Every lake has a story, and the story of Indonesia's Lake Toba is an explosive one. Toba is actually one of the world's largest calderas—the aftermath of a supereruption roughly 74,000 years ago that spewed ash across South Asia and left deposits nearly 2,000 feet thick near the main vent. Some have suggested that following the eruption, a "volcanic winter" shrouded planet Earth in cold and sulfur dioxide. The ensuing conditions may have caused a near-extinction of humans, although this theory is strongly debated. Regardless of its impact on the human race, the event was spectacular. Today, the 62-mile long lake can be seen from space. An island in the middle is a major tourist attraction and home to roughly 100,000 people.
But the story doesn't end there. Researchers at Oregon State University, including graduate student Adonara Mucek, are investigating what happened after the supereruption, a period in Toba's history that might seem relatively tame. If you compared it to a film, it might be the scene following a dramatic car chase. Shan de Silva, an Oregon State geology professor and Mucek's advisor, calls it the "after-party after the big dance."
In geologic terms, it's called resurgence, the process by which the floor of the caldera pushes back up and forms a dome. In the case of Toba, this forms an island, as the caldera hosts a deep lake. Resurgence may not have the fanfare of a massive explosion, but knowing when and how it happens is crucial to understanding future volcanic activity.
"Resurgence is manifested through structural uplift at large calderas, as a result of remnant magma pushing up the caldera floor," Mucek explains. "We know this, but there are a lot of unanswered questions. When does it happen? When does it start? How long does it last? And why does it happen?"
Mucek is particularly motivated to answer those questions because the Indonesian government recently designated Lake Toba as a UNESCO Global Geopark. She and her colleagues are working with the Indonesian government on educational outreach to people living near the lake. Raised in Singapore, Mucek is fluent in Indonesian and has been able to navigate the cultural aspect of her research with relative ease.
In addition to education and outreach, Mucek is combining field work, modeling and spatial analysis to gather clues about Toba's storied past. With support from OSU's Provost Fellowship, an NSF Graduate Fellowship and two GSA student grants, she has traveled to the research site several times to collect sediment samples and lava dome rocks. Analyzing the ages of each will allow her to sequence the geologic events over the last 74,000 years. So far, Mucek knows that the eruption left a giant hole in the ground. Precipitation filled in the hole, eventually creating a lake and depositing sediment. Then, remnant magma started pushing the ground up along some lines of weakness.
Shan de Silva compares this process to a water droplet splashing into a pool. The impact causes a depression, but once the initial force has dissipated, the water flows back in, filling in the depression and creating a peak.
"In calderas, when the roof collapses into the magma chamber to form the caldera itself, the way that's accommodated is the fluid magma moves out of the way. But eventually, once the forces relax, the magma wants to move back to where it was and pushes the floor back up to produce resurgence," he explains.
With a water droplet, this process happens instantaneously. But because rocks and magma are 1021 to 107 times respectively more viscous than water, it can take about 100,000 years for calderas to readjust, so to speak.
About 30,000 years ago, Toba's resurgent dome broke through the surface of the lake. Subsequent activity caused faulting along the already-present lava domes, leading to the Toba we know today.
But what about the Toba of tomorrow? No one alive on Earth will be here long enough to find out, but Mucek's modeling and spatial analysis may soon give us an idea. She is using surface process modeling to compute the evolution of Toba's topographic surface over time to understand the "ingredients" of resurgence. Basically, what mixture of landform development, processes and topographic features leads to resurgence?
Spatial distribution techniques will allow her to determine if the resurgent uplift on Toba's island can be detected from above. Combined with her field work, Mucek is using these methods to help tell the whole Toba story, now and in the years ahead.
"If we can better understand resurgence at Toba, then we can apply it to other super volcanoes. For example, Yellowstone also has resurgence, and other calderas across the world are going through the same process," she said. "It's important to understand these processes that are occurring today to figure out what will happen in the future."