Beneath this Indonesian island lies a magnetic pull so strong it defies simple explanation, pointing to a colossal, altered piece of Earth's mantle.
A magnetic mystery
Deep beneath the volcanic soil of Halmahera, an island in Indonesia's North Maluku province, lies a powerful magnetic anomaly. Aeromagnetic surveys, which use sensitive magnetometers on aircraft to map Earth's magnetic field, first detected this immense feature. The data revealed a concentrated zone of magnetism far stronger than the surrounding crust can explain. The source is not a deposit of magnetic ore but a vast, buried structure with properties that point to origins deep within the Earth's upper mantle.
The island of Halmahera itself comes from intense geological forces. Its K-shape results from the collision of multiple tectonic plates, including the Australian, Philippine Sea, and Eurasian plates. This tectonic pile-up has created a geologic mosaic, where sections of an ancient volcanic arc are mashed together with ophiolites—large slabs of oceanic crust and the underlying upper mantle that have been thrust onto land. The western part of the island is an active volcanic arc, while the eastern arms are built upon this ophiolite basement. It is within this jumbled foundation that the source of the magnetic anomaly is found.
The serpentinite solution
The leading explanation for the Jailolo anomaly is a massive body of serpentinized ultramafic rock. This process, called serpentinization, occurs when ultramafic rocks from the mantle, rich in minerals like olivine and pyroxene, are exposed to water at low to moderate temperatures. The chemical reaction transforms these minerals into a new suite of minerals, including serpentine, brucite, and magnetite. Magnetite (Fe₃O₄) is a highly magnetic iron oxide, and its formation in large quantities during serpentinization can create powerful magnetic signatures.
The process is transformative. The original mantle rock increases in volume by 30-40% and releases significant heat. The source of the Jailolo anomaly is likely a colossal block of this altered mantle rock, an ophiolite fragment that was hydrated by seawater long ago during tectonic processes at the seafloor. Later, plate collisions forced this magnetic block deep beneath the modern island. Models suggest the anomaly is caused by a body of rock with a volume potentially measured in thousands of cubic kilometers, buried several kilometers below the surface. This immense, water-altered piece of the Earth’s mantle now exerts its magnetic influence from the deep, a hidden feature of our planet's dynamic geology.
💡Fun Facts
Serpentinization is an exothermic reaction, meaning it releases heat capable of raising rock temperatures by up to 260°C (500°F).
The process gets its name from the mineral serpentine, whose texture can sometimes resemble the skin of a snake.
Halmahera's strange K-shape is a direct result of being at the convergence of four separate tectonic plates.
The basement rock of eastern Halmahera is composed of dismembered ophiolites, which are sections of the Earth's oceanic crust and upper mantle.
The anomaly is a subsurface geological feature. The island of Halmahera has rugged, volcanic terrain with dense rainforests. Access to different parts of the island varies from developed coastal towns to challenging backcountry.
The Jailolo magnetic anomalyNorth Maluku, Indonesia