(05)Basin&RangeTwo - GEOLOGY 20 - LECTURE 5 - BASIN &...

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1 GEOLOGY 20 - LECTURE 5 - BASIN & RANGE: Part 2 - Extensional Tectonics & Earliest California (Ch. 7 & 1 in Harden - read selectively!) Why is the Basin and Range being pulled apart? What are the forces that would cause this enormous scale of extension? Where else is extension happening in the world? Extensional Tectonics Linear patterns of earthquakes and volcanism extend directly through the ocean basins. This geologic activity reflects plate boundaries where plates are diverging from one another, in the process creating new seafloor. Divergent plate boundaries Plate boundaries where two plates spread apart to create new lithosphere are called divergent boundaries - the primary example of divergent boundaries on Earth is the 60,000 km long underwater mountain chain known as the mid-oceanic ridge system that circles the globe like the seam on a baseball - running up the middle of the Atlantic is the Mid-Atlantic Ridge , whereas the mid- ocean ridge in the Pacific – the East Pacific Rise – extends up through the East Pacific - in cross-section, mid-ocean ridges form an elongate, low mountain chain whose crest is about 2-2.5 km beneath the sea. The axis of the ridge ( axial ridge ) is marked by a narrow, steep-walled valley called a rift valley where the actual process of underwater volcanism occurs. - the axial ridge manifests the actual boundary between the two divergent plates - as the plates pull apart along the axial ridge, the rift valley widens with vertical fractures developing due to the extension - elongate bodies of magma reside beneath the axial ridge – as fractures develop, the magma wells upward through the cracks and erupts as lava on the seafloor and solidifies within the rift valley - this new ocean floor material eventually gets displaced to either side of the mid- ocean ridge as newer molten material rises along fractures in the rift valley. Through this process, ocean basins progressively widen over long spans of geologic time, somewhat analogous to a conveyor belt.
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2 This process of seafloor volcanism and progressive widening of ocean basins along divergent mid-oceanic ridges is called seafloor spreading . It is the primary mechanism for the growth of ocean basins. - as ocean basins grow wider, the youngest volcanic seafloor exists along the volcanically active mid-oceanic ridges. Older seafloor gets progressively pushed outward as the ocean basin grows, so the oldest seafloor resides along the continental margins. - of all tectonic settings, underwater mid-oceanic spreading ridges produce the greatest sheer volume of volcanic material (essentially produces all of the seafloor rock) - 80% of all volcanic activity on Earth takes place under water - eruptions on the seafloor are generally benign since they occur under the pressure of >2 km of water Continental Rifting and the Birth of Ocean Basins How do ocean basins begin? Where do they arise? And by what process? Ocean basins are born when a continent splits and separates into two divergent
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This note was uploaded on 04/22/2008 for the course GEL 20 taught by Professor Osleger during the Spring '08 term at UC Davis.

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(05)Basin&RangeTwo - GEOLOGY 20 - LECTURE 5 - BASIN &...

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