It may seem obvious, but a volcano is only dangerous if there are people living nearby. Antarctica’s Mount Erebus has been erupting continually since 1972, but is harmless since Antarctica is unpopulated. A volcano is otherwise always dangerous if it emits very extensive or explosive eruptions. Every year, about 50 volcanoes are erupting above sea level. The vast majority are basaltic, and they rarely cause major problems since basaltic magma flows easily and evenly and rarely moves any faster than humans who may be trying to outrun it. For example, the top flow speed of Mauna Loa in Hawaii is only 10 mph. A few eruptions are delivered by rhyolitic- or andesitic- magma volcanoes. These types of magma contain more silicon than basaltic magma, and produce viscous magma. They can generate a kind of “plug” in the volcano that’s removed only when the pressure becomes so intense that the volcano explodes. However, the magma is not the only hazard. Pyroclastic clouds of red-hot ash can move down a mountainside with lightning speed after a volcanic explosion. One such cloud destroyed all life in Pompeii in 79 CE. Dangerous gases may also come out of cracks in Lava the volcano. These volcanic gases are responsible for 3 percent of all deaths caused by volcanoes. Finally, heavy rain can trigger an ash avalanche, which can cause deadly slides like the one that killed 23,000 people at Nevado del Ruiz in Colombia in 1985. MAGMA PRESSURE DETERMINES THE VOLCANO’S STRENGTH Volcanoes erupt when there is an overpressure in the magma chamber. Different types of magma affect the progress and severity of the eruption differently.