Around the world there are strange fields of boulders that "sing" when they are tapped. We'll let you listen to some stones singing, and tell you why scientists think these rocks make music.

Throughout the world there are fields full of rocks with an international reputation. Pennsylvania has one. Brittany has one. Baja California has one. Toss a pebble around the field and most of the rocks will make the dull thunking sound that one might expect. Others will make an echoing ring, like the pebble has just hit an empty aluminum can. These fields are called "singing stones" or "ringing rocks" and they all originate in a volcano.

We hear only a fraction of the sound the rock makes. It also emits sound in frequencies too low or high for us to pick up on. The stones are made of basalt, which is a volcanic rock, high in iron, that was brought to the surface of the Earth and quickly cooled off. While it seems like the high iron content - about ten to twelve percent - is the reason why the rocks ring, it's really something else. What we're hearing isn't the sound of metal, but the sound of tension. When ringing rocks were compared with regular stones, stress gauges showed that their inner materials were under considerable strain.

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Scientists John Gibbons and Steven Schlossman sawed slices off a singing rock and compared it with slices off a regular stone. Both the strain and the rock's ring dissipated within a day. Gibbons and Schlossman believe that the stress on the rock caused its materials to be more elastic, and more able to make sound. In addition to initial strain of cooling, water may have seeped into the rocks and expanded bits of clay, putting more and more internal stress on the rock. Although we think of the rocks as ringing like metal, the clay, and its consequences, mean that when we hear the echoing sound, we're actually hearing ceramics. So the rocks have more in common with the ringing of your coffee cup than with cymbals.

[Via If Rocks Could Sing, Ringing Rocks of Pennsylvania.]

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