By Nicole Van Vleck
Volcanic lightning, also known as dirty thunderstorms, is capable of producing some of the most powerful and visually striking lightning storms on Earth. Not all volcanic eruptions produce lightning. When it does occur, a single eruption can create multiple, separate lightning storms. The lightning seems to be set off by explosions during the eruption, although not all explosions cause lightning. During recent eruptions that displayed lightning, scientists observed the most intense lightning occurred at the beginning of the eruption and gradually decreased as the eruption continued, sometimes over a period of weeks.
The earliest recorded sighting of volcanic lightning comes from letters written by Pliny the Younger to the Roman historian Tacitus, describing his eyewitness account of the eruption of Vesuvius in 79 AD. Photographic evidence of volcanic lightning has been around since at least 1944, courtesy of the U.S. military, which captured a photograph of Mt. Vesuvius erupting.
Lightning appears to be a relatively common feature of volcanic eruptions, originating either at the mouth of the volcano or higher up within the ash cloud, possibly originating for different reasons. Scientists are still unclear about what causes volcanic lightning, but they have identified several factors that may play a role.
Lightning is the redistribution of energy among charged particles, and volcanic lightning is no exception. The mystery lies in how volcanic eruptions create charged particles. Do the particles become charged when they break apart or when they collide or both? For instance, the charged particles that create lightning at the mouth of volcanoes might be the result of magma breaking into smaller particles as it exits the volcano, while scientists believe the charged particles within the ash cloud are likely caused by the friction of particles colliding.
Other factors that seem to influence volcanic lightning include the height of the ash cloud and the air temperature. During the 2009 eruption of Mt. Redoubt in Alaska (USA), a team of researchers from the New Mexico Institute of Mining and Technology observed, in general, that the higher the ash cloud the more lighting occurred. Researchers covering the 2010 eruption of Eyjafjallajokull in Iceland discovered that lightning was only present when the temperature at the top of the ash cloud dropped below -20 degrees Celsius, the freezing point for supercooled water droplets in the atmosphere.
Scientists are working on developing a system to monitor volcanic lightning in an effort to detect volcanic activity in remote areas. If successful, this could be used to help alert cargo and passenger air traffic to potential dangers from remote volcanoes that are located along heavily traveled flight routes. There are currently two ways to track volcanic lightning. The first uses low frequency radio waves that can detect lightning from more than 10,000 kilometers (6,200 miles) away. The second is very high frequency (VHF) radio emissions. When using the VHF method, the sensors must be in sight of the volcano, but these sensors can pick up weaker lightning discharges and produces much better resolution.
Due to the remote location of many volcanoes, it is unclear how many eruptions include lightning, but judging by the eruptions that have been documented by scientists as well as photographers, it appears volcanic lightning is neither rare nor is it confined to a specific part of the world. Recent examples of volcanic lightning include: Sakurajima (Japan) 1991, Chaiten volcano (Chile) 2008, Shinmoedake peak in the Kirishima volcano complex (Japan) January 2011, Puyehue volcano in the Puyehue-Cordon Caulle volcano chain (Chile) June 2011, Pacaya Volcano (Guatemala), Krakatoa (Indonesia), Mt. Augustine (Alaska, USA), and Mount St. Helens (Washington, USA). No matter where volcanic lightning occurs, it always provides a striking reminder of nature’s power.