They are fascinating, these powerful forces acting inside the Earth. Volcanic eruptions produce spectacular fire fountains, incandescent lava flows, clouds of ash, or bubbling mud pools. The shape the Earth's surface and generate new crust. However, for the people in the immediate vicinity as well as in the wider surroundings, for the climate and for air traffic, these energetic Earth-forming processes are accompanied by danger and destruction.
Local and regional impacts
The immediate surroundings of a volcano experience the most direct effects of an eruption. Explosively ejected ash disperses in the air. Together with volcanic gases it impairs people's ability to breathe. When the ash falls down, it blankets the land, including agriculturally used fields, houses, roads, industrial plants. Also lava flows may seal the area with a layer of rock. In the case of major eruptions, tephra can spread across hundreds of square kilometers and may accumulate to several metres of thickness. Pyroclastic surges (laterally flowing, turbulent density currents consisting of ash, rocks, and gas) can wipe out houses and trees. If loose rock on a volcanic edifice comes into contact with water, lahars can form that devastate the slopes down which they sweep. If a pyroclastic flow, a surge or a lahar reaches a lake or the sea, they may trigger a tsunami – a sudden rise in the water levels of the lake or sea, which leads to abrupt and usually destructive flooding of the shoreline.
Large-scale to global impacts
The explosive eruption of Eyjafjallajökull in Iceland in 2010 demonstrated that volcanic ash at higher levels in the atmosphere poses a significant threat to air traffic. Hot ash particles can clog the turbines of the aircraft and cause engine failure. Huge financial losses for the airlines due to several days of air traffic re-routing or standstills are usually the result. However, the Eyjafjallajökull eruption also gave rise for a range of investigations on the dispersal of the ash cloud using various techniques. It provided an opportunity to improve our understanding on how ash and gas particles travel in the atmosphere. Thereby, it facilitated a better definition of threshold values at which air traffic will be at risk, and what levels of particles can be considered acceptable.
Large eruptions are not without consequences in the long run either. The emission of volcanic gases modifies the composition of the atmosphere. If the gases reach higher altitudes, the effects are particularly strong and long-lasting. While the gas particles in the troposphere, the lowest layer of the atmosphere and approximately 10 km thick, are relatively quickly washed out by rain, they can remain for up to three years in the overlying stratosphere.
Different gas species have different effects in this layer. If sulphur is injected, the stratosphere heats up. The radiation entering from the outside is reflected back into space, so that less radiation arrives on the Earth's surface. The Earth cools down. This mechanism is impressively exemplified by two Indonesian eruptions in the 19th century. As a result of the Tambora eruption in 1815, there was no summer in the following year. In 1883, Krakatoa produced an aerosol cloud which went around the globe and resulted in a "volcanic winter" that lasted for three to four years. This not only caused famines but also visually spectacular, very intensely coloured sunsets, which have been captured in artists' paintings. The most recent example was the eruption of Mount Pinatubo in the Philippines in 1991, followed shortly afterwards by the eruption of Mount Hudson in southern Chile. In the wake of the strong sulphur emissions of these eruptions, global temperatures decreased for two to three years; the polar bear population in the northeast Canadian Hudson Bay exploded.
In contrast, the input of CO2 and halogens such as chlorine and bromine into the stratosphere causes a natural greenhouse effect. These gases destroy ozone at high altitudes, thus amplifying the depletion of the ozone layer. The consequences of stratospheric ozone destruction are well known: more radiation can penetrate the atmosphere; and the Earth warms up.
Text: Dr. Heidi Wehrmann,GEOMAR Helmholtz-Zentrum für Ozeanforschung Kiel