A meizanologist's diary (100)

15 March: the weather forecast induces Alpinist A to change her plan. Instead of an all-day snowshoe trip, we’ll go for a quick morning traverse of Monju (365 metres), our local miniature Meizan. The sky is already grey when we start out from a temple at the east end of the ridge. And the clouds have dropped yet lower by the time we reach the main summit, an hour or so later.

All three of us have been here many times before. Yet I haven't previously noticed the big boulder to the right of the summit shrine. A helpful placard identifies it as a “lightning fossil”, hinting that the rock has its own magnetic field.

Using my phone’s inbuilt compass, we put this claim to the test. And, gratifyingly, when the phone is brought within a few centimetres of the rock, the needle does swing five degrees or so away from magnetic north. Perhaps there’s a mother lode of magnetite in this rock …

In the woods beyond Monju’s third summit, we pass by a more dramatic reminder of lightning’s power. A bolt from above has split a sapling from its crown almost down to the ground. The wood splinters scattered several metres away are still fresh, suggesting that the tree was struck within the last few months.

On the Japan Sea coast, thunderstorms are more famous for their vigour in winter than in summer. What stirs them up is the temperature difference between cold air flowing in from Siberia and the warm Japan Sea currents, as this NHK programme explains. And right here in our neighbourhood the storms often get an extra fillip from a North Korean volcano…

Blowing cold and hot: how the clouds get their charge.
Diagram courtesy of NHK.

All this adds up to the potential for some serious voltage. In January 1973, a satellite charged with monitoring the test-ban treaty detected a lightning superbolt over the Japan Sea that flashed as brightly as a tactical nuclear weapon. It was hereabouts too, near Kanazawa in 1969, that winter lightning downed one of the Air Self-Defence Force’s accident-prone Starfighters. Another lightning-struck F104 fell into the Japan Sea a few years later.

A big flash over the Sea of Japan.
Image courtesy of NHK.

Rain is spotting down as I put my phone away after capturing the shattered tree. I have to hurry after the two ladies, never ceasing to wonder at how they can walk while conversing at full tilt…

While navigating the muddy path, I’m wondering about that “fossil thunder” boulder that we tested an hour ago. Lightning clearly hits home quite a bit around here - we've seen the remains of blitzed trees on a previous Monju hike too. 

But can lightning really change a rock’s magnetic field? Well, the savants say it can. And, according to this blog, there are other “lightning fossils” on Japanese peaks, such as on Takayama (532m) in Yamaguchi Prefecture, Ryumonzan in Wakayama, and Utsukushigahara in Shinshu – the last named being one of those Hundred Mountains of Japan.

The shrine island of Oshima: strolling over to a magnetic anomaly.

But – hold it – the blogger also names Oshima (above), a rocky island more famous for its shrine and grove of cinnamon trees, as a locus of such strangely magnetised stones. Indeed, we were there with our guest just the other day: it's just an hour's drive away. But on Oshima the suspect rocks are located right by the sea in an old lava flow, not on any summit or eminence. Were they also supposed to have been recrystallized by lightning strikes? Hmm, this may need some further looking into…

By the time we get back to the car, a steady rain has set in, vindicating Alpinist A’s weather sense. It's a good thing we kept our outing short. Fortunately, there is no sign of any electricity in the air.