How volcanoes form — Earth science, 14–17 years
Magma does not rise randomly: its movement, pressure and gas help explain where eruptions happen and what they produce.
What a volcano is
A volcano is a place where molten rock from inside Earth can reach the surface. Underground it is called magma; after it erupts, it is called lava. Pressure from expanding gases can push magma through cracks or a central vent, building layers of lava and ash.
Why eruptions happen
The useful question is not simply “Where is the volcano?” but “Why can magma rise there?” At some plate boundaries, rock melts or pressure lowers enough for melting; elsewhere, hot rising mantle makes magma. Gas dissolved in magma expands as pressure falls, helping drive an eruption.
Worked example: a shield volcano
Imagine basaltic lava leaving a vent at 1 metre per second. In 10 minutes it travels 600 metres, because 10 × 60 = 600 seconds. If each eruption lays down a 2-metre-thick layer and there are 50 eruptions, the cone gains 100 metres. Runny lava spreads widely, so the volcano becomes broad and gently sloping.
Trap: magma is not always explosive
It is reasonable to picture every volcano as a mountain that suddenly explodes, because dramatic eruptions are the ones shown in news reports. But eruption style depends strongly on viscosity and gas. Runny, low-viscosity magma can release gas and flow calmly, while sticky magma can trap gas and erupt violently.
Where volcanoes matter
Volcanic landscapes affect real decisions. Scientists monitor earthquakes, ground swelling and gases to warn communities, while planners map lava and ash hazards. Eruptions can also create fertile soils and geothermal energy, but those benefits do not remove the danger of hot flows, ash and toxic gases.
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