Volcanoes and hot spots

[kingwinner]

Hi everyone! :Waldo:

 

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...

[Turtle]

1) The lava there is not slow. It is safer in the sense that the lava is generally runny & so not explosive.

2) Yellowstone is a super volcano over a hot spot. An earthquake of adequate size is :hyper: capable of fracturing rock over the magma chamber resulting in an explosive erruption. If the magma on the otherhand pushes up gradually the pressure is lessened gradually & an erruption is likely less violent. The magma under Yellostone is current raising a bulge under Yellostone Lake; the last erruption about 600,000 years ago.

3) I could use a bier.

4) The hotspot under Hawaii seems relatively stable in position while it is the Pacific plate moving over it. From base undersea to summit above, these are the tallest mountains in the world. The newest Hawaiian island is now a few thousand feet high & about the same from the surface as it grows just SE of Hawaii.

5) I could use a stone cold bier. :Waldo:

[kingwinner]

1) So shield volcanoes generally produce less explosive eruptions! :Waldo:

 

2) So oceanic hot spots comes up, slowly melting the lithosphere, produces less violent eruptions, while continental hot spots and other volcanic mountains violently break the lithosphere which produces explosive eruptions! :hyper:

 

Hot spots are "pockets of magma" that rises far from a plate boundary! Does this mean that usually there is no magma coming out over a hot spot, but it comes out suddenly in huge amounts (pockets of magma), which over time created separate Hawaiian Islands?

[Turtle]

1) I'm not sure I want to make that generalization based on how few examples of shield volcano I have in mind.

 

2)Ditto the above for the first part & no, not necessarily on the second part. St; Helens sometimes errrupts low viscosity lava; Ape Caves formed as lava tubes from a St. Helens erruption.

 

I think you posted the last part in haste. Go have some popcorn, a root beer & listen to some music; then do a read through of all your material & do it within 45 minutes. Don't expect to start really understanding some of this until you do some field work. :Waldo:

[kingwinner]

Yes, I think I am going a little bit too deep in terms of understanding...

 

Are "simatic minerals" and lava actually the same thing? (does anyone know?)

[Eclogite]

Yes, I think I am going a little bit too deep in terms of understanding...

 

Are "simatic minerals" and lava actually the same thing? (does anyone know?)

Sima and sial are slightly outdated terms now.

The continental crust was often referred to as being composed of sial, or being sialic. This meant the that silicon and aluminium formed a large proportion of it. This is the composition of granite. In contrast the oceans are composed of rocks of basaltic composition, in which silicon and magnesium are major constituents, hence sima.

So you can have a lava of sialic composition, i.e. a granite (though as a lava it would be called a rhyolite), or of simatic composition, i.e. a basalt (the plutonic equivalent of this would be a gabbro.)

[UncleAl]

Lava composition makes a difference. Lavas low in silca and rich with cations (basic, mafic) will tend to be runny, with low viscosity. They make nice fountains. Lavas rich in silica (felsic) will tend to be thick and sticky, with high viscosity. They get stuck trying to flow through conduits, build pressure, and BOOM! Also consider water content and gases.

 

http://www.geology.sdsu.edu/how_volcanoes_work/Controls.html

bottom

 

How does basalt (oceans, mafic) compare with granite (continents, felsic) when melted?

[kingwinner]

So generally, lava that erupted from shield volcanoes contains simatic minerals? On the other hand, hot rocks and ashes that erupted from cinder cone volcanoes contain sialic minerals?