes for more detail. Rift valleys (divergent) •A deep valley formed by the two plates moving away from each other. •Crust warps downward, spreads, eventually “breaks” (boundary comes to surface) Example: Great Rift Valley in East Africa Convergent boundary • Plates move toward each other and collide. • Crust is destroyed here. • Subduction: Where one plate sinks underneath another and is forced into the mantle (then melted) Geologic phenomena at convergent boundaries – Volcanoes – Earthquakes – Mountains • Folded mountains (crust crumples/folds upward) – Trenches/subduction zones* – Volcanic Island Arcs (ocean-ocean boundary)* Types of Convergent boundaries Continent-Oceanic • Ocean plate collides with a continent. • Ocean plate sinks. • Forms a trench. – Trench: also called a subduction zone, where one plate subducts under the other. Continent-Oceanic, continued • Examples: – Juan de Fuca plate and North American plate • Juan de Fuca plate subducts under N.A. plate; created the Cascade Mountain Range in Northern CA, OR, and WA. • Nazca plate and South American plate – Subduction of Nazca plate formed Andes Mountains on west coast of S.A. Types of Convergent boundaries, continued Continent-Continent • Two continents colliding • Pushes the crust upward to form mountains. Continent-Continent Example • The Himalayas and Mount Everest— formed by Indian Plate and Eurasian Plates colliding. Types of Convergent boundaries, continued Oceanic-oceanic • Two ocean plates collide. • The older one sinks under the newer one. • Creates volcanic island arc *Not Hawaii!* Oceanic-Oceanic examples – Mariana Islands (volcanic!): Formed by Philippine plate and Pacific plate – Tonga Trench in South Pacific (Pacific plate subducting under Australian plate) • Fastest moving plate: 24 cm/year!! Transform boundary • Plates move past each other laterally. • Neither plate is destroyed. • Example: – San Andreas Fault (formed by North American and Pacific plates) Geologic phenomena associated with Transform Boundaries: • Earthquakes • Some warping of crust Why Plates Move • Convection is the driving force behind plate movement! How It Works: 1. Magma in the mantle is heated by the core. 2. Heated magma rises toward crust. 3. As magma moves away from heat source, it begins to cool. 4. Cool magma sinks back down toward core, creating a convection cell that helps move crust.
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