Volcanoes Eruptions Volcanic Geoscience Geology

What Causes a Volcano to Erupt?

Ancient civilizations believed volcanic eruptions were the actions of  gods and demigods. The ancient Greeks and Romans believed that the awesome power of volcanoes could only be explained as a punishment and wrath from the gods.  Over the centuries, many explanations were proposed for the cause of volcano eruptions before the modern understanding of the Earth’s mantle and inner structure was achieved.  Volcanic eruptions enable hot, magma, gas and ash to be released into the atmosphere resulting in lower pressure within the volcano.  Three factors which trigger volcanic eruptions.  The buoyancy of the lava, gases within the magma, and new magma added to a magma chamber.

One cause of volcanic eruptions is the buoyancy of the lava. When the rocks transform into molten lava, the mass of the molten rock stays the same, but its volume expands. This results in lava that is less dense than the cooler rock it is created from.  This low density lava is able to rise above higher density lava within the magma chamber. The lava, with enough pressure from within the magma chamber, will rise to the summit of a volcano and cause a volcanic eruption. 

Each type of volcano is formed from lava having a unique composition of silica, minerals, and gases existing within its magma chamber. This results in each type of lava having its own characteristic temperature, buoyancy, and density. These factors dictate the intensity, severity, and type of the eruption that will occur. Lavas containing silica are viscous, trap volatile gases and cause the magma to erupt catastrophically.

The pressure created from gases within the magma trigger volcanic eruptions. The lava creates gases such as water vapor (steam), carbon dioxide, hydrogen chloride, hydrogen fluoride, and sulphur dioxide. It is the expansion of these gases that pushes the lava toward the summit of a volcano. Increasing pressure from this gas themselves pushes the magma higher. In order for the gas to escape and relieve the pressure, the volcano erupts.

A volcanic eruption can also be caused by new magma flowing into a magma chamber. As the new magma enters, the pressure within the magma increases, causing the volcano to erupt. These forces happen rapidly and with so much pressure causing the gases and magma to explode out of a volcano.  This explosion occurs with so much force it travels for miles. 


Foulger, G.R. (2010). Plates and Plumes: A Geological Controversy Wiley-Blackwell.

Douglas Harper (November 2001).”Volcano”. Online Etymology Dictionary. Retrieved 2009-06-11.

“Volcanoes”. U.S. Department of the Interior, U.S. Geological Survey.