Many types of volcanic rock are magnetic. These extrusive igneous rocks form when magma erupts onto the Earth’s surface as lava and cools quickly. The magnetic property comes from specific iron-bearing minerals locked within the structure, not all rock. This permanent magnetism provides a profound tool for understanding the planet’s dynamic history.
How Volcanic Rock Acquires Magnetism
Volcanic rock acquires its magnetic signature through Thermal Remanent Magnetization (TRM). This process begins when magma or lava exists in a molten state at extremely high temperatures. While the rock is hot, the magnetic domains within its iron-rich minerals are in a state of thermal agitation, meaning their microscopic magnetic moments are randomly oriented.
As the lava cools, it passes a specific temperature threshold called the Curie Point, which differs by mineral composition. Above this temperature, the material cannot hold a permanent magnetic field. As the temperature drops below the Curie Point, the magnetic domains lock into alignment with the Earth’s ambient magnetic field, freezing the field’s direction and polarity into the rock structure.
This magnetic signature is preserved through geological time, recording the Earth’s magnetic field at the time of formation. For example, pure magnetite has a Curie Point of approximately 580 degrees Celsius. Once the rock cools past this temperature, the acquired TRM is highly stable, making volcanic rocks excellent recorders of paleomagnetic data.
Iron-Rich Minerals That Hold the Charge
The ability of volcanic rocks to acquire and retain a magnetic charge depends entirely on the presence of certain iron and titanium oxide minerals. The most significant of these is magnetite (Fe3O4), a ferrimagnetic mineral that is an efficient carrier of permanent magnetism. Basalt, a common volcanic rock making up the majority of the oceanic crust, typically contains a high concentration of magnetite, often up to 1 or 2% of its total volume.
Another important mineral is titanomagnetite, a solid solution formed by a mix of magnetite and ulvöspinel (Fe2TiO4). The ratio of titanium to iron in this compound directly impacts the magnetic properties, including the Curie Point. Ti-rich titanomagnetite has a lower Curie Point, while Ti-poor versions behave more like pure magnetite with a higher Curie Point.
The concentration and type of these iron-bearing minerals determine the overall magnetic strength of the rock. Rocks like basalt and some andesites are strongly magnetic because they are rich in these compounds. In contrast, rocks that form from more silica-rich magmas, such as obsidian or pumice, are generally non-magnetic or only weakly magnetic because they lack the necessary iron-titanium oxides in significant amounts.
Unlocking Earth’s Geological History
The stable magnetic signature preserved in volcanic rock is the foundation of paleomagnetism, the study of ancient magnetic fields. By sampling and analyzing the orientation of the magnetic domains in rock formations, scientists can determine the direction and polarity of the Earth’s magnetic field millions of years ago. This record has provided irrefutable evidence for some of the most profound geological theories.
The study of volcanic rocks along the mid-ocean ridges led to the discovery of magnetic striping on the seafloor. As new oceanic crust forms from cooling magma at the ridge crest, it locks in the current magnetic polarity, creating parallel bands of alternately magnetized rock. These symmetrical stripes of normal and reversed polarity extending outward from the ridge crest provided the first clear geophysical evidence supporting the theory of seafloor spreading.
This magnetic record confirmed that the Earth’s magnetic field has periodically reversed its polarity throughout geological time. The ability to precisely date these reversals in volcanic rocks has allowed geoscientists to construct a geomagnetic polarity timescale. This timescale is an invaluable tool for dating rock formations globally and for understanding the movement of tectonic plates.