How is a volcano formed?

Question

How is a volcano formed?

Answer 1

after millions of years of lava flows to the surface of the earth...

..depending on the viscosity of the lava....

..low visc...wil form a ragged mass...as it spreads out like a water flow...

...hi visc..will form a beautiful cone...

Answer 2

Subduction of tectonic plates.

Answer 3

What everybody has said is correct.

Different volcanoes form at different plate lines.  Convergent (subduction zone) you will get a strato-volcano, which is that perfect triangular cone type of volcano.  They are called strato volcanoes because they are layered (lava and pyroclast or tephra),  This happens because of high viscosity lava (consists of gases and water vapor).  Highly viscous because at a subduction zone there is an immense build up of pressure in the mantel which forms gases and at some point needs to be relieved of the pressure and this is how you have volcanoes.  Vesuvius, Popocatepetl, Mt St Helens, and Rainier.

At divergent plate boundaries or hot-spots you will find more fissure, or shield volcanoes like the volcanoes in Hawaii.  These are basaltic volcanoes which consist of mainly lava and very little ash.  Low viscosity lava which makes it more watery or flowey (river like)  Not as dangerous as a strato volcano.

Cinder cones or scoria cones are randomly scattered throughout different plate boundaries or hot-spots.  They are very small and almost perfect cone shape and often a little oval shaped.  They are explosive but the lava often cools before it even hits the surface.  The rock formed from this is called scoria hence its name.  Looking at one of these volcanoes from far away looks almost like a pile of gunpowder or ground led with a crater or a deep dent in the middle.

Answer 4

there is something called plate tectonics and there within this there is something called a convergent boundary of the plates. plates are just another term for land masses broken up into pieces. in a convergent boundary line, on plate converges with another and one slides under while the other goes ontop (the oceanic plate rides under and the continental plate on top b/c of the different densities). the plate that subducts then turns into magma since it is heating up and melting. this then rises and erupts. just for some extra info, magma is considered lava when it is not in the ground.

Answer 5

One way is that magma slowly comes up through the crust and erupts out into open air.  Another way is that, when tectonic plates move, magma can flow out between the cracks and form mountains (usually chains of mountains).  Consider, for example, the Pacific's Ring of Fire, which is multiple volcanoes and volcano chains surrounding the Pacific plate.