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Columnar joints in basalt are typical structures formed during magmatic cooling and contraction. However, their formation mechanisms, internal structural characteristics, and cooling histories remain debated. This study aims to constrain the internal structures and cooling histories of large-diameter basalt columns through integrated rock magnetic and paleomagnetic analyses.
Detailed rock magnetic and paleomagnetic analyses were conducted on 49 oriented samples collected from two Pliocene basalt columns, each reaching up to 1.5 m in diameter, in the Bo Phloi section, Kanchanaburi, Thailand. Rock magnetic experiments included hysteresis-loop measurements, isothermal remanent magnetization (IRM) acquisition, first-order reversal curve (FORC) analysis, anisotropy of magnetic susceptibility (AMS) measurements, and temperature-dependent magnetic susceptibility measurements. Stepwise thermal demagnetization was performed to isolate stable components of remanent magnetization.
Hysteresis loops and IRM acquisition curves indicate that the magnetic assemblage is dominated by pseudo-single-domain (PSD) titanomagnetite grains, with magnetic saturation reached at fields below approximately 300 mT. The two-stage increase in IRM acquisition with increasing field suggests contributions from magnetic components with different coercivities. FORC diagrams further support the predominance of PSD magnetic grains and reveal systematic differences between the margins and interiors of the basalt columns. For both basalt columns, AMS results show sub-vertical minimum susceptibility axes (K3) and sub-horizontal maximum (K1) and intermediate (K2) axes, with generally low degrees of magnetic anisotropy (Pj < 1.05). These AMS fabrics indicate a primary near-horizontal magma-flow fabric during emplacement, but provide no evidence for vertical melt migration or internal convection. AMS parameters also reveal systematic spatial variations. Samples from the column margins exhibit lower magnetic susceptibilities (χ), lineation (L), and anisotropy degree (Pj) values and are predominantly characterized by oblate fabrics (T > 0), whereas interior samples show higher χ, L, and Pj values and predominantly prolate fabrics (T < 0). These differences reflect contrasting cooling conditions between the margins and interiors of the basalt columns. The margins cooled more rapidly, leaving less time for magnetic minerals to crystallize, grow, and develop preferred orientations. In contrast, the interiors cooled more slowly and likely remained at elevated temperatures in a plastic or partially molten state for longer periods, allowing magnetic minerals to crystallize, become concentrated, and develop preferred orientations under thermal contraction stresses. Paleomagnetic results indicate that stepwise thermal demagnetization isolates a stable, single-component remanent magnetization carried by PSD titanomagnetite. Six marginal samples from basalt column A exhibit relatively scattered virtual geomagnetic pole (VGP) distributions and anomalous directions, whereas the remaining 43 samples show relatively clustered VGPs after tilt correction. Systematic variations in remanent magnetization directions and VGPs indicate that cooling did not proceed symmetrically or uniformly from the column margins toward the cores. Instead, the columns appear to have undergone an asymmetric, unidirectional regional cooling process, possibly influenced by a localized heat source.
Integrated rock magnetic and paleomagnetic analyses lead to the following conclusions: (1) The basalt columns in Kanchanaburi are dominated by PSD titanomagnetite. Their AMS fabrics, characterized by subvertical K3 axes and subhorizontal K1 and K2 axes, record a primary near-horizontal magma flow during emplacement. (2) The column margins cooled more rapidly, resulting in finer magnetic grains, lower magnetic anisotropy, and predominantly oblate fabrics, whereas the interiors cooled more slowly, allowing magnetic minerals to crystallize, grow, and develop stronger preferred orientations, resulting in higher anisotropy and predominantly prolate fabrics. (3) Systematic variations in paleomagnetic directions and VGPs among the 49 samples indicate that post-jointing cooling was neither uniform nor symmetric but instead proceeded asymmetrically and unidirectionally across the basalt columns.
These findings contribute to a better understanding of the cooling processes of basaltic lava and provide new insights into long-term variations in the geomagnetic field.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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