There are no small
rock fragments such as gravel at this outcrop. All the rocks exist as large blocks. This gives the impression that some unusual process is at work to create a pile of
boulders without the small rock debris that we often see at outcrops. Frost wedging
is thought to have created this jumbled arrangement. I saw a small boulder at
Grinnell Glacier in Glacier National Park that imitated the same
appearance as Ringing Rocks. The boulder was disintegrating in a small pile of
|Fractured Boulder at Grinnell Glacier
The most interesting
aspect of Ringing Rocks is the sounds they make when struck with hammer. Click
on the links below to hear the rock music.
Ringing Rocks Video #1
Ringing Rocks Video #2
Why do the Ringing Rocks
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.
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
Directions to Ringing Rocks
1 Lageson, David R., James G. Schmitt, Brian K. Horton, and
Thomas J. Kalakay. "Influence of Late Cretaceous magmatism on the Sevier orogenic wedge, western Montana."
Geology 28.8 (Aug. 2001): 723-6.
3 Lapp, David R. The Physics of Music and Musical Instruments.
Medford, Mass: The Wright Center for Innovative Science Education,
Tafoya, Claire. "Unusual percussion instrument." E-mail
to the author. 24 July 2009.
Suits, B H. Frequencies for equal-tempered scale. 23 July 2009 <http://www.phy.mtu.edu/~suits/notefreqs.html>.
Robert L'Hommedieu July 2009