If you strike a rock, you’d expect to hear a dull ‘thud’. Or maybe a ‘chink’. Definitely not a ringing sound. So you’d be surprised to know that ringing rocks actually do exist. Nestled in the midst of the 128-acre Ringing Rocks County Park in Pennsylvania, is a field of unique boulders. Spread out across seven to eight acres, the boulders produce a distinctive metallic ‘clang’ when struck with a hammer or another piece of rock. Native Americans have known about the rocks for centuries, and passed on their knowledge to the first White settlers in the mid-1700s.

The sound produced by the rocks is so unexpected that it could get you wondering if they are really made of stone. They actually sound hollow and metallic. The strange phenomenon has baffled scientists and geologists for years. Several experiments have been conducted on the ringing rocks, but the exact reason for the unusual sound remains unknown.

Richard Faas, a geologist from Pennsylvania, tested a few of the rocks in his lab in 1965. He discovered that when struck, each individual rock produced low frequency tones that aren’t audible to the human ear. The tones from multiple rocks interact with each other and it’s the collective sound that we get to hear.

Photo: Chris Tengi

Faas was able to determine the nature of the sound, but he still couldn’t explain why these rocks were so special. They are made of a volcanic substance called diabase – consisting of iron and other hard minerals like all stones. But something about the composition makes them different. Some scientists believe that the stress within the rocks is responsible for the ringing.

It’s not just the ringing that makes this boulder field unusual. Most fields of rocks are created as a result of an avalanche from a collapsing mountainside. But this field is located at the top of a hill, not at the bottom. So it isn’t the result of a landslide. And there isn’t any evidence pointing to glaciers either. So how did the rocks get there?

Photo: J Morton Scott

There’s another very odd thing about the field – there’s almost no vegetation or plant life in the area. Not even insects. The 10 foot deep field is hotter than the forest surrounding it and provides very little in terms of food or shelter. A few have claimed that compasses don’t work in the barren area, but no trace of high background radiation, abnormal magnetic fields or strange electromagnetic activity has ever been found. So I suppose that rules out the supernatural angle as well.

But we don’t really need to know the science behind the ringing rocks in order to admire them. Ringing Rock Park attracts thousands of tourists each year, who can’t seem to get enough of striking the boulders. Some of them have even found ways to play the rocks like musical instruments.

It all began way back in 1890, when Dr. J. J. Ott, a Bucks County local, tried to create music with the rock’s acoustic qualities. He arranged a concert at Stony Garden for the Buckwampum Historical Society, accompanied by the Pleasant Valley Band, a brass ensemble. This was perhaps the first ‘rock’ concert ever. In more recent times, local musicians regularly have jam sessions at the park – striking the boulders with various tools like other rocks, sticks, railroad spikes and hammers.