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Thread: Continental and oceanic islands

  1. #1 Continental and oceanic islands 
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    How do continental and oceanic islands form? I have always assumed that continental islands formed because pieces of land broke off the main continent (continental drift etc.). Is this incorrect? Did they form by a different means? Is it something to do with tectonic plate boundaries (similar to oceanic islands)? I know that some came about due to the flooding of coastal regions (where some highland remained above sea level). But is there more to know?

    Also, I have a similar uncertainity about the formation of oceanic islands (at destructive/consuming boundaries). They are supposed to have formed at subduction zones. Can I clarify that a subduction zone is when an oceanic and continental plate collide (and the oceanic plate is overridden)? Does this mean that the magma (which forms the island arc) rises between an oceanic and continental plate? This confuses me since I always believed that it was the collision between two oceanic plates which caused these islands to form?


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    WYSIWYG Moderator marnixR's Avatar
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    the way i understand it, continental islands have either split off through plate spreading (e.g. Greenland, Madagascar) or are the unsubmerged part of a continental edge (Great Britain, Papua New Guinea)

    as for oceanic islands in island arcs near subduction zones, my impression was that the subducting plate started melting on its descent, and rising melt punctures the other plate, thereby forming volcanic islands


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  4. #3  
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    North American volcanoes are far inland, above where already subducted plate diverges down away from continental plate. I think those volcanoes owe to pressure of melting rock constrained by the plastic plate beneath it. They contribute relatively insignificant bulk to the Rockies.

    A subducting plate is a bit sticky, so it crumples whatever's above it, like how your sweater sleeve bunches when you pull on a jacket.

    A subducting plate is a sort of conveyor belt of any old junk sitting on it - you know how groceries pile up at the end of the checkout belt. The Rocky mountain chain is assembled of such debris and even lesser continents. It's the world's largest dust pan.

    If Hawaii were much farther north, it would ride to the Aleutian arc and stop there. Or alternately you could say the Aleutians would go to Hawaii. Anyway the only islands there now are stunningly volcanic, set back from the actual subduction trench.
    A pong by any other name is still a pong. -williampinn
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    Quote Originally Posted by Pong
    North American volcanoes are far inland, above where already subducted plate diverges down away from continental plate. I think those volcanoes owe to pressure of melting rock constrained by the plastic plate beneath it. They contribute relatively insignificant bulk to the Rockies.
    Cascades are a better example perhaps, Mt Baker, St Helen, Hood etc. all within a hundred miles of the coast.
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  6. #5  
    WYSIWYG Moderator marnixR's Avatar
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    obviously if you have subduction of an oceanic plate below a continental one, the result won't be islands
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  7. #6  
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    It may, where the overriding plate is also below sea level. All those islands north of Olympian peninsula are a single continental plate (or "terrane") crumpled partially above sea level by the force of subduction. Folds also form valleys which, half submerged, yield fjords. The ocean floor between islands is the same geology. This is a recent addition to North America. Formerly it was a lowland marsh continent near the equator.

    P.S. Such a thrill to live where the action is!
    A pong by any other name is still a pong. -williampinn
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  8. #7  
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    There must be a hundred different ways to form an island. The main ways are already mentioned: rifting and volcanic arcs. You can add all sorts of smaller processes too such as sediment buildup at the mouth of a river. Is there a specific island we're talking about?
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  9. #8 Re: Continental and oceanic islands 
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    Quote Originally Posted by emetzner
    How do continental and oceanic islands form? I have always assumed that continental islands formed because pieces of land broke off the main continent (continental drift etc.). Is this incorrect? Did they form by a different means? Is it something to do with tectonic plate boundaries (similar to oceanic islands)? I know that some came about due to the flooding of coastal regions (where some highland remained above sea level). But is there more to know?
    In some cases, extension/subsidence can occur 'inland' from a subduction zone. In the case of the California basin, it's above sea level, so no island is created. In the case of the sea of Japan (IIRC), it's below sea level and creates an island.

    Can I clarify that a subduction zone is when an oceanic and continental plate collide (and the oceanic plate is overridden)?
    Subduction simply refers to a convergent margin where one place subducts beneath the other. Because continental crust is less dense than oceanic crust, the latter tends to subduct. By definition, ocean island arcs should arise from oceanic-oceanic subduction. That may be what's occurring around Indonesian portion of the Pacific ring of fire, but don't quote me on that.

    Does this mean that the magma (which forms the island arc) rises between an oceanic and continental plate? This confuses me since I always believed that it was the collision between two oceanic plates which caused these islands to form?
    In the case of the Andes, magma generated from the subducting oceanic plate mixes with the continental crust to form intermediate-to-felsic magmas. I would assume that oceanic-oceanic subuction would predominantly generate basaltic-andesite in the early stages, and then as subduction continues, would mix with the newly formed island to form more felsic magmas.

    It's worth keeping in mind that some islands can also form via hotspot activity, such as Hawaii and Iceland.
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  10. #9  
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    The Aleutian arc puzzles me. Because though above water it's starkly volcanic with perfect cones; below it looks quite different. It looks like a relatively level-topped isthmus. The depth of that top as I understand it corresponds with undersea continental shelf worldwide, and is the low-water mark of ice ages.

    http://maps.google.ca/maps?hl=en&q=a...355957&t=h&z=6

    Is erosion that powerful? And is that lower sea level actually the norm? It appears the flattening only happens at or above ice-age low water, as the long submerged Hawaiian hotspot track bears the same chop-off depth where volcanoes dared grow too high. Is this wave action?
    A pong by any other name is still a pong. -williampinn
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