take mount etna that constantly erupts, magma is molten rock brought up from the earths crust. as you go deeper into the crust it gets hotter, so when a crack occurs in the earths crust from the tectonic plates, and theres enough gas/pressure the molten rock/magma will start to push through
Not quite, the upper and lower mantle (what is under the crust) is actually solidus, solid with a very small amount of meltling (around 1% of mass if that). As the pressure is released through movement the melting point of certain minerals within the rock decreases and so the solid rock starts to melt, but still only a few % of the overall volume of the rock. This melt then rises up through the cracks and ends up in a magma chamber, which contrary to belief isn't a pool of moltern rock, it's an area with much higher partial melting (the melted material just sits between the solid minerals, much like oil in sandstone).
So essentially it's the opposite, it's a decrease in the pressure that can cause the melting.
Using diagrams like those below you can then work out (using the mineral content of the extruded magma/lava) where abouts the magma was likely to have come from as well as how long it had been at certain points in the crust and what phase of the eruption it is likely to be (for example if the volcano has almost emptied its chamber) as well as a load of other bits and pieces.
If you're more interested have a look at these diagrams and they may explain it better...
http://cgc.rncan.gc.ca/mindep/method/alteration/images/fig04a.jpg
http://www.geol.lsu.edu/henry/Geology3041/lectures/02IgneousClassify/images/PQAPF.GIF
(God they were the bain of my degree! Hate them!
)
It's constantly getting recycled - as new crust is formed in locations along (for instance) the mid-atlantic ridge like Iceland there are subduction zones (like along the South American East coast) where the older crust gets forced down under the continental land mass and it melts
Yup, an easy way of seeing it is to see the ages of the ocean floor
http://geoinfo.amu.edu.pl/wpk/pe/a/harbbook/c_iii/globaltectonics/AtlanticAge.jpg
All oceans are like that too.
in regards to Katla it's well overdue though it erupts every 60 years last one being 1918, scientist predict within the next 6 months it will blow. compared to eyj which is a mild eruption katla would be fierce and dangerous, taken into account the amount of pressure is under that glacier this would affect our climate, in a nutshell this is our change to prepare. the true fact is that no-one has really seen a dangerous disaster in real life, only by news, so everyone shrugs it as though it won't happen, another volcano which is overdue is yellowstone which erupts every 650,000 years is overdue and has a big magma chamber. basically all volcanoes are becoming active due the a biblical cycle that happens.
Not really, Katla is unlikely to be as large an eruption as Pinotobu (91) and St Helens (83), it's just it may end up affecting a much more populated area.
As for Yellowstone, it's not in the same league as the others and I hate that 650k prediction. It came from a Horizon program and is pure scare mongering at it's best. Look at previous eruptions and you'll see that in fact it normally erupts every 600k-1M years. There is a possibility that it may erupt in the near future however it is certainly not "overdue".
