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Thread: Volcanoes and hot spots

  1. #1 Volcanoes and hot spots 
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    Hi everyone!

    1) I was told that shield volcanoes (like these in Hawaii) are the safest ones, is it because the lava, or simatic minerals, erupted from these volcanoes flows very slowly?

    2) In location of hot spots, "plumes MELT the lithosphere allowing the lava to flow onto the surface without a violent eruption" (quote from a text book). What I think is that magma must melt the lithosphere before it erupts, whether a violent eruption or a gentle one, or else the magma can't get onto the surface, true? That's why I don't understand what the quoting is referring, why by melting the lithosphere above a hot spot, the eruption is less violent, as the quote states?

    Another question I would like to ask is, why are the islands (e.g. Hawaii and its neighboring islands) formed by a hot spot separated by some distance? It should form a continuous island instead of individual islands if magma is constantly coming out


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  3. #2 Re: Volcanoes and hot spots 
    Forum Cosmic Wizard SkinWalker's Avatar
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    Quote Originally Posted by kingwinner
    1) I was told that shield volcanoes (like these in Hawaii) are the safest ones, is it because the lava, or simatic minerals, erupted from these volcanoes flows very slowly? [...] That's why I don't understand what the quoting is referring, why by melting the lithosphere above a hot spot, the eruption is less violent, as the quote states?
    If memory serves correct, it's because the "hot spot" is heating a thinner layer of lithosphere and it's staying hot. In otherwords, the magma is there and just keeps melting lithosphere material as it slides past (which is why there are a 'chain' of islands that trail the hotspot. With volcanos in other places -say, the Pacific Northwest- the magma rises up and builds in pressure until it is able to penetrate the lithosphere, often through a much thicker layer.

    It would be akin to shaking a bottle of coke then poking a hole in the cap. The hotspot would be as if the cap wasn't on when you shook it. Both bottles of coke will spew, but the one with the cap will be more violent.

    Quote Originally Posted by kingwinner
    Another question I would like to ask is, why are the islands (e.g. Hawaii and its neighboring islands) formed by a hot spot separated by some distance? It should form a continuous island instead of individual islands if magma is constantly coming out
    Magma production is likely to not be consistent. I would hypothesize that there are periods in which volcanic activity either stops or subsides. Also, erosion and physics of the lava flow itself probably play a role in shaping the islands. The islands furthest from the hotspot are the smallest, since they've been exposed to more erosion and, perhaps, isostatic movement. The pressure of the hotspot being gone, the earth's crust would drop, sinking the island.

    But isostatic might not be the right term, since it usually refers to the rebound of the crust following the absence of the weight of a glacier. Maybe Ophiolite will correct me and expand on what I've posted.

    The bottom line is: don't buy real estate on the smaller Hawaiian Islands if you want to pass it down from generation to generation in your family... your descendents might end up owning a coral reef!


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  4. #3  
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    Thanks a lot, SkinWalker! Your explanation and analogy make things very understandable! :wink:

    1) So what actually make shield volcanoes relatively safe, compared to the 2 other types of volcanoes? Is it simply because shield volcanoes produce less violent eruptions and the lava can flow freely?

    Besides, are simatic minerals and lava meaning the same thing? (does anyone know?)

    2) Ocean hot spots allow magma to melt the lithosphere. On the other hand, for continental hot spot volcanoes and volcanoes along Ring of Fire (like Cascade Range), does magma usually break the lithosphere to erupt instead of melting it, creating an explosive eruption?
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  5. #4  
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    The whole situation is rather interesting. In the excellent and well written popular geological text, "Fire Mountains of the West' the authors go into the subduction zone beginning at the southernmost end of the Juan de Fuca plate, which creates the northern California volcanoes and extends up into Canada, the northernmost end of the Cascade volcanoes. I highly recommend it.

    The hotspots in the West Coast are two. First, the Columbia River basalts, which began to erupt in SE Washington about 10 M years ago, or so. As this hot spot moved, with the NorthAm continent basically moving over it, it chewed out the Snake River plain and the volcanic eruptions there. Currently that hotspot lies under the Yellowstone plateau and is presumably still moving to the ENE.

    The second hotspot is the Long Valley caldera in California east of Mammoth Mtn. Where that one came from is anyone's guess, but possibly when the Pacific plate was moving under the West Coast one of those Hawaii type hot spots simply overrode the continent and then created it. That might as well have been how the Yellowstone hotspot began, too, as the western edge of the NorthAm continent overrode a hot spot on the east moving Pacific plate.

    An interesting insight into all of this are the Deccan lava flows in west central India. They seem to correspond to a series of seamounts extending out into the Indian ocean, as a good topographic map of the Indian ocean will show. When that hotspot, apparently moved on shore, the vast Deccan lava flows, associated with the KT extincitions began. It's similar, but larger than the Columbia River plateau basalts and tens of millinos of years earlier.

    The Siberian traps in the Arctic also seem to have been of the same kind of origin and dated to an far earlier extinction, the Permian if memory recalls.

    The Icelandic hotspot seems to be another one, this time on the mid atlantic ridge, which created Iceland.

    But what creates hotspots in the first place and sustains them, is not at all clear. Altho many feel they are upwellings of magma from the deeper earth below the mantle. Altho no one really has any good idea why they occur where they do. One more interesting geological mystery.
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  6. #5  
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    Just recalled that some have suggested that oceanic hot spots begin when a large asteroid punched thru the oceanic crust. But there does not seem to be an obvious way of substantiating that cause. I recall that was suggested as the origin of the Columbia basalt flows, altho overriding a hot spot seems to be more reasonable.

    But what created that hot spot in the first place? GOK!
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