Technical Reference
Yes. This is physically plausible and consistent with standard magnetic-survey principles.
A magnetometer does not measure the contribution of a single magnetic source in isolation. At a given observation point, the measured field represents the combined contribution of the Earth’s main magnetic field, local geological sources, nearby ferromagnetic objects, and other magnetic disturbances.
Conceptually:
**Observed magnetic field
= Earth’s main magnetic field
local field from subsurface magnetic rocks
local fields from nearby iron or steel objects
other magnetic disturbances**
Magnetic fields are vector quantities. Contributions acting in approximately the same direction can reinforce one another, whereas oppositely directed components can partially cancel.
Iron and steel are ferromagnetic materials capable of producing significant local magnetic anomalies. The U.S. Environmental Protection Agency notes that ferromagnetic materials alter the local geomagnetic field and that the polarity and magnitude of a measured magnetic anomaly depend in part on the orientation of the source. Magnetic surveys routinely exploit this property to detect buried ferrous objects such as tanks, utilities, and other steel structures. [1]
Steel objects can contribute through both induced magnetization and remanent magnetization. In the simplest linear form, induced magnetization may be represented as:
M = χH
where:
M = magnetization
χ = magnetic susceptibility
H = magnetizing field
Field and laboratory measurements of ordinary steel objects demonstrate that bulk steel can possess substantial magnetic susceptibility and remanent magnetization. Measurements of unrusted steel drums, for example, yielded bulk susceptibility values of approximately 90–125 SI and remanent magnetizations of approximately 325–2,750 A/m. [2]
A steel stake can therefore act as an additional local magnetic source. Where magnetically strong rock already exists beneath a survey point, a magnetometer records the combined field generated by the geological source and the nearby steel objects.
The magnetic contribution from an elongated ferromagnetic object depends not only on the amount of metal present but also on its geometry relative to the observation point.
Relevant variables include:
distance between the object and the sensor,
orientation of its long axis,
direction of its magnetization,
angle relative to the ambient geomagnetic field,
position of the object relative to the measurement point.
The U.S. EPA specifically notes that source orientation affects the polarity of magnetic anomalies. A ferromagnetic source oriented so that its magnetic field reinforces the external field can generate a positive anomaly, whereas an opposing orientation can generate a negative anomaly. [1]
Consequently, the same steel stake can make a different contribution to a fixed magnetometer reading if it is:
tilted from a vertical position,
laid horizontally,
rotated,
moved toward the sensor,
moved away from the sensor.
The measured magnetic value can therefore rise or fall even when the intrinsic magnetic properties of the subsurface rock remain unchanged.
The relevant physical change is the spatial configuration of the magnetic sources and therefore the vector sum measured at the observation point.
This behavior is also supported experimentally by magnetic investigations of steel objects, in which source shape, orientation, bulk susceptibility, and remanent magnetization affect the resulting anomaly. [2]
For a compact magnetic source that can be approximated at sufficient distance as a magnetic dipole, magnetic-field strength decreases approximately with the inverse cube of distance:
B ∝ 1/r³
This is the standard far-field relationship for a magnetic dipole. [3]
The practical consequence is important for near-surface magnetic measurements.
A ferromagnetic object positioned close to a magnetometer can have a disproportionately large influence on the recorded field. Moving the same object farther away can rapidly reduce that influence.
If several steel stakes are present, the measured local field may be represented schematically as:
**subsurface geological field
stake 1
stake 2
stake 3
...**
Each stake contributes according to its magnetization, position, and orientation.
Changing the position or orientation of only part of such an array can therefore change the total field measured at a fixed point without requiring any change in the underlying geological source.
Near-surface magnetic surveys routinely detect discrete ferrous objects precisely because their local fields can produce high-amplitude positive or negative anomalies relative to geological background values. [1]
Chlorite-bearing alteration and strong rock magnetism do not form a universal one-to-one relationship, but both can occur within the same volcanic hydrothermal system.
Chlorite is a common secondary mineral formed during alteration of igneous rocks. The U.S. Geological Survey describes chlorite-group minerals as common hydrothermal alteration products of ferromagnesian minerals in igneous rocks. [4]
Chlorite is particularly characteristic of propylitic alteration, commonly occurring with minerals such as epidote, calcite or other carbonates, albite, and locally iron-bearing phases.
USGS investigations of hydrothermally altered volcanic rocks in the Silverton volcanic terrain documented regional propylitic alteration involving chlorite, calcite, and epidote. The same study also examined the magnetic mineralogy of these altered rocks and documented magnetite-bearing propylitic samples. [5]
The relevant geological relationship is therefore not based on chlorite alone. It is based on the broader volcanic host-rock and hydrothermal-alteration system in which chlorite and strongly magnetic Fe-bearing minerals may coexist or occur in adjacent alteration domains.
Volcanic rocks can contain ferrimagnetic Fe-Ti oxides, particularly magnetite and titanomagnetite. When hydrothermal fluids circulate through such rocks, primary silicate minerals can be altered and secondary minerals such as chlorite may develop, while Fe-Ti oxide minerals may be preserved, modified, dissolved, or locally formed under different hydrothermal conditions.
A well-documented example occurs in basaltic rocks from the Krafla geothermal system in Iceland. The investigated drill-core basalts contain chlorite and chlorite-smectite alteration, while petrographic observations indicate approximately 5–10 vol.% titanomagnetite in some samples. [6]
This demonstrates that a hydrothermally altered volcanic rock can simultaneously contain:
chlorite-bearing secondary alteration minerals
and
significant quantities of ferrimagnetic Fe-Ti oxides.
A volcanic hydrothermal system can therefore provide a common geological framework for both chlorite-bearing alteration and strongly magnetic rock.
The Icelandic Krafla–Stardalur studies provide a particularly useful rock-magnetic example.
Both Krafla and Stardalur basalts have undergone hydrothermal metamorphism within the chlorite zone and contain comparable Fe-Ti oxide mineralogies. However, altered Stardalur basalts display exceptionally high magnetic properties.
Reported values include:
Natural remanent magnetization (NRM): 18–121 A/m
Magnetic susceptibility: 47–118 × 10⁻³ SI
The study attributes the strong remanent magnetization at Stardalur to magnetite-related mineralogy, including secondary magnetite and magnetite formed during alteration processes. [6]
This provides direct evidence that volcanic rocks affected by chlorite-zone hydrothermal alteration can, under appropriate mineralogical conditions, retain or develop very strong magnetic properties.
The occurrence of chlorite-bearing alteration therefore does not preclude strongly magnetic rock within the same altered volcanic system.
Hydrothermal alteration does not affect magnetic minerals uniformly.
Its effects depend on variables including:
temperature,
fluid composition,
redox conditions,
sulfur activity,
water-rock ratio,
original host-rock mineralogy,
alteration history.
Under different conditions, primary magnetite or titanomagnetite may be preserved, chemically modified, replaced, dissolved, or supplemented by secondary magnetic minerals.
The Krafla study documents fluid-rock reactions affecting titanomagnetite and titanomaghemite, including dissolution and replacement by secondary phases. It also shows that hydrothermal alteration can produce large variations in both magnetic susceptibility and natural remanent magnetization within rocks belonging to the same broad geothermal system. [6]
Comparable heterogeneity is documented in USGS work on propylitically altered volcanic rocks near Silverton, Colorado. Magnetite abundance varies substantially among altered samples, while chlorite is present across many of the propylitic lithologies. [5]
A hydrothermal alteration system should therefore be regarded as magnetically heterogeneous rather than as a single uniform magnetic body.
Strongly magnetic and weakly magnetic rocks can occur within relatively short spatial distances as a result of differences in mineralogy and alteration history.
Magnetite-rich rock and chlorite-rich rock do not need to occupy exactly the same alteration zone.
Volcanic hydrothermal systems commonly develop alteration zoning in response to variations in temperature, fluid composition, permeability, and water-rock interaction.
A documented Indonesian example occurs at the Brambang porphyry copper-gold system on Lombok.
The hydrothermal alteration sequence includes a higher-temperature potassic assemblage characterized by:
quartz–biotite–actinolite–magnetite
and an adjacent propylitic assemblage characterized by:
chlorite–epidote–calcite.
The study places the chlorite-rich propylitic zone outside the magnetite-bearing potassic zone as part of the same evolving hydrothermal system. [7]
This is a direct geological example of a single volcanic-hydrothermal system containing spatially associated:
magnetite-rich alteration
and
chlorite-rich alteration.
Accordingly, the close occurrence of chlorite-bearing hydrothermally altered volcanic rocks and magnetite- or titanomagnetite-rich magnetic rocks is geologically well supported.
Steel objects can possess both induced and remanent magnetization and can therefore act as independent local magnetic sources.
Where magnetically strong subsurface rocks already create a local magnetic field, the value recorded by a magnetometer represents the combined vector field of the geological source and nearby ferromagnetic objects.
Changing the:
position,
distance,
orientation,
inclination,
or rotation
of steel stakes can change their contribution at a fixed measurement point.
The measured value can consequently increase or decrease even when the underlying geological source remains unchanged.
This behavior is consistent with standard magnetostatics, empirical measurements of steel objects, and established ground-magnetic survey practice. [1–3]
Chlorite is a common secondary mineral in hydrothermally altered igneous rocks and is widely associated with propylitic alteration. [4,5]
Volcanic host rocks can also contain magnetite and titanomagnetite. Hydrothermal alteration can preserve, modify, destroy, or locally generate magnetic Fe-bearing phases, producing substantial magnetic heterogeneity within a single system. [5,6]
Field and laboratory studies demonstrate that chlorite-zone altered volcanic rocks can possess high magnetic susceptibility and strong natural remanent magnetization, while alteration zoning can place chlorite-rich and magnetite-rich rocks in adjacent parts of the same hydrothermal system. [6,7]
The scientifically supported relationship is therefore:
**volcanic host rocks
hydrothermal circulation
→ chlorite-bearing alteration
and, under appropriate mineralogical conditions,
→ magnetite/titanomagnetite-bearing magnetic rocks**
The two features can be treated as geologically related products or components of the same volcanic-hydrothermal environment.
[1] U.S. Environmental Protection Agency. Magnetic Method. Environmental Geophysics. U.S. EPA. Updated May 6, 2026. The technical guidance describes magnetic susceptibility, ferromagnetic sources, positive and negative magnetic anomalies, and the dependence of anomaly character on source orientation.
[2] Ravat, D. (1996). “Magnetic properties of unrusted steel drums from laboratory and field-magnetic measurements.” Geophysics, 61(5), 1325–1335. DOI: 10.1190/1.1444056. The study provides laboratory and field measurements of magnetic susceptibility and remanent magnetization in bulk steel objects.
[3] OpenStax. University Physics, Volume 2, Section 12.4, “Magnetic Field of a Current Loop.” The magnetic-dipole far-field solution gives the characteristic inverse-cube dependence of magnetic-field strength with distance.
[4] Poppe, L.J., Paskevich, V.F., Hathaway, J.C., and Blackwood, D.S. (2001). A Laboratory Manual for X-Ray Powder Diffraction. U.S. Geological Survey Open-File Report 01-041. The USGS chlorite-group reference identifies chlorites as common hydrothermal alteration products of ferromagnesian minerals in igneous rocks.
[5] Yager, D.B., Choate, L., and Stanton, M.R. (2008). Net Acid Production, Acid Neutralizing Capacity, and Associated Mineralogical and Geochemical Characteristics of Animas River Watershed Igneous Rocks Near Silverton, Colorado. U.S. Geological Survey Scientific Investigations Report 2008-5063. DOI: 10.3133/sir20085063. The report documents chlorite-bearing propylitic alteration and variable magnetite abundance in hydrothermally altered volcanic rocks.
[6] Oliva-Urcia, B., Kontny, A., Vahle, C., and Schleicher, A.M. (2011). “Modification of the magnetic mineralogy in basalts due to fluid–rock interactions in a high-temperature geothermal system (Krafla, Iceland).” Geophysical Journal International, 186(1), 155–174. DOI: 10.1111/j.1365-246X.2011.05029.x. The study documents chlorite-zone alteration, titanomagnetite-bearing basalt, alteration-driven changes in magnetic mineralogy, and the exceptionally high NRM and susceptibility values reported from hydrothermally altered Stardalur basalts.
[7] Faesal, A., Aminuddin, M.I.K.A., and Ubaidillah, A.S. (2022). “Host rock petrology, hydrothermal alteration characteristics & ore mineralogy of porphyry copper-gold deposit, Brambang, Lombok, West Nusa Tenggara Indonesia.” Materials Today: Proceedings, 66(5), 3071–3076. DOI: 10.1016/j.matpr.2022.07.373. The study documents a quartz–biotite–actinolite–magnetite potassic zone and an adjacent chlorite–epidote–calcite propylitic zone within the same Indonesian hydrothermal system.