Unformatted text preview: When Plates Collide Plate Tectonics – Plates meet at boundaries. • Divergent • Convergent • Transform Convergent Boundaries ! ! What gets created at the spreading centers must get destroyed (Subduc>on). Three types: These blocks have equal volume and are ﬂoa>ng in this jar of water. Which one has greater density? A B C= I don’t know Which has the greatest density? A. Old oceanic lithosphere B. New oceanic lithosphere C. New con>nental lithosphere When plates collide, the denser plate sinks! • Con>nental Lithosphere ~ 150 km thick. – Grani>c crust + uppermost mantle • Lighter (less dense) . • More buoyant – Floats higher. • Oceanic Crust~ 7 to 100 km thick. – Basal>c crust + uppermost mantle • Heavier (more dense). • Less buoyant – Sinks lower. Convergent boundaries: Three types 1. Ocean-‐con>nent subduc>on zone Figure 2.23 A Video Oceanic to Con>nental Convergence The Cascades: A Con>nental Volcanic Arc The Cascades result from subduc>on of the Juan de Fuca Plate underneath the North American Plate Convergent boundaries: Three types 2. Ocean-‐ocean subduc>on zone Figure 2.23 B Video Deep ocean trenches are the surface expression of subduc>on Mariana Trench: Deepest part of the world – 11 km (~36,000 a) deep! Aleu>an Islands: A Volcanic Island Arc That ring of fire… 1. How many tectonic plates are represented by this ocean
2. How do you know? Convergent boundaries: Three types 1. Con>nental-‐Con>nental Convergence • Con>nental collisions follows aaer ocean-‐con>nent convergence… • Con>nental lithosphere is too buoyant to subduct Continent to Continent
Convergence Video The Himalayas: Result of a recent con>nental collision (45 Ma-‐ present) World’s tallest mountain range! Mount Everest = 8,848 metres (29,029 a) Final Boundary Type: Transform
Fig. 2-14, p. 24 Multiple boundary types Con>nental collisions • Con>nents crash together – forming huge mountain ranges. – E.g. Himalayas, Alps, Appalachians Draw a transect ...
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