In February of 1942,  Mexican farmer Dionisio Pulido thought he heard thunder  coming from his cornfield. However, the sound wasn’t coming  from the sky. The source was a large, smoking crack  emitting gas and ejecting rocks. This fissure would come to be known as  the volcano Paricutin, and over the next 9 years, its lava  and ash would cover over 200 square km. But where did this new volcano come from, and what triggered  its unpredictable eruption? The story of any volcano  begins with magma. Often, this molten rock forms  in areas where ocean water is able to slip into the Earth’s mantle  and lower the layer’s melting point. The resulting magma typically remains  under the Earth’s surface thanks to the delicate balance  of three geological factors. The first is lithostatic pressure. This is the weight of the Earth’s crust  pushing down on the magma below. Magma pushes back with the second factor,  magmastatic pressure. The battle between these forces  strains the third factor: the rock strength of the Earth’s crust. Usually, the rock is strong enough  and heavy enough to keep the magma in place. But when this equilibrium is thrown off,  the consequences can be explosive. One of the most common causes  of an eruption is an increase  in magmastatic pressure. Magma contains various elements  and compounds, many of which are dissolved  in the molten rock. At high enough concentrations, compounds  like water or sulfur no longer dissolve, and instead form  high-pressure gas bubbles. When these bubbles reach the surface, they can burst with the force  of a gunshot. And when millions of bubbles  explode simultaneously, the energy can send plumes of ash  into the stratosphere. But before they pop, they act  like bubbles of C02 in a shaken soda. Their presence lowers  the magma’s density, and increases the buoyant force  pushing upward through the crust. Many geologists believe this process  was behind the Paricutin eruption in Mexico. There are two known natural causes  for these buoyant bubbles. Sometimes, new magma  from deeper underground brings additional gassy compounds  into the mix. But bubbles can also form  when magma begins to cool. In its molten state, magma is a mixture of dissolved gases and melted minerals. As the molten rock hardens, some of those  minerals solidify into crystals. This process doesn’t incorporate  many of the dissolved gasses, resulting in a higher concentration  of the compounds that form explosive bubbles. Not all eruptions are due  to rising magmastatic pressure— sometimes the weight of the rock  above can become dangerously low. Landslides can remove massive quantities  of rock from atop a magma chamber, dropping the lithostatic pressure  and instantly triggering an eruption. This process is known as “unloading” and it’s been responsible  for numerous eruptions, including the sudden explosion  of Mount St. Helens in 1980. But unloading can also happen  over longer periods of time due to erosion or melting glaciers. In fact, many geologists  are worried that glacial melt caused by climate change  could increase volcanic activity. Finally, eruptions can occur when  the rock layer is no longer strong enough to hold back the magma below. Acidic gases and heat escaping from magma can corrode rock through a process  called hydrothermal alteration, gradually turning hard stone  into soft clay. The rock layer could also be weakened  by tectonic activity. Earthquakes can create fissures  allowing magma to escape to the surface, and the Earth’s crust  can be stretched thin as continental plates  shift away from each other. Unfortunately, knowing  what causes eruptions doesn’t make them easy to predict. While scientists can roughly determine  the strength and weight of the Earth’s crust, the depth and heat of magma chambers  makes measuring changes in magmastatic pressure very difficult. But volcanologists are constantly  exploring new technology to conquer this rocky terrain. Advances in thermal imaging  have allowed scientists to detect subterranean hotspots. Spectrometers can analyze  gases escaping magma. And lasers can precisely track the impact  of rising magma on a volcano’s shape. Hopefully, these tools will help us better understand these volatile vents and their explosive eruptions.