A volcano sits quietly, showing no above ground activity for hundreds, perhaps thousands of years, and suddenly-Kaboom! Ash shoots skyward, and soon a river of molten lava is pouring down the mountain. What caused the explosion?
Most volcanoes are found on fault lines where the earths crust is a little more unstable or thinner than in the middle of a tectonic plate. This means that the magma under the solid portion of the crust is closer to the surface. The magma finds it’s way into underground cracks, mixing with gases and water as it goes.
The more gases magma contains, the more pressure is built up underground. More pressure causes the magma to go further and further into whatever cracks it can find, melting rock as it goes and causing the cracks to widen.
Newly melted rock mixes with the old, along with new minerals, gasses and suddenly uncovered water, which causes a greater build up of gasses. This continues until the flow of liquid material is stopped by a blockage, usually a solid wall of rock, and the pressure builds.
Imagine air going into a balloon. The air goes in and keeps going until it is stopped by the surface of the balloon itself. If you keep blowing air in, it will expand as far as it can, and then POP! The pressure was to much and the air found a way out.
This is basically what happens when a volcano erupts, the pressure that has been building for a very long time has finally become stronger than the rock or whatever was blocking it’s way and the gasses blast their way out of the volcano. The explosion doesn’t come from the magma, but from the gasses that are associated with this molten rock.
Eruptions vary in size, depending on how long it has taken the gases to break free. Some may come in the form of a small flow of lava, (magma is called lava when it is found above ground).
Others have taken so long to break through and built up so much pressure, that they may shoot a column of gasses and ash several hundred feet into the air. The higher the pressure builds before eruption, the greater the explosion.
Gasses may cause the eruption to be in a single vent, a series of vents, or along a crack in the earth’s crust. Earthquake activity due to the gasses forcing their way through cracks, almost always signals when an eruption is going to occur. Depending on the size and frequency of the quakes, scientists have gotten very good at predicting if and when an explosion will occur.
Volcanic eruptions, caused by pressure building up under the earth’s crust, are one of the most visible ways that show nature is constantly changing the surface of the earth.