The Icelandic volcano Eyjafjallajokull dominated the media for weeks during its series of eruptions this spring. Its massive ash plumes stranded millions of global airline travelers and captured headlines around the world.
The event was as breathtaking as it was disruptive, but does it signify something far more serious? Are volcanic eruptions on the rise?
Not according to Lee Siebert, director of the Smithsonian Global Volcanism Program (GVP). Charged with documenting, analyzing and disseminating information about Earth's active volcanoes, the GVP boasts 40 years of data to indicate it's business as usual under the crust.
"If you plot data from the last 200 years, there's a clear increase in the number of eruptions over time," Siebert said, "but that's not a function of the actual number of eruptions but rather due to reporting effects."
Specifically, the GVP found that the increase in volcanic eruptions paralleled the rise in global population. It paralleled human encroachment of volcanic areas and improvements in communication technologies. Think of it as the "if a tree falls in the forest" effect. With more people around, and better technology, it became harder for a volcanic eruption to go unnoticed.
"If you plot the data over time, you do see peaks and valleys with an overall upward trend," Siebert said, "but a lot of those specific peaks or troughs can be attributed to individual reporting events. For example, there are prominent troughs that correspond to World War I and World War II, times when people's attention was focused on other issues."
During those periods of global strife, Siebert said, we simply didn't record eruptions with the same regularity. Conversely, reporting jumped following the dramatic eruptions of Krakatoa in 1883 and Mount Pelée in 1902. Sensitized by such intimidating examples of volcanism, the world took greater notice of smaller occurrences, at least for a time.
Observation bias aside, global volcanism has remained steady throughout recorded history. You have to view the planet from a geologic perspective, spanning millions and billions of years, to glimpse any major changes — or pinpoint an eruption higher than a seven on the Volcanic Explosivity Index (VEI).
"Those are the Yellowstone-type eruptions, sometimes referred to as 'supervolcanoes,'" Siebert said. "None of those have occurred in historical time."
The theory of plate tectonics divides the surface of the Earth (or lithosphere) into an interconnected puzzle of gigantic sections, or plates. Sometimes these plates rub against each other at transform boundaries. Other times, however, subduction occurs, a process that pushes one plate under another and, consequently, raises mountains, stirs earthquakes and generates volcanic activity.
"The tectonic plates are driving volcanism for the most part," Lowenstern said. "And over geologic time, those processes are turning on and off. Subduction zones begin and end. Hot spots are generated and thrive for millions of years and then just stop. So as a result of those effects, you get increased volcanism or decreased volcanism in certain areas and even globally."
The formation and melting of glacier ice can also theoretically affect volcanism. But to find a possible example of this, Siebert said, you'll have to look back to the beginning of the Holocene epoch 10,000 years ago.
"There were depressions in the Earth's crust due to glacial ice during the ice ages," Siebert said. "That ice melted back, resulting in a rebound effect that has been related by some to an increase in volcanic eruptions during that time."
Still, Siebert and Lowenstern stress that the relationship between melting glacier ice and increased volcanism is far from cut-and-dry.
"It's a lot more complicated because ice is melting in one place, and the water is going somewhere else," Lowenstern said, "So you might have a decrease in pressure in the northern latitudes as a result of ice melting, but you also might have an increase in the ocean depths in the south that might keep magma from erupting. And even then, it's not a simple relationship between increased pressure and decreased volcanism."
Several other factors also influence the planet's volcanism, some of which scientists don't fully understand. So while a few studies predict future climate change may generate a rebound effect, climate typically plays an indirect role.
"Volcanism typically influences climate," Lowenstern said, "not the other way around."