An eruption occurs whenever molten rock, debris, and gases from Earth’s interior reach the surface. The formation of a volcano begins when magma first breaks through the surface of Earth and erupts as lava. Solidifying quickly, this lava has the potential to become mountains of igneous rock.

If pressure caused by expanding gases and magma builds beneath the surface, eruptions can be explosive, potentially capable of wiping out cities and towns. Other eruptions can be quiet, occurring with very little consequence as soupy lava oozes from fissures and cracks within the earth.

Viscosity of Magma

The viscosity of molten rock has a lot to do with how a volcano forms and the characteristics of an eruption. Viscosity is a liquid’s resistance to flow. Molten rock can vary in viscosity. Some is highly viscous and has the consistency of wet cement. Less viscous magma flows more easily with a consistency like running water.

The viscosity of molten rock is affected by its chemical composition and temperature. Molten rock that is high in silica produces highly viscous, light colored lava that does not flow very easily. This lava can produce granite. Molten rock having low silica content is usually darker, flows more easily, and forms rock like basalt.

The temperature of molten rock also varies largely. Higher temperatures lead to less viscous lava that flows easier than lava with lower temperatures.

Explosive Eruptions

Picture yourself blowing air through a straw into a glass of water. As gases travel from your lungs to the water, you see gas bubbles rise to the surface. As they reach the surface, the bubbles release the gas and make a small popping sound.

If you blew bubbles into a glass of mud, larger bubbles would rise to the surface. When they reached the surface, these larger bubbles would release the gas as an explosion, spreading mud in all directions.

Magma high in silica is thick and sticky like the mud in the glass. The high viscosity of this magma prevents it from releasing trapped gas as effectively as magma with lower viscosity. Trapped gas results in pressure build-up, which can lead to explosive eruptions. The explosive nature of some of these eruptions can blast solid pieces of rock and large amounts of volcanic ash thousands of meters into the air. This is often combined with a pyroclastic flow, which is an avalanche of gas, ash, and rock flowing down the side of a volcano, leaving destruction in its path! These kinds of eruptions are common in subduction zone volcanoes, such as at Mount St. Helens and others in the "Ring of Fire" around the Pacific Ocean.

Effusive Eruptions

Magma with low silica content is less viscous, which leads to effusive, or quiet, eruptions. These eruptions are not as explosive, as the erupting lava can flow quite easily and can continue to flow many kilometers from its source, as it does on the island of Hawai'i. Low silica magmas are called basaltic magmas which cool to form black rocks.