A magnetic anomaly is a localized variation in the strength of the Earth’s magnetic field, measured at the surface or in the air. It represents the difference between the actual magnetic field intensity at a specific point and the intensity predicted by a smooth, regional model of the Earth’s main field. These deviations can be positive (stronger than expected) or negative (weaker). Scientists quantify these localized signals in nanoteslas (nT) to map features originating from the planet’s shallow crust.
The Earth’s Primary Magnetic Field
The Earth’s main magnetic field accounts for the vast majority of the magnetic signal measured globally. This primary field is generated deep within the planet by the movement of molten iron and nickel in the liquid outer core, a process known as the geodynamo. Convection currents in this electrically conductive fluid, combined with the Earth’s rotation, induce electric currents that sustain the magnetic field.
The resulting global field is largely dipolar, resembling the field produced by a simple bar magnet tilted relative to the planet’s axis of rotation. Although its intensity ranges widely (from about 25,000 to 65,000 nT), its large-scale structure is smooth and predictable. Scientists mathematically remove this predictable core field, along with transient external fields from the sun, to isolate the small, localized magnetic signals originating from the crust.
Geological Sources of Local Disturbances
Magnetic anomalies are caused by variations in the magnetic properties of crustal rocks. Rocks contain magnetic minerals, such as magnetite, and the concentration of these minerals determines how strongly a rock influences the local magnetic field. Two main types of magnetization contribute to a rock body’s total magnetic signature: induced and remanent magnetization.
Induced magnetization is temporary, representing the magnetic field a rock acquires in the presence of the Earth’s current magnetic field. The strength of this induced magnetism is proportional to the rock’s magnetic susceptibility. Rocks with a high content of iron-bearing minerals will easily become magnetized and create a stronger induced field, but this induced field vanishes if the external magnetic field is removed, making it less stable over geological time.
Remanent magnetization is permanent and often forms the most significant component of anomalies, especially in igneous rocks like basalt. When molten rock cools, magnetic mineral grains align with the Earth’s magnetic field at that moment in time. Once the temperature drops below the Curie temperature (about 570°C for magnetite), this orientation becomes permanently locked into the crystal structure. Geological structures rich in magnetic minerals, such as volcanic intrusions or buried iron ore bodies, produce the strongest and most recognizable magnetic anomalies.
Mapping Magnetic Anomalies
Measuring these subtle subsurface signals requires a highly sensitive instrument called a magnetometer, which detects the total intensity of the magnetic field. Geophysicists primarily use scalar instruments, such as proton precession or optically pumped cesium vapor magnetometers, which can measure field changes with sensitivities of 1 nanotesla or less. These sensors are mounted on aircraft for rapid aeromagnetic surveys, towed behind ships for marine surveys, or carried by ground crews, allowing for efficient data collection over large areas.
The choice of survey method depends on the required resolution and coverage area. Aeromagnetic surveys provide broad, regional coverage, while ground surveys offer higher-resolution data that is more sensitive to near-surface features. High-sensitivity alkali-vapor magnetometers are often used in a gradiometer setup, employing two sensors to measure the magnetic field gradient, which helps focus on these localized, shallow features.
The raw data collected during a survey represents the total magnetic field and must undergo several processing steps to isolate the anomaly. The first step involves correcting for transient, external fluctuations in the field, known as diurnal variations, caused by solar activity. Next, the predictable, large-scale primary field originating from the Earth’s core is mathematically subtracted from the total measurement. The residual signal that remains after these subtractions is the magnetic anomaly map, which precisely reflects the magnetic properties of the underlying crustal rocks.
Using Anomalies to Understand Earth
Magnetic anomaly maps are used in mineral exploration because many ore deposits contain high concentrations of magnetic minerals, particularly iron oxides. Mapping the extent and shape of a magnetic anomaly helps scientists define the boundaries and depth of buried iron ore bodies, which often produce signals hundreds or even thousands of nanoteslas stronger than the background field. These surveys also help map basement rock structures beneath sedimentary basins, providing structural context useful for petroleum exploration.
The most significant application of magnetic anomalies came from surveys conducted over the ocean floor. These surveys revealed alternating, symmetrical bands of positive and negative anomalies running parallel to the mid-ocean ridges, providing definitive evidence for seafloor spreading.
As new molten material rises and cools at the ridge, it acquires a permanent remanent magnetization aligned with the Earth’s field at that time. The alternating polarity of these stripes demonstrated that the Earth’s magnetic field has reversed its poles numerous times throughout geological history. The width of each stripe corresponds to the known duration of a magnetic polarity chron, allowing geophysicists to calculate the rate of seafloor spreading. This preserved record confirmed the mechanism of plate tectonics.