supervolcano

[Ian Robinson]

Originally Posted by multiweb

Wow! Why is that? I would have thought that if it happened deep in the ocean with all the pressure and the cold water it would just "fizz" quietly? Are pressure and cold a bad mix with Magma?

If you know anyone who works / or worked / in the steel industry (at Kembla or Newcastle or Whyalla) ask them what happens when molten steel at 1600oC is poured into a BOS furnace or laddle or mould that's got a bit of water in it.... it's spectacular and very very dangerous.

Now imagine billions of cubic metres of sea water under terrific pressures making contact with and instantly becoming superheated steam with billions of tons of magma at 1600 oC..... doesn't bare thinking about , would be earth shaking and cataclysmic .

[renormalised (Carl)]

Originally Posted by multiweb

Nasty!

Very

[matt]

Originally Posted by renormalised

dragons and all sorts of nasties:d

:p

[Ian Robinson]

Originally Posted by renormalised

Not that we know of.

Operative words " know off " .... the deep ocean is not well explored or charted.

So this doesn't mean there are no deep ocean supervolcanoes.

[AstralTraveller (David)]

Originally Posted by Ian Robinson

If you know anyone who works / or worked / in the steel industry (at Kembla or Newcastle or Whyalla) ask them what happens when molten steel at 1600oC is poured into a BOS furnace or laddle or mould that's got a bit of water in it.... it's spectacular and very very dangerous.

That happened at Pt Kembla works when I lived at Coniston. We were woken but what we thought was a small earth tremor. It turned out that some slag had been poured onto some water. I'm not sure of the quantities involved but I can't imagine there was more than 1 rail wagon of slag, so <100t. We were probably about 3-4km from it.

[Baddad (Marty)]

Hi Marc & Ian Rob,

I have the DVD of Krakatoa. The one you refer to. "Son" or Child of the volcano is growing and expected to erupt soon. What the time frame is? I do not know.

Spectacular and devastating? It is likely.

Cheers Marty

[AstralTraveller (David)]

Originally Posted by Ian Robinson

Makes you wonder what the effects would be of any supervolcanoes on the sea bed ..... if they blew ....

In the case of Krakotoa at least, I think the caldera was underwater, but not deep water. My memory is that this sort of volcanism occurs in conjunction with subduction zones. So the eruption may be under water but it would still be on the continental shelf. I think oceanic crust does not have the right geology to cause such volcanism.

[renormalised (Carl)]

Originally Posted by AstralTraveller

In the case of Krakotoa at least, I think the caldera was underwater, but not deep water. My memory is that this sort of volcanism occurs in conjunction with subduction zones. So the eruption may be under water but it would still be on the continental shelf. I think oceanic crust does not have the right geology to cause such volcanism.

You're partially correct....Krakatoa was an island, the subsequent caldera ended up underwater (as also in Santorini). The volcanism is island-arc volcanism associated with subduction and is usually acidic (Andesitic, Ryolitic rocks). Oceanic volcanism, like Hawaii, is either associated with MOR's (Mid Ocean Ridges) or mantle plumes and is mostly basic in nature (basaltic rocks).

[renormalised (Carl)]

Where you get super-volcanoes is when a mantle plume rise to within a few kilometres of continental crust and just sits there. The heat from the plume basically acts like a blow torch and burns a gash into the surface. It's very much like how a rift valley begins to form, only that it doesn't completely rift apart the opposite sides of the scar. As the continent moves over the plume, the scar in the surface extends along the line of travel of the plate riding over the plume. If there is already a pre-existing weakness in the overlying rock, then the plume will tend to burn its way through that area of weakness. What you get in the area being affected is uplift (doming) and then splitting of the crust, leaving a depression between either side of the uplift.

Every now and again, apart from the usual activity in these areas (like in Yellowstone, at present), you get the violent super-eruptions when injections of magma occur in the top of the mantle plume or in an under-rift magma chamber. Depending on the size of the melt and it's composition, the eruption size will vary. A supervolcanic eruption is defined as an eruption which displaces 1000km^3 or greater of material from the vent (or vents) of the volcano. They're classed as VE8 in intensity and are usually of the Plinian type. Most are associated with caldera complexes where more than one eruption has occured, forming nested and adjacent calderas. They're also usually very large and almost impossible to spot, even from the air. Most of the time, they're spotted using high altitude recon planes and satellites.

[multiweb (Marc)]

Is that what's happening with Hawaii? There's a hot spot under that keeps popping up islands as the plate moves over it?

[renormalised (Carl)]

Originally Posted by multiweb

Is that what's happening with Hawaii? There's a hot spot under that keeps popping up islands as the plate moves over it?

Correct...if you look at the Hawaii-Emperor Seamount chain, you'll see that there is a dog leg in the chain just north west of the main Hawaii group. The seamounts then run north....what's happened is that originally, the Pacific Plate was moving in a northerly direction and then about 20 million or so years ago, it started to move in a more north-westerly direction. The big island is around 7 million years old and from the seafloor to the peak of the main island is 34000 feet high. However, if you measure its height from the base of the edifice, Mauna Loa would top 19.5 kilometres.

[AstralTraveller (David)]

To continue with too much information..... Islands formed from hot-spot volcanism have a relatively short life. Once the island detaches from the hot-spot it starts weathering away, and basalt weathers quite quickly. Further the ocean crust is domed up over the hot spot; the depth to the sea-floor is less than elsewhere. As the crust to which the island is attached moves away from the hot spot it sinks, taking the island down with it. So the Hawaiian island get smaller going east to west and then there are seamounts.

In tropical climes the island will form a reef system - fringing in young systems then lagoon followed by barrier. Once the hard rock is below water level the coral may survive as an atoll attached to the top of the seamount. Not all tropical islands leave atolls, it depends on the rate of subsidence. As the island sinks (or sealevel rises - its all the same to the coral) the coral must either 'keep up, catch up or give up' as my lecturer used to say. Not all succeed.