Volcanoes have been producing new land mass for millions of years. However, they can also be extremely destructive. It isn’t unusual for people to wonder what determines how explosive a volcanic eruption will be. There is an answer to the question, which lies in the properties of the magma that makes up the roots of the volcano.
Deep in the earth, the temperatures are so intense that rock is in a molten state. It is not only less dense than the solid rock that lies above it, it is also being forced upward by pressures below. The fluid rock, upon finding a crack or weakness in the solid stone above it, bursts through. This is simplified, but it is what a volcanic eruption is. The moment the magma reaches the surface, it is lava. However, the eruption can be quiescent or extremely explosive.
Even under pressure, the more liquid the magma is, the more apt it is to flow easily. It can still spout hundreds of feet into the air, however it is rarely explosive, because the fluid rock is apt to run easily and for other reason yet to be explained. This is the case at the Hawaiian Islands. When the magma reaches the surface, there is little to prevent it from flowing. The lava moves quickly, usually, covering many feet per minute and sometimes much more. This is the kind of magma that is characteristic of quiescent volcanic eruptions. They can be spectacular, and the intense heat can cause a great deal of destruction, however the volcano doesn’t explode in a cataclysm.
It is an entirely different story when it comes to thick magma. When the liquid rock is thick, it doesn’t move freely. Pressures can build up, until the fluid is forced out explosively. This kind of eruption can eject millions of tons of solid rock out of the ground, pulverizing some of it into fine ash and still with enough force to hurl house sized boulders many miles. Mount St. Helens is an example of an explosive eruption. When it blew, it forced ash many miles into the sky, literally knocked down thousands of acres of growing, healthy trees and unleashed a super-heated blast of gas and rock near the surface.
Part of the difference between the two kinds of eruption is because of the gases in the rock. Rock, whether fluid or solid, contains gases. However, when it is liquid and is under pressure, it will nearly always move toward a balanced state, which is to escape toward lower pressure. Therein is the crux of the matter.
Fluid magma and lava releases the gas easily, into the lesser pressure of the atmosphere. However, thick magma and lava holds it in. The difference is similar to boiling water and thick pudding, respectively. The gases, trying to escape, exert an enormous pressure against anything preventing it.
It is mostly the composition of the lava and magma that makes the difference between it being thin or thick. To be more precise, it is primarily the amount of silica that the fluid rock contains, that makes the magma thick or thin. Silica is a major constituent of sand, and the more there is, the thicker the magma or lava becomes. This is similar to mixing flour to gravy or soup to make it thicker.
The problem is that when the magma gets thick, the gases must force themselves out of the fluid rock. An explosive eruption is born.
Scientists don’t have a way to accurately predict when an eruption will occur. However, by measuring the amount of silica below the eruption point, they can predict if the eruption is going to be explosive or not, or to what degree. Nature loves to throw in a twist, though, and this is a case in areas where it happens. This is because even if the silica content can be measured, a volcano can have magma that is low in silicon. Years later, the same volcano can be high in silicon.
Still, this is what determines how explosive a volcanic eruption will be.