Everybody is familiar with ocean tides that cause the ocean level to go up and down, usually twice a day. We can go down to the beach and watch this tidal action along any coast in the world. On a global level, the ocean tides are actually waves whose crests are half a world apart and traveling from east to west. At most points on the globe, the tidal crest comes by about every 12.5 hours.

The engine driving this phenomenon is the gravitational attraction of, primarily, the moon, but also the sun. The earth completes one rotation every 24 hours, and the ocean tides can be imagined as a watery bulge that remains relatively stationary while the planet rotates inside.

The biggest tides occur about every two weeks, when the sun and moon are aligned (either full or new moon). Hence they are called the "fortnightly" tides.

Few realize that the solid earth also exhibits tidal behavior, with bulges on opposite sides of the globe, also driven by the moon. At the Hawaiian Volcano Observatory, we can actually measure these tides with our tiltmeters and strainmeters.

The earth's surface tilts up to 0.03 microradians in response to the apparent passage of the moon overhead. A tilt of one microradian is the tilt of a solid bar one kilometer (0.6 miles) long with one end raised by the thickness of a dime. To emphasize how small the tidal tilts are, our tiltmeters automatically alert us to the possibility of volcanic activity when tilts change more than 0.5 microradians in 5 minutes.

Who would have thought that the moon had that kind of power, not only to be able to cause the world's oceans to bulge, but also to squeeze terra firma twice a day? But it does, so it should not come as a complete shock that reputable scientists have suggested that these squeezings might influence whether a volcano will erupt or not.

The idea is that if a volcano is full of magma, the squeezing at the fortnightly tidal maximum might be just enough to overcome the resistance of the crust, push magma out, and get an eruption going. Once started, the eruption would continue on its own.

More than 25 years ago, a pair of earth scientists compared the records for 680 eruptions that occurred since 1900 and found that "the probability of an eruption is greatest at times of maximum tidal amplitude." In plainer language, volcanoes are more likely to erupt at the fortnightly (or 14-day) "high" tide.

A specific look at 52 Hawaiian eruptions since January 1832 shows the same sort of pattern.

"Nearly twice as many eruptions have occurred nearer fortnightly tidal maximum than tidal minimum." Hawaiin Volcano Observatory scientists have noted that the Pu'u 'O'o fountaining episodes each occurred remarkably close to fortnightly tidal maximums and that the first set of eruption pauses in 1990 (periods during which the eruption turned off for up to a few days) occurred remarkably close to fortnightly tidal minimums.

Great! Now let's start predicting eruptions based only on this information. The fortnightly tidal maximum occurs at full and new moons, every 14 or so days. The next tidal maximum will be the new moon on November 15 -- will Mauna Loa erupt then? Almost certainly not.

Although this is a fascinating correlation, there are just too many tidal maximums and too many volcanoes to base predictions on tidal cycle alone. In the Hawai'i example of 52 eruptions since January 1832, there have been nearly 3,900 tidal maximums, of which roughly 3,850 went by without causing an eruption. Statistically, this is about a one percent chance that any tidal maximum will affect the start of an eruption.

The correlation is more important as a clue to how volcanoes work. The effect of the tides suggests that a volcano can remain in a state of near eruption for a period of time before some threshold is exceeded and an eruption starts. There are probably many possible mechanisms for exceeding that threshold -- the lunar tides are but one. (source - Hawaii Volcano Observatory).