(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

The settlement of Kalaupapa on Molokai. Photo by Baron Sekiya | Hawaii 24/7

Last fall, residents of Kalaupapa settlement on the Island of Moloka`i began reporting unusual sulfurous odors coming from the direction of Kauhako Crater lake, a small, deep lake located just a few kilometers (miles) away. An understandable question was quickly raised as to whether this might be a sign of impending volcanic activity. After all, hydrogen sulfide (H2S) is a known volcanic gas, and people were smelling the gas with the rotten egg odor from a volcanic crater for the first time in over 50 years. Might something be up, volcanically speaking?

Not likely. Kauhako Crater, which hosts the lake, is an extinct eruptive vent that formed at the top of Pu`u `Uao lava shield some time between 340 and 570 thousand years ago. Long ago, Moloka`i drifted away from the hot spot that supplies the magma currently driving eruptions on Hawai`i Island. So, while not impossible, renewed volcanism on Moloka`i at this stage in its life would be an extremely rare event.

Fortunately, the National Park Service (NPS) had been studying the water quality and biological aspects of Kauhako Lake for some time as part of their Inventory and Monitoring Program, so they responded immediately to the H2S reports. But before we talk about what they found, we should say something about the lake itself.

The settlement of Kalaupapa on Molokai. Photo by Baron Sekiya | Hawaii 24/7

Kauhako is identified as the only truly deep lake in the Hawaiian Islands. The lake’s modest footprint of 0.35 hectares (0.86 acres) and its maximum depth of about 250 m (820 ft) give it the largest depth-to-area ratio for any lake on Earth. In profile, the lake looks more like a tall test tube or, perhaps, a slender champagne glass than a typical dish-shaped lake.

Upon closer study, the lake becomes even more interesting. Measurements of Kauhako carried out in the early 1970s showed it to be meromictic lake. This term is used to describe a lake comprised of multiple layers that usually don’t intermix. In the case of Kauhako, the top layer is made up of brackish (salt and fresh) water up to a few meters (yards) thick. This oxygen-containing upper layer supports, among other things, microscopic phytoplankton and small native shrimp. The lake’s surface is situated at sea level but has no obvious connection to the ocean, 1.6 km (1 mi) away.

Investigators exploring the 800-foot-thick bottom layer found it to be of essentially sea-water composition, although, unlike the Pacific Ocean, it is devoid of oxygen. Interesting chemistry happens in this layer, however. Through a series of reactions, sulfate ions react with ordinary organic matter—leaves and other debris that fall into the lake—to generate hydrogen sulfide. Because the lower and upper water layers don’t mix, this H2S remains trapped (dissolved) in the lower water layer and slowly accumulates over time.

Now we return to last November, when the NPS responded to Kalaupapa resident reports of H2S odor wafting in from Kauhako. When they began their descent from the crater rim, NPS staff could easily detect the tell-tale rotten egg odor of H2S. The reason for this became apparent as they reached the lake’s surface. The top water layer had effectively disappeared, either through mixing with the bottom layer in a rare overturning of the lake or, because of the dry conditions that had recently been present, caused it to evaporate completely. Whatever the precise cause, the salty, sulfide-rich waters, formerly trapped, were at the surface of the lake, where H2S could escape to the atmosphere. The fresh-water aquatic life—phytoplankton and shrimp—were noticeably absent, unable to live in this new environment.

Since that time, the lake has seemingly struggled to regain its former layered character. As a precaution, NPS, in consultation with HVO, began monitoring hydrogen sulfide, carbon dioxide, and oxygen levels within Kauhako Crater. As a further affirmation that this was a distinct lake “event” rather than a volcanic one, HVO, using remote satellite methods and other instrumentation, has been monitoring for obvious changes in surface temperature, deformation, and seismicity.

This occurrence at Kauhako Crater reminds us that natural systems are complex, and whether the lake is filled with water or lava, there’s often a lot going on beneath the surface.