Geomagnetic field intensity determination from Pleistocene trachytic lava flows in Jeju Geopark

A composite of 28 trachytic lava flows were recovered from the Jeju Geopark Drilling Project (JGDP) in Jeju Geopark, one of the new seven wonders of Nature declared by UNESCO in 2011. Each trachytic lava flow has a tendency to increase in magnetic grain size from the rapidly cooled brecciated margin and vesicle streaked zone downward into the massive crystalline flow interiors. The brecciated margin and vesicle streaked zone of individual trachytic lava flow contains exclusively fine‐grained magnetite as inclusions in plagioclase. High‐fidelity paleointensity determinations were obtained from 26 (out of 224 examined) samples from JGDP cores. Temporal variation of virtual axial dipole moments (VADMs) calculated from the absolute paleointensity estimates follows the trend of sint‐800 data for the interval from ∼80 to ∼360 ka. High VADM from flow 21 possibly represents real intensity peak, as previously recognized high VADM in Japan at ∼336 ka, in Trans‐Mexican volcanism ∼339, and in Hawaii ∼340–350 ka. Perhaps such a strong magnetic intensity near ∼325–350 ka might be smoothed out in relative paleointensity records. Key Points A composite of 28 trachytic lava flows were recovered in Jeju Geopark Brecciated margin and vesicle streaked zone contains fine‐grained Fe‐oxide High strength geomagnetic field intensity appears at ~325‐350 ka