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SEAFLOOR SPREADING – YES!! 1. HOW DOES THIS RELATE TO THE FORMATION OF THE OCEAN FLOOR? 2. IS THE EARTH GETTING BIGGER? 3. HOW DOES THE FORMATION OF NEW OCEAN FLOOR RELATE TO THE DEFORMATION OF THE EARTH’S SURFACE? Intrusive Mafic Igneous Rock -- Gabbro Sequence of Crystallization Bowen’s Reaction Series Less Dense More Dense ISOSTACY – REACHING A BALANCE IN DENSITY DISTRIBUTION OF EARTHQUAKES ON EARTH AREAS OF SPREADING – AREAS OF CONVERGENCE EARTH IS CONSTRUCTED OF SOLID TECTONIC PLATES OCEAN-CONTINENT COVERGENCE Driving Mechanisms Slab pull-Ridge push • Ridge push – The higher elevation of the MOR pushes the adjacent lithosphere away. • Slab-pull – Once an oceanic plate begins to subduct, it pulls the rest of the plate with it • Plates may be slowed or hastened by convection in asthenosphere Plate Velocities Plate velocities • Plate velocities may be determined by… – Assuming fixed hot spot locations – Measuring volcano age / distance along a hot spot track. Tectonic plates Tectonic plates: Earth’s lithosphere is divided into ~20 plates • All plates move continuously relative to one other. - Motion ranges from 1 to 15 cm/yr • Plates interact along boundaries - Divergent - Convergent - Transform Three types of Convergent Margins WHAT HAPPENS TO A PLATE WHEN IT SUBDUCTS? Formation of Intermediate Composition Magmas Partial Melting and Crustal Rock Assimilation Andesitic/Rhyolitic Volcanism Convergent Margins Cascades Sierra Nevada Mts. Central America OCEAN-OCEAN COVERGENCE Mt. Fuji Volcanic Activity at Ocean-Ocean Boundary Accretion of Continents Through Accretion on the Margins CONTINENT-CONTINENT COVERGENCE MAJOR MOUNTAIN RANGES COMMONLY COINCIDE WITH THE MARGINS BETWEEN CONVERGENT PLATES The Wilson Cycle Fig. 13.20 W. W. Norton Fig. 13.22 W. W. Norton Palisades Sill: Rifting during Breakup of Pangea