These May Be Divergent Boundaries Characterized By

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Tips for Drawing Cross Sections Identify plate boundaries on the map – these may be divergent boundaries characterized by mid-ocean ridges or convergent boundaries characterized by a trench and either a volcanic island arc or continental volcanic arc o Mid-ocean ridges are LONG and mostly continuous except where they are broken by transverse faults o Straight lines on a map unbroken by transverse faults and without obvious ridges may be sonar tracks from ships; it’s ok to ask for clarification with these o Don’t confuse seamounts and ridges produced by hotspot activity with mid-ocean ridges. Remember – mid-ocean ridges are basically continuous except for transverse faults o Don’t confuse trenches with continental slopes. Continental slopes may look dark by contrast with the continental shelf but a trench will be very dark, significantly darker than the abyssal plain. Trenches will also be bordered by either volcanic islands or continental volcanoes. Determine if continental margins are active or passive - is there a plate boundary adjacent to the continent or are any plate boundaries on the map a long way away? Make sure that parts of your diagram which should break the surface of the ocean (e.g. islands, continents) actually break the surface and that parts that shouldn’t break the surface (e.g. mid-ocean ridges) don’t break the surface. Make sure you draw your layers with appropriate thickness o Ocean crust should be thin, continental crust should be thick o If a transect crosses an island followed by shallow ocean followed by more land, that shallow ocean is probably a continental shelf and therefore sits on continental crust o At divergent boundaries, the ocean crust and lithosphere should taper right up to the peak of the mid-ocean ridge to show the asthenosphere reaching up right to the peak of the ridge Make sure to label all layers and appropriate features o For a passive margin, this includes continental shelf (which sits on continental crust), continental slope, continental rise (boundary between continental and oceanic crust occurs underneath the continental slope and rise), and abyssal plain o For an active margin/convergent boundary, this includes a trench and a continental volcanic arc or volcanic island arc o For a divergent boundary, this is the mid-ocean ridge o Don’t forget to label crust, lithosphere, asthenosphere Make sure to answer any other parts of the question! .

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PLATE TECTONICS.Docx
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Copyright © by SIAM. Unauthorized Reproduction of This Article Is Prohibited. 82 PHILIPPE GUYENNE and DAVID P
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Partial melting occurs when some minerals melt Continental Rifting Trench Spreading while others remain solid because their melting point has not been reached The Crust Center continental Subduction Basalt Volcanic rifting Granite Arc Accretionary process Oceanic Prism Crust a Granite q The role of partial melting Basalt Kk 3 Types of Plate Boundaries MODERN CONTINENTS EVOLVED FROM PANGAEA DIVERGENT CONVERGENT New lithosphere forms as plates pull apart. One plate dives beneath another (subduction) or two Convergent Divergent TRANSFORM plates collide without either Main Types of Plate Motion Plates grind past each other. subducting. Transform No change in Lithosphere Divergent Boundary Mid-Ocean Ridge Seafloor spreading leads to Spreading Center the formation of new crust that, compared to continental crust, is relatively enriched in iron and magnesium and depleted in silica (SiO2) (because it reflects the chemistry of the mantle). As two plates continue to move apart, the rock in the seafloor grows older as its distance from the rift zone increases, and as it ages, it cools and becomes denser and is buried under marine sediments that are deposited on the seafloor. 1 TRANSFORM Transform Boundaries Connect OCEANIC FRACTURE ZONE BOUNDARY Two Spreading Transform Boundary with INACTIVE (FRACTURE ZONE) (side-to-side plate Centers and ACTIVE (TRANSFORM FAULT) portions. movement) Occur where two plates slide past each other. Motion called shearing. Connect two spreading centers (less commonly, two subduction zones). Probably the most FAULT famous transform “a place where the crust is broken and the broken edges boundary in the are offset relative to each other world is the San Andreas Fault.
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