Welcome to Ring Rocks Park in Pottsgrove, PA. You will need to bring a hammer with you. This boulder field contains one of the most interesting and rare phenomena on the planet. Only a few places on Earth have this same trait. When the boulders are struck, instead of the making a dull clang, the boulders create a clear ringing note that has been compared to the ringing of bell. These rocks are known as sonorous rocks or lithophonic rocks. The boulder field was first identified in 1742 when a road was cut between Pottstown and New Gosenhoppen (Pennsburg).
The boulders were formed around 200 million years ago during the Jurassic Age. Ancient Lake Lockatong had covered most of Bucks County and then had gradually filled with sediments that had hardened into shale. Molten rock within the earth was forced into the shale and hardened into a ledge of olivine diabase rock, sandwiched within the shale. But over the eons, the shale’s upper layer eroded and exposed the diabase. Subsequent severe freezing during the last Ice Age, when glaciers came close to but did not reach Bucks County, causing so-called periglacial conditions, broke the diabase into boulders.
What makes the boulders ring is still a mystery. The most extensive research on the boulders was conducted in the 1965. Richard Faas took “live”, boulders that ring, and “dead”, boulders that don’t ring, to experiment on. He found that when the rocks were struck they created a series of tones at frequencies lower than the human ear can hear. An audible sound is only produced because these tones interact with each other.
The most plausible explanation that has yet to studied is that internal stress of the rock causes the sounds. In the 1960s a Rutgers University professor did an informal experiment where specimens of 'live' and 'dead' ringing rock boulders from the Bucks County park site were sawn into thin slices and then measured for changes in shape (Gibbons 1970). The rock slices were measured with delicate foil strain gauges which could measure minute changes in size. According to the professor, the dead rocks showed no change after the rocks were sawn, however the live rocks showed a distinctive expansion or 'relaxation' within 24 hours after they were cut. This relaxation indicates that the rock was under internal elastic stresses which were released by the mechanical sawing of the rock. This suggest that the ringing ability comes from the internal stress of the boulders. It works similar to how string instruments work. When a violin string is relaxed the violin will not produce any notes. However, when you pull the strings tight, you will be able to hear all the notes on the scale.
Your Mission:
1.) Describe in your own words what the “live” boulders sound like. (Use a hammer to hit the boulders to create the sound.)
2.) Estimate the size of the boulder Field.
3.) What is the theorized reason for the sounds the boulders produce.
4.) What type of rock are they formed out of?
5.) The diabase boulder have a unique appearance to the normal brown shale stone that is common to Pennsylvania. You can find boulders that have been split open in the boulder field. Please find a boulder that exposes it's insides and describe the appearance of the diabase that is exposed.
Optional: 6.) Include a photo with you at the Ringing Rocks in your found it log.
Sources used in this geocache listing includes:
https://en.wikipedia.org/wiki/Ringing_rocks#Diabase_Ringing_Rock_Boulder_Fields_in_Pennsylvania_and_New_Jersey
https://marciabonta.wordpress.com/2015/06/24/stony-garden/