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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.

What causes a volcano erupt?

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 ...

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).

What makes a volcano erupt? | Reference.com

www.reference.com/science/volcano-erupt-4c02278a54b905b5

Volcanoes erupt due to pressure that the weight of the rocks puts on the magma and forces it to the surface out any available vent or exit provided to it.

What causes a volcano to erupt and how do scientists predict ...

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 ...

Volcanoes and volcanic eruptions - BBC

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.

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 ...

What Makes A Volcano Erupt? - Catalogs.com

www.catalogs.com/info/earth-science/what-makes-a-volcano-erupt.html

There are few geological events that are as dramatic as a volcanic eruption except maybe a hurricane, tornado or a tsunami. Each is drama.

How Do Volcanoes Erupt? - Universe Today

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

May 26, 2016 ... Volcanic eruptions are driven by a combination of mechanisms, which ... The crust makes up a tiny volume of the Earth, ranging from 10 km in ...

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.