Flores Island, East Nusa Tenggara, occupies a tectonically and magmatically active sector of the eastern Indonesian island-arc system. Its geology is characterized by Neogene to Quaternary volcanism, intrusive activity, structurally controlled hydrothermal circulation, widespread alteration, and locally significant concentrations of ferrimagnetic Fe-Ti oxides.
The purpose of this note is to establish the geological and geophysical conditions under which localized magnetic anomalies may occur in Flores, with particular emphasis on the eastern part of the island. It does not assign a specific origin to any individual magnetic anomaly. Rather, it defines the range of lithological, hydrothermal, and rock-magnetic conditions documented by geological and geophysical investigations in Flores and therefore available as a physically defensible regional background.
Flores forms part of the active Indonesian volcanic-arc system generated by convergence of the Indo-Australian plate with the southeastern margin of Eurasia and the Banda–Sunda arc system. The island contains numerous Quaternary volcanic centres and is dominated over large areas by volcanic and volcaniclastic successions of broadly basaltic-andesitic to andesitic composition.
Geothermal investigations conducted by the Geological Survey of Japan and Indonesian agencies describe Flores geothermal prospects as occurring principally within young andesitic and basaltic volcanic terrains associated with post-volcanic fractures, faults, caldera structures, and hydrothermal manifestations. Reservoir temperatures inferred or measured at several Flores geothermal systems fall approximately within the 200–280 °C range.
The structural architecture is consequently heterogeneous at scales ranging from kilometres to metres. Fault zones, fracture networks, volcanic contacts, breccias, dykes, intrusive bodies, alteration fronts, and collapse structures may juxtapose units with strongly contrasting density, electrical resistivity, magnetic susceptibility, and remanent magnetization.
Such heterogeneity is a prerequisite for short-wavelength geophysical anomalies.
The Oka–Larantuka / Oka Ile Ange sector of East Flores has been the subject of formal geological, geochemical, and geophysical investigation by Indonesian geological authorities.
The Geological Agency of Indonesia archives an integrated investigation entitled Laporan Penyelidikan Terpadu Geologi, Geokimia, dan Geofisika Daerah Panas Bumi Oka, covering Oka, Larantuka, and the surrounding area. The archived investigation comprises geological, geochemical, and geophysical components and includes numerous accompanying maps.
More recent magnetotelluric work in East Flores has demonstrated that the subsurface electrical structure is sufficiently heterogeneous to require explicit three-dimensional modelling. Arafat and Daud analysed MT observations in the East Flores geothermal area and showed that conductive seawater produces a measurable coast effect because of the extreme electrical-resistivity contrast between the island and the surrounding sea. Their modelling incorporated seawater with a resistivity of approximately
[
\rho_{\mathrm{sea}} \approx 0.3\ \Omega,\mathrm{m},
]
and showed frequency-dependent distortion of MT responses, particularly at lower frequencies. Forty-five MT stations with nominal spacing of approximately 500–1500 m were used in the study area. Geological interpretation placed the principal geothermal prospect around the Mount Anging sector and identified structurally controlled thermal manifestations.
Subsequent work on the Oka Ile Ange geothermal area has employed more than 100 gravity observations together with more than 120 MT stations to constrain density and resistivity structure, identifying fault systems and a subsurface depression associated with anomalously low resistivity and low density.
The significance of these studies is not that magnetotelluric anomalies are equivalent to static magnetic anomalies; they are not. Their significance is that East Flores is demonstrably not a geophysically homogeneous volcanic block. The subsurface consists of structurally compartmentalized volcanic, intrusive, altered, fractured, fluid-bearing, and electrically contrasting materials.
Hydrothermal alteration is extensively documented elsewhere on Flores and provides an appropriate island-scale analogue for altered volcanic rocks in East Flores.
Regional work by the Geological Agency of Indonesia in central Flores identifies Tertiary–Quaternary volcanic and sedimentary successions, including tuff, andesite lava, volcanic breccia, and green tuff. Late Miocene intrusive activity produced hydrothermal alteration and mineralization. Documented alteration minerals include:
chlorite;
zeolite;
kaolinite;
silica;
jasper.
Detailed geothermal drilling and alteration studies at Mataloko likewise demonstrate vertically and laterally heterogeneous alteration mineral assemblages in volcanic rocks. The broader Flores geothermal literature documents clay alteration, zeolitization, silicification, argillic alteration, fault-controlled hydrothermal circulation, and high-temperature subsurface systems.
Chlorite is therefore geologically unexceptional in a Flores hydrothermal setting. Its presence is consistent with alteration of volcanic protoliths under appropriate fluid-temperature-rock conditions and need not imply any exotic mineralogical process.
Importantly, chlorite itself should not be treated as the principal source of a strong magnetic anomaly. The more relevant phases are Fe-bearing ferrimagnetic minerals, principally magnetite and titanomagnetite, where present.
The magnetic behaviour of volcanic rocks is controlled disproportionately by relatively small volumetric fractions of strongly magnetic minerals.
For crustal rocks, the bulk magnetization may be written as
\mathbf{M}{\mathrm{ind}}
+
\mathbf{M}{\mathrm{rem}},
]
where
\kappa\mathbf{H}_{0},
]
(\kappa) is magnetic susceptibility, (\mathbf{H}{0}) is the ambient geomagnetic field, and (\mathbf{M}{\mathrm{rem}}) is natural remanent magnetization.
Magnetite is commonly the dominant magnetic mineral controlling crustal magnetic anomalies. Remanent magnetization can equal or exceed induced magnetization in volcanic and intrusive rocks, and its direction need not coincide with the present geomagnetic field.
Flores provides direct evidence for iron-rich and magnetite-bearing material. Geological Agency investigations in central Flores report iron mineralization and iron sands containing approximately
[
4.22% \text{ to } 50.83% \ \mathrm{magnetite}.
]
The same regional investigation documents hydrothermal alteration including chlorite within the volcanic succession.
These observations do not demonstrate that every altered volcanic unit on Flores is strongly magnetic. Hydrothermal alteration may either preserve, redistribute, oxidize, or destroy primary magnetic minerals. Indeed, geothermal systems can produce magnetic lows where hydrothermal processes demagnetize volcanic rocks. An integrated geophysical study at Mataloko identified a narrow negative residual magnetic anomaly approximately coincident with a low-resistivity hydrothermal zone and interpreted it as hydrothermal demagnetization.
Accordingly, a realistic Flores subsurface should be expected to contain magnetic contrasts rather than uniform magnetization.
For an observation point (\mathbf{r}), the measured crustal magnetic perturbation is the vector sum of contributions from individual magnetic bodies:
\sum_{i=1}^{N}\mathbf{B}_{i}(\mathbf{r}).
]
Thus, even where the mineralogical composition varies only moderately, geometric factors can produce significant differences in the measured anomaly.
The field contribution of each body depends on:
f
\left(
\mathbf{M}{i},
V{i},
\mathbf{r}{i},
\mathrm{geometry}{i}
\right),
]
where:
(\mathbf{M}_{i}) = total magnetization vector;
(V_{i}) = magnetic volume;
(\mathbf{r}_{i}) = source-observer displacement;
geometry includes source shape, orientation, and measurement geometry.
This dependence is particularly important in shallow surveys. For compact magnetic sources at distances appreciably greater than their dimensions, the field approaches the magnetic-dipole limit:
[
|\mathbf{B}|
\propto
\frac{MV}{r^{3}}.
]
Consequently, shallow magnetic sources generate steep spatial gradients. A localized magnetic body at metre-scale depth can therefore produce a measurable anomaly over a correspondingly restricted surface footprint without requiring a regionally extensive magnetic formation.
A magnetic body does not invariably produce a positive anomaly.
Because magnetization is a vector, the anomalous field generated by a remanently magnetized rock may project either parallel or antiparallel to the local geomagnetic field at a particular observation point.
The first-order total-field perturbation is
[
\Delta T
\approx
\hat{\mathbf{F}}{0}\cdot\mathbf{B}{a},
]
where (\hat{\mathbf{F}}_{0}) is the unit vector parallel to the regional geomagnetic field.
Therefore,
[
\Delta T > 0
]
for a reinforcing projection,
whereas
[
\Delta T < 0
]
for an opposing projection.
USGS investigations have documented volcanic rocks in which remanent magnetization dominates the observed magnetic anomaly and may have a direction substantially different from the induced component. Negative anomalies associated with oppositely directed remanent magnetization are a well-established phenomenon in magnetic geophysics.
This property permits adjacent magnetized bodies to produce mutually cancelling contributions at one observation point while generating entirely different anomaly amplitudes elsewhere.
Magnetic geophysical surveying is not hypothetical in the East Flores volcanic environment.
The Geological Agency of Indonesia archives a dedicated magnetic-method investigation of Mount Lereboleng in East Flores undertaken to examine the internal structure of the volcanic edifice.
More broadly, Oka–Larantuka geothermal investigations explicitly incorporated geophysical methods, while subsequent East Flores studies have used MT, gravity, three-dimensional inversion, and coast-effect modelling to resolve subsurface structure.
Accordingly, geological interpretation of a small magnetic anomaly in East Flores would have to consider at minimum:
[
\text{primary volcanic lithology}
]
[
+\ \text{Fe-Ti oxide abundance}
]
[
+\ \text{hydrothermal alteration}
]
[
+\ \text{remanent magnetization}
]
[
+\ \text{faulting and structural rotation}
]
[
+\ \text{source depth and geometry}.
]
No single parameter is sufficient.
An important distinction is required.
The documented East Flores coast effect is primarily an electromagnetic induction problem affecting magnetotelluric measurements. It arises from the extreme conductivity contrast between seawater and continental or volcanic crust. It should not be invoked as a direct explanation for a static, metre-scale magnetic anomaly caused by magnetized rock.
Nevertheless, it demonstrates that electromagnetic and electrical geophysical measurements in East Flores must be interpreted in a three-dimensional coastal environment.
The two phenomena should therefore be separated analytically:
[
\text{MT coast effect}
\neq
\text{local rock-magnetic anomaly}.
]
The first concerns induced electromagnetic response in a conductive island/coastal geometry.
The second concerns magnetization of crustal materials.
They may coexist in the same field area without sharing a common immediate cause.
On the basis of the published geological and geophysical record, the following configuration is physically compatible with a Flores Island setting:
a volcanic or volcaniclastic host sequence;
hydrothermal alteration of part of that sequence;
chlorite-bearing altered volcanic lithologies;
local concentrations of magnetite and/or titanomagnetite;
strongly heterogeneous magnetic susceptibility and remanent magnetization;
fault-, fracture-, or cavity-controlled juxtaposition of magnetic bodies;
metre-scale magnetic gradients above shallow magnetic sources;
positive and negative magnetic contributions produced by differing source positions and remanence directions.
What cannot be inferred solely from the regional geology is the exact magnetization, geometry, depth, or temporal behaviour of any individual subsurface magnetic body. Those quantities require site-specific measurement.
The regional geology establishes plausibility, not proof.
Geological Agency of Indonesia — Integrated Geological, Geochemical and Geophysical Investigation of the Oka Geothermal Area, Larantuka, East Flores.
Badan Geologi archive record — Oka integrated investigation
Arafat, Y. & Daud, Y. (2020). “Coast effects on 3-D inversion of magnetotelluric data for geothermal exploration in East Flores, Indonesia.” IOP Conference Series: Earth and Environmental Science, 538, 012071. DOI 10.1088/1755-1315/538/1/012071.
Universitas Indonesia publication record
Geological Survey of Japan, AIST — Bulletin of the Geological Survey of Japan, Vol. 53, No. 2/3: Flores geothermal investigations.
Geological Survey of Japan — Flores geothermal volume
Harahap, B. H., Abidin, H. Z., Utoyo, H., Djumhana, D. & Yuniarni, R. “Prospect of Mineral Deposits in the Central Flores Island, Eastern Indonesia.” Jurnal Geologi dan Sumberdaya Mineral. DOI 10.33332/jgsm.geologi.v16i1.46.
Geological Agency of Indonesia journal article
Nasution, A., Taniguchi, M., Kikuchi, T. & Muraoka, H. (2003). “A Lithostatically Pressurized Clay Cap of the Vapor-dominated Reservoir Deduced from Hydrothermal Alterations of Borehole Cuttings in the Mataloko Geothermal Field, Flores Island, Eastern Indonesia.”
J-STAGE article record
Geological Agency of Indonesia — Magnetic-method investigation of Mount Lereboleng, East Flores.
Badan Geologi archive record — Lereboleng magnetic investigation