In the northern part of the Indonesian island of Sumatra lies the Toba caldera, a massive crater formed by what scientists think is the largest volcanic eruption ever experienced by humanity. The eruption, called the Youngest Toba Tuff supereruption, took place about 74,000 years ago.
By dating zircon, a diamond-like gemstone, and other minerals in the area such as quartz, Reid and Vazquez have pieced together clues about the activity of magma below the surface prior to the supereruption.
Zircon is the oldest dated mineral on Earth. With a hardness rating of 7.5, it is resistant to chemical and mechanical weathering and can withstand metamorphism (structural changes due to heat, pressure, and other natural processes). All of these factors make it an ideal mineral for geological dating, especially for magma. Because zircon does not gain or lose uranium or lead even at magmatic temperatures, zircon typically contains high uranium and low lead levels, and scientists may use the ratio of these two elements in the zircon to determine the age of the sample.
The way in which zircon crystals in the Youngest Toba Tuff magma appear to have nucleated and grown over time, the researchers found, provides evidence of intermittent changes in the composition of the underground body of magma that eventually erupted. Certain characteristics of the zircon also indicate repeated episodes of magma recharge—fresh influxes of magma that often trigger eruptions—occurring tens of thousands to hundreds of thousands of years before the supereruption.
The team’s findings are significant for modern-day humans, given that aerosols and ash that erupted from Youngest Toba Tuff are thought to have entered the atmosphere, causing global cooling and the near extinction of the human race. A supereruption of equal or greater magnitude today could therefore have similarly drastic consequences. By better understanding the conditions that led up to the Youngest Toba Tuff supereruption, scientists can help paint a clearer picture of the future. (Geochemistry, Geophysics, Geosystems, https://doi.org/10.1002/2016GC006641, 2017)
—Sarah Witman, Freelance Writer