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.