Why don’t all rocks make a ringing sound like Ringing Rocks?  Why is this phenomenon so uncommon?  When in their natural history did this pile of boulders acquire the physical characteristics necessary to resonant at audible frequencies and how much longer will they retain this ability?   We don’t know the answers to these questions and without detailed research we won’t know the answers but we can make some general statements and speculations about the necessary conditions for the rocks to ring.

 When struck with a hammer, the rocks make a sound which persists after the initial blow.  This persistence of sound is called resonance.  It implies that the rock continues to vibrate at a natural frequency and produce compression waves that we perceive as sound.  The human ear can detect sounds with a frequency of 20 – 20,000 hz so the rock must have a natural resonant frequency in this range.  Resonance requires a rigid material, freedom from internal reflection within the material, isolation from vibration dampening contacts, and a length equal to some integer multiple of half the wavelength of the sound wave.³   

            The rocks are part of a mafic pluton.  Mafic rocks are denser than felsic rocks and contain higher quantities of iron.  Since most crustal rocks are felsic, the ringing rocks are denser than most of the rocks we normally experience.  Since the ringing rocks formed at the same time from the same pluton, they have an almost identical composition and internal structure.  Sedimentary and metamorphic rocks are less likely to be so homogenous.  Igneous rocks have interlocking grains that form during cooling of the pluton making them potentially more rigid than sedimentary rocks with cemented grains and metamorphic rocks with foliations.  Their interlocking grains may also reduce the occurrence of internal reflection of sound waves compared to sedimentary and metamorphic rocks.  The physical weathering of the pluton has resulted in a jumbled boulder pile in which large blocks are in limited contact with their neighbors.  This helps isolate the rocks and minimize dampening of their vibrations.  Chemical weathering of their surface has created a relatively uniform surface, a patina of rusted minerals and resulted in rounded corners that may also play a role.  Observation tells us that the ringing rocks are large boulders and their massive size is probably a clue.  A professional musician has reviewed the videos and she was able to recognize 4 pitches all above middle C:  E, F#, G#, and B.⁴ These pitches have frequencies ranging from 329.64 to 493.92 hz and wavelengths of 70 to 105 cm.⁵  In order to resonant at those wavelengths, the rock must have a length equal to some integer multiple of half the wavelength of the sound wave.³    The minimum rock length necessary to produce these notes is 35 to 52.5 cm. We may not be able to fully explain why these rocks make musical sounds but maybe that is just as well.  A little mystery makes them all the more appealing and their sounds that much more enchanting.