Ringing Rocks in Montana

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The Ringing Rocks are an outcrop of rocks in Montana near Butte that ring when struck with a hammer.  The rocks that make up the outcrop are weathered from a mafic pluton on the edge of the Boulder Batholith.  The Boulder batholith formed 80-70 Ma and originated at the trailing edge of the Eldorado-Lombard thrust.   A thin shallow mass of igneous rock intruded into pre-existing rock and at about the same time there was volcanic activity in the Elkhorn Mountains.  Volcanic ejecta of the same age and composition as the batholith covered the area of the batholith.  The ejecta was eroded away exposing the batholith. 1  The batholith is oriented in a northest southwest direction and is approximately 75 miles wide by 100 miles long.  It runs from south of Butte to near Helena.  The Butte mining district lies in the boulder batholith region.  Valuable minerals were concentrated in the Butte area due to the action of hydrothermal fluids on the magma.  Volatiles in the magma created fractures and deposited minerals in the fractures creating ore veins.  Around 5 Ma another pulse of hydrothermal fluids further enriched the veins. 2 

The Ringing Rocks are a large jumbled pile of boulders.  These boulders are variable in size from several meters to a meter.  They are light maroon color and are generally smooth except for some small pits and grooves on their surfaces.  The boulders are not spherical but roughly rectangular with rounded corners.  Their overall appearance is suggestive of rusted metal.  Fresh surfaces reveal black rock with a dull red surface coating.  The rock is a gabbro containing intermediate size interlocking grains of amphibole and pyroxene.  Occasional small blue crystals where seen in one sample, possibly labradorite.   The reddish surface coating is interpreted as iron oxide.2

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Fresh surface Benson, Rod. Rock Music in the Boulder Batholith. 22 July 2009

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 jumbled rocks.

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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 Ring ?

 

           

       

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. 

Directions to Ringing Rocks

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References

 

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.

           

2          Field notes

 

3          Lapp, David R. The Physics of Music and Musical Instruments. Medford, Mass: The Wright Center for Innovative Science Education,

 

4                    Tafoya, Claire. "Unusual percussion instrument."  E-mail to the author. 24 July 2009.

 

5                    Suits, B H. Frequencies for equal-tempered scale. 23 July 2009 <http://www.phy.mtu.edu/~suits/notefreqs.html>.

 

Robert L'Hommedieu    July 2009

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Bob L'Hommedieu