A Cretaceous geological remnant
The disorienting forces at Ilhabela originate deep in geologic time, a remnant of the Earth's distant past. The island is dominated by a complex of alkaline igneous rocks that formed during the Early Cretaceous period. These rocks are part of the Serra do Mar Alkaline Province, a series of intrusions that occurred as the supercontinent Gondwana was actively breaking apart. Radiometric dating of similar formations along the coast places their origin around 129 to 130 million years ago.
As South America and Africa separated, magma pushed up into the crust, cooling slowly to form rocks like syenite and gabbro. This process concentrated dense, iron-rich minerals, particularly magnetite (Fe₃O₄), throughout the island's rock formations. It is this high concentration of magnetite that gives the rocks their unusually powerful magnetic properties. The local magnetic field generated by these Cretaceous formations is strong enough to override the Earth's global magnetic field, which is what causes navigational instruments to behave erratically. Dozens of shipwrecks are scattered along the coastline, with some historical accounts attributing the incidents to navigational instruments being altered by an inexplicable magnetic field.
Navigational interference and the South Atlantic Anomaly
The first documented European visit to the island was a Portuguese expedition on January 20, 1502. For centuries since, sailors have contended with the island's strange magnetic effects. The phenomenon is not a historical curiosity; modern electronic compasses and smartphone magnetometers are also susceptible to the powerful local field. A compass needle, instead of aligning with the planet's magnetic north, will align with the much closer and stronger field of the magnetite-laden rocks, rendering it useless for reliable navigation in certain areas. In some spots, the local field can cause a "reverse polarization," where a compass may point south instead of north.
Ilhabela's local magnetic variance is a much larger geophysical phenomenon: the South Atlantic Anomaly (SAA). The SAA is a vast region where the Earth's magnetic field is significantly weaker than average, a "dent" in the magnetosphere that stretches from South America to Africa. In this zone, the inner Van Allen radiation belt dips closer to the surface, down to an altitude of about 200 kilometers. This lowered shield increases the exposure of orbiting satellites to high-energy particles, which can cause malfunctions and data loss. While the Ilhabela anomaly is a crustal feature and the SAA originates from the planet's liquid outer core, Ilhabela sits directly within this broader region of planetary magnetic weakness.