The explosive eruption of Mt Pinatubo in the Philippines on June 12, 1991.
| Photo Credit: USGS
Deep beneath the earth’s surface, temperature and pressure are high enough to melt parts of the mantle and crust to form molten rock called magma. Because magma is less dense than solid rock, it tends to rise and pool in subterranean reservoirs.
When magma ascends, the water vapour, carbon dioxide, sulphur dioxide, and other volatile compounds dissolved in it stay trapped by the weight of overlying rock. But close to the surface the pressure drops and allows these compounds to exit and form bubbles, like when a soda bottle starts to fizz when you open the cap. Bubbles increase the internal pressure.
At tectonic plate boundaries or above mantle plumes, fractures and faults in the crust allow this pressure to be released. Magma brimming with expanding gas surges through this weak zone, and the sudden pressure release also accelerates gas expansion, shooting lava, ash, and gases into the sky.
If a bottleneck in the rock abruptly stops fast-moving magma, it can reverberate against the rock and create low-frequency seismic waves.
A volcano is said to be active if it’s erupting right now or will soon. There are also zombie volcanoes like Uturuncu in Bolivia. Although it hasn’t erupted in 2.5 lakh years, it still emits gases and triggers earthquakes in its surroundings.
Recently, scientists used a new technique to find that gases are collecting in magma chambers beneath Uturuncu’s crater but also that it isn’t likely to erupt any time soon.
Published – June 03, 2025 10:42 am IST