Strange magnetic ‘anomaly’ identified for the first time on new maps of Lake Rotorua

Scientists have now fully mapped a legendary lake in New Zealand that was once a key setting for a famous Māori love story. Their analysis reveals never-before-seen details about the deep hydrothermal systems hidden beneath the disarmingly peaceful surface.

Peaceful waters and violent histories

Lake Rotorua (At Rotorua now at Kahumatamomoe in Māori) is a special place. It is the second largest lake on New Zealand’s North Island and was formed by the violent explosions of a now dormant ancient volcano. About 200,000 years ago, that volcano experienced a massive eruption, causing the underlying magma chamber to collapse, creating a large caldera 10 miles wide. Then, about 65,000 years ago, the crater filled with water to form Lake Rotorua, which has been at its current level for the past 22,000 years.

Despite its beauty, Lake Rotorua is still home to hydrothermal activity that reminds us of the violent volcanic processes that created it. Some have even described that clouds of steam sometimes float over the water and that the lake’s high sulfur content gives it a “magical green-blue” appearance.

Now researchers from GNS Science, a New Zealand-based institute, have mapped the bottom of Lake Rotorua and revealed new details about the ancient base. According to their work, the lake hides thousands of ‘smallpox’ – essentially smaller cats – some of which were 50 meters (164 feet) in diameter. These pockets mark where gas bubbles have disturbed the sediment at the bottom of the lake.

The map also shows evidence of an ancient river that predates the lake. You can see the river meandering off the coast of Sulfur Point and then meandering westward from Mokoia Island before bending to the northeast. The island, which was formed by a rhyolite dome created by slow-flowing lava, is now a lush green environment that serves as a refuge for much of New Zealand’s endangered wildlife and rare birds.

It is also steeped in cultural significance for the Te Arawa people, who consider it a sacred space. Importantly, Mokoia Island is the setting for one of New Zealand’s most famous legends, the love story of Hinemoa and Tūtānekai.

The cards reveal the hidden history behind this serene scene.

Image credit: Troy Wegman/

Magnetic anomalies

In total, the maps cover about 55 square kilometers and cover about 68 percent of the lake bottom. The mapping process was achieved through various techniques, some of which were carried out by the New Zealand Navy; they used multibeam sonar to map the physical features of the lake and also conducted magnetic surveys of the lake bottom. This latter technique revealed interesting magnetic anomalies towards the center of the lake.

Under normal conditions, volcanic rocks contain magnetite, a mineral (not to be confused with the Pokémon of the same name) that is highly magnetic and aligns with the Earth’s magnetic field. However, in places like Lake Rotorua, hot water can flow through the rocks and transform magnetite into pyrite (fool’s gold), which has virtually no magnetic signal.

“Normally with volcanic rocks you get very positive deviations when you run a magnetometer over them, but in this case we get negative deviations, probably due to very low magnetic sensitivities,” says Dr. Cornel de Ronde, director scientist at GNS , told LiveScience.

This anomaly appears to coincide with a “subtle heat flow anomaly,” researchers explained in a statement, which points to a “possible igneous intrusion and another hydrothermal system nearby – exciting clues about the hydrothermal processes at play beneath the lake floor .”

This could explain why there is a negative anomaly in that part of the lake.

“We are not only excited about the way these new maps broaden our collective knowledge of the region,” the researchers said, “but also celebrate the iconic landscape and will contribute to further research in the future.”

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