From Mount Ontake, an Ominous Reminder that Earth Is Still Random

On Saturday Mount Ontake, Japan's second-tallest volcano, erupted, initially stranding more than 250 hikers and killing at least 30. The volcano continues to emit smoke today, and the release of toxic gases has hindered rescue efforts in the area. No one saw it coming, not even Japan's Meteorological Agency (JMA), thought to host the most advanced monitoring network in the world.

"There were no other signs of an imminent eruption, such as earth movements or changes on the mountain's surface," a JMA official told the Guardian. Yet, given the type of eruption unleashed by Mount Ontake (a phreatic eruption), there should have been a large earthquake that would have triggered the warning system.

A phreatic eruption is called as such because the magma underneath the ground has heated the ground water so quickly it turns into steam, which forces an explosion and a corresponding earthquake. This explosion is the gray cloud of evaporated water, rock, ash, and large balls of lava known as "volcano bombs." The latter projectiles get their name for how far they travel through the air and the aerodynamic shape they acquire by doing so.

According to the JMA, Mount Ontake didn't have one of these large earthquakes. This isn't some bullshit line from an agency that dropped the ball; the JMA system is considered to be the most extensive and sophisticated in volcano and earthquake monitoring worldwide. Japan has more volcanoes than any other country in the world, with the mainland boasting 109 volcanoes and the total figure rising to 118 if you count the volcano islands nearby. According to the JMA, 110 of these are active.

Japan has so many volcanoes because it is located right off what is called a "a triple-plate subduction boundary." In plainspeak, this means the island is located next to where three of the Earth's tectonic plates come together—tectonic plates being hard pieces of the Earth's outer layer, scientifically referred to as the lithosphere.

There are seven (or eight depending on the definition you are using) major plates covering the Earth, with lots of smaller ones. When these plates move against each other (the layer underneath is more viscous than solid so these plates shift regularly) they cause earthquakes and volcanic eruptions on Earth's surface. And when these plates mash together they can also create mountains, like the Himalaya.

Japan is located where two major plates and one of the largest of the smaller plates rub against each other. The movement and friction between these plates is responsible for the nation's earthquakes (and tsunamis), volcanoes, mountains, islands, and even Japan itself.

To deal with all of this tectonic activity, the JMA has the largest observation network in the world for monitoring seismic waves, e.g. vibrations in the Earth caused by movements deep underground. The JMA operates 187 seismographs and 627 seismic intensity meters of its own, while collecting data from more than 3,600 seismic intensity meters managed by National Research Institute for Earth Science and Disaster Prevention ( NIED), a multinational effort based out of Japan.

According to the JMA, roughly 15 volcanic eruptions and more than 1,500 earthquakes happen each year just in Japan's vicinity. The agency has its hands full.

Besides activity on Mount Ontake on Saturday, last Wednesday two earthquakes hit the Fukushima Prefecture. Earlier this month, the Suwanose-jima volcano on Ryukyu island emitted an ash plume, as did the Sakurajima volcano on the island of Kyushu, which also had its threat level elevated as it was exploding multiple times a day.

In addition to all this activity, the small island of Nishinoshima, which was overtaken by lava coming from a newly-formed volcano last year, continues to grow bigger each day as lava keeps spewing. If this lava falls off the newly formed island into the ocean, officials are worried it will cause a tsunami.

Of the hundred-plus seismographs in the country, 47 are placed near the most active volcanoes, with Mount Ontake having a few. The seismometers in place around Mount Ontake had actually picked up some minor rumblings underground since Sept. 10, but JMA officials didn't think much of it. They seemed a minor and a regular occurrence compared to those in history (like when Mount Ontake erupted in 1991) and others happening around the country on a regular basis.

Mount Ontake erupting without a large earthquake to trigger an alert is something new. With many thousands of pieces of monitoring equipment spread across Japan, yesterday was an ominous demonstration that Earth can still be random.