Earth Sciences, Natural Disasters
Etna’s Changing Landscape
At 11,000 feet, Mount Etna is tall enough and cold enough to preserve snow in its ash. Credit: Meg Reitz.
Boris and Alfio, geologists at Sicily’s National Institute of Geophyscis and Volcanology picked us up in their four-wheel drive jeeps. Etna is a stunning image. She rises 3,300 meters right from the seafloor, towering over the towns located around her flanks, providing fertile land for farming and beautiful hiking and skiing. Alfio calls her their “Sicilian Mother”: bountiful and beautiful, but able to flare up at a moments notice.
We drive up the base of Etna studying the lava flows visible on the road cuts. Lava from a 1690 eruption traveled 45 km to Catania, destroying much of the city, before pouring into the Ionian Sea. As we make our way up the lava gets younger: 1700s, 1983, 1991-2, until we finally reach the tourist center where lavas in 2001 and 2002 lavas destroyed several buildings. There is a cable car that takes people from the tourist camp to 2,500 meters. The cable car was first built in the 1970s so people could more easily reach the summit. Periodic lava flows have destroyed it four times in 40 years. The current one was rebuilt after the 2002 eruption.
At base camp, we stop to pick up Doug and Diane, two videographers accompanying us up the mountain. Boris and Alfio also grab a caffé (an Italian staple). We pass through the gates for authorized personnel only, getting annoyed looks from the tourists who have to pay to ride the cable car or trudge up themselves.
We’ve driven about halfway up, when we notice two large hills covered with ash towering over us. In 2000, the area was a flat expanse of ash without these features. Within a year, magma beneath Etna had generated these two massive cones.
Boris says that every time he comes up to Etna he takes dozens of photos and that in the seven years has accumulated hundreds of photos of features that are no longer part the landscape. We so often think of mountains as slowly growing features that may set off an earthquake every few decades, but rarely change within our lifetime. And here is Etna that, like all active volcanoes, changes completely every few years, even without a major eruption.
We park the jeeps around 2,800 meters and begin to hike across thick deposits of windblown ash. We can see traces of snow that fell this year or several years ago, preserved under the ash. The walking gets tough as the ground turns to lava called A’a (for its Hawaiian counterpart).
A’a is crumbly, sharp, and painful to grab onto if you lose your balance.
Further up we start to see rocks of hydrothermal origin. These are composed of minerals that crystallize from water heated inside Etna (sulfur is the most common mineral). We’re still far from Etna’s active caldera, so these are rocks that were ejected from the caldera during Etna’s numerous explosions, or burps as Nano calls them.
We make the last scramble across a 40 degree slope to edge of the Etna’s most active caldera, where enormous fountains of lava erupted in 2008.
Over a period of eight months, 66 lava fountains gushed into the air. (Compare this to Mauna Loa’s 46 lava fountains in three years.)
So here we are, standing right next to it.
The rocks are coated in soft ash from explosions earlier this year, in April. They are warm to the touch from the magma just beneath the surface. Walking around, we come across vents of hydrogen sulfide under our feet. If the breeze blows the wrong way for too long, the smell of rotten eggs is overwhelming, burning your eyes, nose, and throat. Boris said he’s breathed in so much hydrogen sulfide, he has destroyed much of his sense of smell.
The trek back down Etna is treacherous, but beautiful. It’s a relief to finally make it back to the soft ash and our jeeps. Those of us here for the first time – myself, my parents who are visiting from Massachusetts, Doug, and Diane are nearly speechless with awe and wonder.
The next morning Boris calls to tell us that the caldera edge we were hiking along had collapsed into the caldera. The powerful, scary Etna had changed the landscape once again. I agree with Alfio: a Sicilian Mother after all.
Tags:Calabrian Arc ProjectEarthquakesLamont-Doherty Earth Observatory
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During the ~8-year period between the 1991–93 and 2001 flank eruptions, the eruptive activity of Mt. Etna was confined to the summit craters. Deformation and tomography studies indicate that this activity was fed by a magma accumulation zone centered NE of the summit, at a depth of 5 to 9 km below sea level. The most significant gravity changes measured during the same period were induced by mass redistributions at shallower depth below the southeastern flank of the volcano, where minor ground deformation was observed (i.e., vertical displacements within 2 cm).
The mismatch between the position of pressure and mass sources is difficult to explain under the assumption that both are directly related to magma dynamics. Past studies have suggested that the gravity changes observed during 1994–2001 may primarily reflect changes in the rate of microfracturing along the NNW–SSE fracture/weakness zone (FWZ) that crosses the SE slope of Etna.
We use the finite element method to shed new light on the complex relations between stress, strain and mass changes that occurred at Etna during the studied period. In particular, following previous results on the degradation of the mechanical properties of rocks, we perform a set of simulations assuming that the part of the medium containing the FWZ is characterized by a lower Young's modulus than would be expected from interpolation of tomographic data. We find that the presence of the FWZ creates a distortion of the displacement field induced by the deeper pressure source, locally resulting in a weak extensional regime. This finding supports the hypothesis of a cause–effect relationship between pressurization beneath the NW flank and tensile extension beneath the SE slope of the volcano. We propose that this extensional regime enhanced the propagation of pressurized gas, that, in turn, amplified the tensile strain across the FWZ.
We also find that decreasing the value of Young's modulus in the FWZ allows for a larger amount of extension at depth, with no change in the magnitude of surface displacements. This result provides an indication of how the changes in the rate of microfracturing at depth, which are needed to induce the observed gravity changes, might have occurred without large ground deformation.