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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 ...

www.telegraph.co.uk/science/0/what-causes-a-volcanic-eruption

Nov 17, 2016 ... In a volcanic eruption, super-heated magma from within the earth's upper ... while last week's episode focused on what causes a wildfire.

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 ...

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.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.universetoday.com/31124/how-volcanoes-erupt

May 26, 2016 ... The 2006 Cleveland Volcano Eruption viewed from space. ... What is it exactly that makes molten rock rise from the Earth's interior and explode ...

www.ask.com/youtube?q=What+Makes+Volcanoes+Erupt&v=0u3IyeYRzmA
Nov 23, 2016 ... In this video we look at how the continuous movement of Earth's tectonic plates results in volcanic activity. In a volcanic eruption, super-heated ...