Magnetic Mystery Unveiled: Mapping Lake Rotorua’s Hidden Depths and Hydrothermal Secrets

Lake Rotorua, nestled within the crater of a dormant volcano on New Zealand’s North Island, renowned for its cultural significance and hosting a Māori love story, has undergone its first detailed mapping. Researchers from GNS Science in New Zealand have revealed previously unseen features on the lake floor, including eruption craters, an ancient river, and an intriguing magnetic anomaly in the southern part of the lake. The extensive maps, covering approximately 68% of the lake’s floor, were collected through a combination of data obtained by the Royal New Zealand Navy using a multibeam echo sounder and magnetic surveys.

The magnetic anomaly, a surprising find, contradicts the usual patterns seen in volcanic rocks. Instead of positive anomalies indicative of highly magnetic minerals like magnetite, researchers observed negative anomalies, believed to result from the transformation of magnetite into pyrite, or fool’s gold, due to hydrothermal processes. The study also identified additional signs of hydrothermal activity in the same region, such as craters and a heat flow map revealing the presence of hot water rising from beneath the lake floor.

Cornel de Ronde, a principal scientist at GNS Science, likened the impact of seeing the detailed maps to “wearing glasses for the first time when you didn’t realize you needed them.” Despite the significant hydrothermal activity, the lake’s immense size moderates water temperatures near its bottom, maintaining a relatively cool average of 57 degrees Fahrenheit (14 degrees Celsius). While swimmers may not perceive the subtle fluctuations in temperature, advanced instrumentation aids scientists in uncovering the dynamic geological processes occurring beneath the surface of Lake Rotorua.