Lesson 2.3

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Clemson University *

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1010

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Geology

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Oct 30, 2023

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docx

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Uploaded by DeaconFangKingfisher36

GEOL 1010 Lesson 2.3—Plate Boundary Processes Theory of Plate Tectonics: - The outer layer of the Earth (lithosphere) consists of separate plates that move with respect to one another. - “Grand unifying theory” of geology - Explains so much about these things: o Earthquakes, mountain building, paleontology, faulting/rifting, volcanism, distribution of resources, maybe even the evolution of life A Few Key Points - Lithosphere broken up into 12 major and many minor plates o Move/glide over weak asthenosphere - Plates move relative to one another - Plate boundaries do not necessarily correspond to edge of continent - Most rock deformation occurs at plate boundaries. Not much happens in plate interiors What main differences do you see between the Active and Passive Margins of South America? - Elevation between active (very mountainous) and passive - Active side has a trench and passive has a broad wide continental shelf Three Types of Plate Boundaries - Divergent - Convergent - Transform Convergent Plate Boundaries - Plates move toward one another. Which one forms depends on type of crust converging o Ocean-ocean or ocean-continent: Subduction o Continent-continent: Collision Subduction Zones - Where an oceanic plate dives back into the mantle - Oceanic lithosphere is destroyed - Balanced by creation of oceanic lithosphere at mid-ocean ridges Processes/Features of Subduction Zones - 4 features all subduction zones have: o Trench o Volcanic Arc o Accretionary Prism/Wedge o Deep, large earthquakes

GEOL 1010 Trenches - Mariana Trench—contains the deepest spot on Earth, called Challenger Deep Volcanic Arcs - A row of volcanoes in the overriding plate, that parallels the subduction zone - Called a Volcanic “arc” because subductions zones are always curved, thus so is the row of volcanoes - Ex. Zoom in on Aleutian Islands on Google Earth Volcanic Arcs—Why do volcanic arcs form along subduction zones? - Important fact to start with: oceanic sediments and crust contain water  As descending slab (plate) heats up, wet ocean crust is “dehydrated” (water is liberated)  Water moves UP into the mantle above the descending slab  This water causes the mantle to melt—magma rises to surface and erupts forming volcanoes

GEOL 1010 Accretionary Prism/Wedge - Seafloor sediment scraped off subducting plate - Transferred onto overlying plate - Creates a thick mound—sediment at the bottom buries deeply enough to be metamorphosed Deep, Large Earthquakes - Earthquakes occur all along the subducting slab, to depths of ~600 km - The inclined area underground where subduction-caused earthquakes occur is called the Wadati-Benioff Zone Subducting Plates - We can also use seismic waves to image the cold subducting slabs underground o Because P-waves travel faster through cold rock than hot rock - What happens to subducted plates? o We think they pile up at the core-mantle boundary slab graveyard

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