Inside Iceland’s volcanic history

The surrounding soil and rock provided insulation that kept the lava tunnel hotter than paths the lava might have taken on the surface. Basalt glass formed at the solid-liquid border, resulting in tunnel walls with smooth horizontal grooves, seen here in the layered structure. In other parts of the world, the chemical composition of the local lava means obsidian forms instead.

Credit: Craig Bettenhausen/C&EN

Because of the tunnel’s insulation and smooth inner surface, the lava drained out as the eruption subsided instead of hardening inside. The process created a long cave 10–30 m in diameter. As it passed through, the lava deposited a surface layer bit by bit: it’s smooth horizontally, ribbed vertically, and glassy at the centimeter scale. At first, the entire interior surface had the striated, glassy appearance still seen in many sections. Over time, chunks fell from the walls and ceiling, leaving gaps like this one, which stretches across roughly 2 m, similar in size to a chaise longue.

Credit: Craig Bettenhausen/C&EN

As molten rock rose from the high-pressure environments in the Earth’s mantle into the lower-pressure zones in the crust and on the surface, some components of the melt vaporized. That escaping gas left holes (seen in this closeup photo) in the solidifying material, often near the upper sections of the cooling lava flow. Out in the open air, rapid cooling and ambient pressure resulted in rough, porous stones such as pumice. In some parts of the lava tunnel, intermediate pressures and cooling rates created rocks with a glassy surface but thousands of voids at centimeter scale.

Credit: Craig Bettenhausen/C&EN

Partial tunnel collapses revealed the rock behind the glassy deposit, exposing formations that are chemically distinct from the volcanic basalt. Most of the local rock is composed of silicates. Accumulations of iron, sulfur, and copper on the exposed surfaces created the pockets of red-orange, yellow, and blue seen here.

Credit: Craig Bettenhausen/C&EN

Raufarhólshellir has a few “skylights,” where collapses opened holes in the tunnel roof, like the two letting sunlight into this section. But most of the length of the tunnel is pitch dark. Without light, the tunnel supports very little life.

Credit: Craig Bettenhausen/C&EN

The tunnel’s owners and operators have strung up lights for tours. As a result, some spots, which for thousands of years were bare, now have moss (green patches) taking advantage of the few hours a day of artificial light.

Credit: Craig Bettenhausen/C&EN

Aside from those interlopers, the cave’s few native organisms are pale, hydrophobic microbes, seen here in the right half of the photo as yellow and white patches on the cave’s ceiling. The majority are chemolithotrophs or chemoautotrophs. The names indicate that these organisms subsist on chemical energy they glean by oxidizing inorganic compounds containing iron, sulfur, hydrogen, and nitrogen.

Credit: Craig Bettenhausen/C&EN

Lava stalactites (seen here) are a curious feature found in lava tunnels around the world. Plenty of cave systems have dagger-like stone columns hanging from the ceiling that result from minerals deposited by water dripping over the course of thousands of years. Despite the similar appearance, lava stalactites formed rapidly from dripping lava. And though water does drip day and night from many of the lava stalactites in Raufarhólshellir, they don’t grow. The local groundwater doesn’t have enough calcium to make deposits as it drips.

Credit: Craig Bettenhausen/C&EN

Scientific insights gleaned from scrutinizing Raufarhólshellir have helped researchers find and understand the inner workings and geochemistry of hundreds of other lava tunnels across Iceland. This site is mostly a tourist attraction now, but research continues here and could carry the formation's influence far beyond Iceland. Astrobiologists from NASA visited recently to sample the hardy microbes living deep inside; they’re hoping for insight into what kind of life scientists might find on Mars.

Credit: Craig Bettenhausen/C&EN