The notorious E15 in pictures

My sedimentologist friend is fond of saying that he likes looking at rocks “because they tell stories.” Iceland’s volcanic rocks are still in the process of writing their story, and I had the privilege of reviewing both the ancient and the hot-off-the-press chapters, alongside meteorologists and volcanologists at the AGU Chapman Conference on Volcanism and the Atmosphere held in Selfoss, Iceland, in June 2012. During a field trip to Eyjafjallajökull, 115 scientists spent the day taking in the rugged, inhospitable landscape sculpted by water, fire, ice, and ash. The experience of scrambling around on rocky features that look permanent but are rapidly changing can only be described as humbling.

The summit cone of Iceland’s Eyjafjallajökull (or E15—count the letters) volcano rises high above the otherworldly gray plains of Iceland’s Eastern Volcanic Zone. In early 2010, ground inflation suggested that magma was stirring beneath the glaciated slopes of the volcano. A “nice tourist eruption” in which a basaltic fissure opened and released red fire fountains on the northeast flanks preceded the summit eruption on 14 April of that year. Potentially triggered by injection of basalt into a silicic magma chamber, the intermediate-alkaline magma flow worked its way through the glacier above, and the ash plume reached heights that halted jet travel for weeks. On the north side of the mountain, glacial melt charged down the slopes to the lake below, broke the terminal moraine holding it in, and caused striking changes in the landscape.

The approach to Eyjafjallajökull follows a valley carved by glaciers. The lava that flowed along the tops of the foothills to the west (former sea cliffs) also flowed onto the valley glacier, which became a floodplain when the glaciers retreated 10 000 years ago. Glacial runoff continues to form river channels, which are impassible to all but the most intrepid travelers.

Before 2010, a 60-m-deep lagoon with floating icebergs abutted a calving glacier and filled the area beneath the north side of Eyjafjallajökull. The glacial moraines shown here—deposits carried by glaciers during the last ice age—held the lake in. After the 2010 eruption, the Eyjafjallajökull glaciers melted, and the subsequent flooding broke the terminal moraine barrier, which allowed the lake to flood the valley. The lake’s surface would have been tens of meters below the alluvial plain that remains today.

When the mountain glaciers began to melt during the eruption, water shot out of the crevassed sides of the slopes, further eroding the deep channels. Water flux measured 2500 cubic meters per second in the first 20 minutes of flooding. Then, the water meter got washed away. Now the glacial tongue reaches to the base of the northern slopes of Eyjafjallajökull, and small streams tumble down the gorges.

The force of the water as it roared down the lake bed and eventually out to the sea is indicated by the size of the boulders on the valley floor. Large, person-sized boulders are primarily found near the base of the mountain where the flow started. Those transition to football- and smaller-sized rocks and fine grains as the water became less forceful because it moved farther away from its source.

Icebergs, too, were deposited on the alluvial planes by the floods. They eventually melted where they were left, leaving crater-like pits. In some cases, those kettle pits remain filled with water and even algal growth: Ridges on the other side (inset) indicate water levels in the pit over time.

Eyjafjallajökull is a subglacial volcano: It erupted beneath a glacier. The spires of basalt on the upper edges were built underneath the glacier when chilled lava shattered due to rapid contraction. The features shown here are from older eruptions—the background rock is Quaternary. It formed more than 10 000 years ago, when the last ice age ended, but less than 780 000 years ago, when Earth’s magnetic field changed.

Lava flows from the 2010 eruption did not reach the northern valley floor, yet there are some new lava rocks on the former lake bed. These chunks of shiny black obsidian formed higher up on the slopes and were carried down in the flows. Their smooth, glassy quality indicates that they cooled rapidly above ground when the magma came into contact with water and air. The size of green olivine crystals in similar rocks (and, in general, of crystals in rocks) indicates how much time the rock had to cool and, therefore, the depth at which it formed: Larger crystals, which take longer to form, indicate that the rock cooled slowly underground. Smaller crystals form rapidly, indicating that the rock cooled quickly.

Hardly any ash from the 2010 eruption is visible on the surface. The top layer of soil near the volcano is all remobilized ash from wind and ice melt. But the tephra flow from 10–20 May left a fine gray layer, 5 cm thick, that is apparent just 10 cm below the surface in some areas.

Ignimbrites are deposits that settle out from pyroclastic flows, or density currents of volcanic particles that travel along the ground. These ignimbrites from Tindfjallajökull volcano represent a 52 000-year-old time marker in sediment cores in the North Atlantic Ocean. The lava flow on top is a satellite cone of Eyjafjallajökull. A deposit can be identified as coming from a density current when it contains mixed grains: In an ash cloud deposit, particles would be deposited in order of size. Here, ash and pumice are mixed together.

Forty kilometers east of Eyjafjallajökull lies Katla volcano, under the Mýrdalsjökull ice cap. Although Katla sits above a separate magma chamber, historic eruptions of Eyjafjallajökull have preceded eruptions of its neighbor. The Eastern Volcanic Zone, which includes those two volcanoes, is and has been the most active volcanic region in Iceland. It will be exciting to see what happens next.