A strange “magnetic anomaly” has been discovered in a legendary New Zealand lake

New Zealand is rich in natural beauty. An example of this is Lake Rotorua, which borders the city of the same name and is the second largest island in the country's North Island, with an area of ​​79.8 square kilometers. It is the place of a Maori legend that tells the story of forbidden love, but in addition to the legends, it is also famous for its dazzling colours.

Lake Rotorua is known for its turbid blue-green water due to the presence of sulphur, making it a place of scientific interest.

For the first time, researchers from institute For GNS Science They mapped the depths of this lake in detail and They discovered the presence of a “magnetic anomaly” that helps understand the work of volcanic lakes in the region.

Discovery in Lake Rotorua

Researchers They have mapped about 68% of the lakewhich Found in an ancient crater of an extinct volcano. The main approach was to use a multibeam echosounder (a type of sonar) to detect the depth and shape or bathymetry of the lake floor.

It is believed that Lake Rotorua was formed after a volcanic eruption that destroyed the Earth about 220,000 years ago. The volcano responsible for the eruption is dormant beneath the lake.

Mapping results revealed Signs of hydrothermal activity beneath the lakeincluding a series of small calderas that created a hole in the bottom of the lake, in addition to Strange “magnetic anomaly”..

According to researchers, These small craters are likely the result of hydrothermal eruptions: They are created by discharging hot gas and water and still emit almost imperceptible heat fluxes. They noticed this along the coast Sulfur a point (a geothermal wetland south of Lake Rotorua characterized by the presence of sulphur), where warm water rises into the lake and cold water rushes down.

Despite this intense activity. Temperatures near the bottom of the lake are around 14°CThis is because there is a large amount of cold water that works to neutralize the heat coming from below.

The “magnetic anomaly” of the lake

this “Magnetic anomaly” It aroused the interest of researchers, indicating the presence of pyrite instead of the expected magnetite In the volcanic rocks of the lake.

It turns out that volcanic rocks often contain magnetite, which is highly magnetic. They think so Warm lake water (hydrothermal fluids) Converting magnetite into another mineral, pyrite, which has practically no magnetic signal (This can be seen in the blue tones in the image below, which cover craters caused by hydrothermal eruptions.) Which is associated with low or no magnetic intensity). This hydrothermal process would significantly reduce the magnetic signal and explain the failure.

“Normally, for volcanic rocks, when you pass a magnetometer over them, you get very positive anomalies, but in this case we Get anomalies negative“This is probably due to the very low magnetic susceptibility,” said Cornel de Ronde, a geologist at the institute. GNS .

For researchers, this is an indication of that The ancient volcano, even dormant, still affects Lake Rotorua. For Runde, this research is “an excellent example of a comprehensive approach to understanding our geothermal-hosting volcanic lakes, allowing us to better model the complex working of Rotorua’s geothermal system.”

News reference:

New maps reveal the hidden depths of Lake Rotorua. GNS Science2024.