In some corners of the US, you can find gigantic piles of boulders that make distinct musical chimes when hit with a hammer.

Ringing Rocks outside of Butte, Montana. Probably the only such area where the public is encouraged to hit public land with hammers. Image credit: Bureau of Land Management

Ringing rocks, also known as sonorous rocks or lithophonic rocks (also used in idiophonic musical instruments called lithophones) are rocks that resonate like a bell when struck. In the US, you can find them in Ringing Rocks Park, Bucks County, Pennsylvania, and in Pluton, Montana.

How these rocks got their musical abilities is unknown, but it is believed that a combination of the composition of these particular rocks and how the pile has eroded may contribute to this unique quality.

Just check this out. (Note that the rocks no longer ring if removed from the pile)

Several early scientists became interested in the ringing rocks; however, none were able to formulate a credible theory on the ringing ability of the rocks or the formation of the boulder fields. In 1965 geologist Richard Faas of Lafayette College in Easton, Pennsylvania, took some off the rocks to his lab for testing and found that, upon being struck, the rocks created a series of tones at frequencies lower than the human ear can hear. He concluded that, out in these special rock fields created by nature, an audible sound is only produced because these tones – or stones, really – interact with each other. But although Faas’s experiments explained the nature of the tones, they did not identify the specific physical mechanism in the rock which actually made them.

In the Pennsylvania/New Jersey area, the locations of over a dozen ringing rock boulder fields have been identified, alas the majority are either on private property or have been obliterated by urban development. At present, there three sites north of Philadelphia that are readily accessible to the public: Ringing Rocks County Park, Stony Garden, and Ringing Hill Park.

Locations of several known ringing rock boulder fields in Pennsylvania and New Jersey. Image credit: Andrews66

The ringing rock boulder fields of southeastern Pennsylvania and New Jersey are all a form of felsenmeer (a German term meaning “sea of rock”). These barren block fields occur in periglacial environments where outcrops of resistant rock are exposed with a slope of less than 25°. Frost wedging breaks up the upper portion of the rock formation, and the slight dip of the field allows the fine weathering materials to be flushed away before soil can develop. The accumulation of snow and ice often lifts and rotates the boulders, leaving a considerable amount of vacant space between them.

So far, there has been only one published scientific experiment on the source of the ringing ability. In the 1960s, a Rutgers University professor did an informal experiment whereby he sawed “live” and “dead” ringing rock boulders from the Bucks County park site into thin slices and then measured them for changes in shape. He found that while the dead rocks showed no change after the rocks were sawed, live rocks – the ones that make sound – showed a distinctive expansion or “relaxation” within 24 hours of being cut, indicating that the rock was under internal elastic stresses released by the mechanical sawing of the rock.

This led the professor to making the observation that the live rocks were generally found toward the middle of the boulder fields, where they did not come in contact with soil and the shade of the surrounding trees. He then theorized that the stresses were caused by the slow weathering rate in the dry “microclimate” of the fields, making the outside skin of the boulders expand due to the conversion of pyroxene to montmorillonite (a clay mineral). Boulders along the periphery of the fields, on the other hand, weather too quickly and tend to break apart before the stresses can develop.

Although more rigorous testing needs to be done to verify these results, they do suggest strongly that the ringing ability is a direct result of internal stresses.

In the US, you can also find ringing rocks between Butte and Whitehall in the mountains of southwest Montana. The Ringing Rocks Pluton found here is actually the deep-seated vent of a volcano which last erupted 76 million years ago. A pluton basically means magma mixing in a conduit (the volcano’s central vent), specifically between olivine basalt and granitic magmas – a process that created a hybrid rock type crystallizing against the outer wall of the volcano’s crater and being exposed to the surface after millions of years of uplift and erosion. Here in Montana, periglacial freezing during the Pleistocene Epoch eventually shattered the high standing walls of the volcano, allowing a large tor of ringing boulders to come to the surface.

Ringing Rocks Pluton in Montana, USA. The hybrid ringing rocks (lower left) crystallized against the outer wall of the volcano’s crater and were exposed to the surface after millions of years of uplift and erosion. Image credit: Andrews66

Apart from the US, similar musical stones can be found in Skiddaw in the English Lake District; in Kiandra, New South Wales; and in the Bell Rock Range of Western Australia. They have a sound very similar to that of their US counterparts.