This is a type of volcano called a stratovolcano. You can see how this very tall volcano has been built up by layers of ash and lava from previous eruptions.
The volcano is currently dormant. But it is about to re-awaken!
Because it is lighter than solid rock, magma (molten rock) has been rising to form a huge pool, called a magma chamber, beneath the volcano.
The volcano will erupt when gases held within the magma burst out explosively, now that they are under less pressure than when the magma was deeper within the earth.
The explosive escape of the gases from the magma causes a violent eruption which literally \"blows the top\" off the volcano. Gas, blobs of magma (now called lava), chunks of rock, and fine debris called volcanic ash are blasted upwards. (The ash, because it is very light, may reach heights sufficient to endanger jet airplanes.)
Because the fragments of rock and ash (pyroclastics) are heavier than air and may not mix with enough air to remain high aloft, they may surge down the sides of the volcano as dangerous \"pyroclastic flows.\"
Within the flow, heavier particles sink and the lighter particles and gases are displaced upwards. The flow meanwhile surges along like a boiling cloud. Pyroclastic flows can travel at speeds greater than 100 miles per hour and can be as hot as the inside of a kiln. Such flows killed 29,000 people in the 1902 eruption of Mount Pelée, Martinique.