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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.

Why are there volcanoes? Why do they erupt? | Volcano World ...

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).

Why do volcanoes erupt? What creates eruptions? - eSchooltoday

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

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

USGS FAQs - Volcanoes General - How Do Volcanoes Erupt?

www2.usgs.gov/faq/categories/9819/2729

Some volcanic eruptions are explosive and other are not. How explosive an eruption is depends on how runny or sticky the magma is. If magma is thin and ...

Explainer: why volcanoes erupt - The Conversation

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

Aug 4, 2015 ... Piton de la Fournaise or “Peak of the Furnace” on Reunion Island is one of the world's most active volcanoes, shown erupting in August 2015.

How Do Volcanoes Erupt? - Universe Today

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

May 26, 2016 ... Volcanoes come in many shapes and sizes, ranging from common cinder cone volcanoes that build up from repeated eruptions and lava ...

Why do volcanoes erupt? | Reference.com

www.reference.com/science/volcanoes-erupt-6ec5e7566259e2a5

Hot melted rocks collect under the Earth's surface and when the pressure becomes too hard, the Earth's skin breaks, and a volcano erupts. The melted rock  ...

Why do volcanoes erupt? | All media content | DW.COM | 03.02.2017

www.dw.com/en/why-do-volcanoes-erupt/av-37406277

Feb 3, 2017 ... Researchers have captured dramatic footage of a stream of lava pouring into the Pacific from the Kilauea volcano in Hawaii. The intensity of the ...

www.ask.com/youtube?q=Why Do Volcanoes Erupt&v=0dVR_obFbQA
Feb 5, 2015 ... Subscribe here: http://www.youtube.com/user/MagicboxEngRhy? sub_confirmation=1 MAGIC BOX ANIMATION PROUDLY PRESENTS "I ...

How It Happens | Volcanic Eruptions - Video - NYTimes.com

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.