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Several factors trigger volcanic eruptions, including the buoyancy of magma, a build-up of pressure from the gases in the magma and an injection of new magma into a full magma chamber.
When a part of the earth's upper mantle or lower crust melts, magma forms. A volcano is essentially an opening or a vent through which this magma and the dissolved gases it contains are discharged.

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

Volcanoes erupt because of density and pressure. The lower density of the magma relative to the surrounding rocks causes it to rise (like air bubbles in syrup).

www.scientificamerican.com/article/what-causes-a-volcano-to

When a part of the earth's upper mantle or lower crust melts, magma forms. A volcano is essentially an opening or a vent through which this magma and the ...

www.vtaide.com/png/George/volcanoes.htm

A volcano begins as melted rock (magma) which rises from deep inside the earth toward the surface. As the gas-filled magma rises ...

theconversation.com/explainer-why-volcanoes-erupt-44732

Aug 4, 2015 ... It is increased magma movement within a volcano that causes an eruption. These movements are triggered by different processes that happen ...

eschooltoday.com/volcanoes/how-do-volcanoes-erupt.html

Find out what the earth make-up is, and how extreme pressure build up inside the ... molten matter to be expelled out to the surface, causing a volcanic eruption.

www.nytimes.com/video/science/100000003431211/how-it-happens-volcanic-eruptions.html

Jan 5, 2015 ... The eruption of a volcano can be one of the planet's most destructive events — spewing huge amounts of rock, gas and ash that can kill people.

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

A secondary school revision resource for GCSE Geography covering key facts about volcanoes and volcanic eruptions.

www.iflscience.com/environment/are-explosive-volcanic-eruptions-triggered-frictional-heating

Jan 6, 2016 ... A team of volcanologists led by the University of Liverpool have released a perhaps controversial Nature study on the causes of volcanic ...

www.spacedaily.com/news/volcano-03a.html

Nov 27, 2003 ... "One of the central problems of volcanoes is: Why do they erupt and why ... At the same time, the loss of dissolved gases makes it more difficult ...