What causes volcanoes to erupt?

[Hunter1234]

I would appreciate if you could provide a source. Thanks!

 

[jedishrfu]

Here's a short answer:

http://volcano.oregonstate.edu/why-are-there-volcanoes

and haere's a more involved answer:

http://www.bbc.co.uk/schools/gcsebitesize/geography/natural_hazards/volcanoes_rev1.shtml

 

[waternohitter]

What Causes A Volcanic Eruption?

That Mt. Helen picture is really something.

 

[Astronuc]

Mount St. Helens Is Recharging Its Magma Stores, Setting Off Earthquake Swarms
http://www.wired.com/2016/05/mount-st-helens-recharging-magma-stores-setting-off-earthquake-swarms

When it comes down to it, volcanoes spend most of their existence not erupting. If you look at almost any volcano, it might have a bout of eruption for days to months at a time, then go quiet for decades, centuries or more. So when you think about the activity at any given volcano, you should not only concern yourself with what might be happening when the volcano is actually coughing stuff up (erupting), but also when, at the surface, things look perfectly calm.

http://www.csmonitor.com/Science/2016/0507/Rumbles-heard-from-Mount-St.-Helens-Another-eruption

More than 130 small tremors have been documented in the Mount St. Helens region in Washington, according to the US Geological Survey. The quakes began in mid-March and have ramped up to nearly 40 localized earthquakes a week.