|Ringing Hill Park. Photo: Sherry M/TripAdvisor|
Although scientists don’t have a definitive answer as to why the rocks ring, they do have a name for ringing rocks. They are called with sonorous rocks or lithophonic rocks. These are the terms that are given to rocks that resonate with a bell-like sound when they are struck.
Sonorous rocks are rocks that resonate like a bell when struck. These chime-like sounds come from geological phenomena known as ringing rocks. Physically, they look no different from regular rocks, but it’s not until you lightly strike them with a hammer that the rocks reveal their sonic secret.
Stones do not usually ring, but when these particular stones are struck lightly with a hammer, they will vibrate and make a sound not unlike that of a bell. Why they do this is still a mystery to science. The rocks themselves are composed of diabase, the same type of rock that makes up most the earth's crust. Another part of the mystery is while all the rocks seem to be made of the same material (mostly iron and hard minerals) only one-third of them generate the ringing sound when hit. Rocks that ring are known as "live" rocks, and those that don't are referred to as "dead" rocks.
Although there have been over a dozen diabase ringing rock boulder fields identified in the Pennsylvania/New Jersey area, the majority are either on private property or have been obliterated by urban development. There are three sites north of Philadelphia which are readily accessible to the public: Ringing Rocks County Park, Stony Garden, and Ringing Hill Park.
Geology:These boulder field in southeastern Pennsylvania and central New Jersey formed from a group of diabase sills in Newark Basin. The sills were formed when stretching of the Earth's crust allowed mafic magma to travel up from the upper mantle inject into the sedimentary basin 200 million years ago (early Jurassic Period).
The basal olivine unit is similar to the one found in the Palisades Sill in New Jersey and New York. The olivine diabase unit is significantly harder, denser, and more resistant to weathering than the upper portions of the diabase sill.
Most observers did not make the distinction between the upper "normal". diabase which is found over wide areas, and the thin olivine diabase unit found at the base of the sills which actually produces the ringing rock boulder fields. A main factor in the confusion is the outward appearance of the rocks. Both the normal and olivine diabase rocks are dark grey to black in color. Microscopic examination is often required o identify the differences.
Although the Newark series diabase sills crop out in a belt throughout the length of the Appalachian Mountains, only a narrow band of outcrops in southeastern Pennsylvania and New Jersey develop ringing rock boulder fields. The probable reason is that these areas were at the southern edge of the Pleistocene glaciers, and would have been subject to extreme periglacial conditions. Periglacial boulder fields are a common feature in Pennsylvania and New Jersey.
All of the observed ringing rock boulder fields in southeastern Pennsylvania and New Jersey are a form of felsenmeer. 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.
What We Know About the Rocks’ Ringing Ability:There has been a great deal of controversy concerning the ringing ability of the boulders; conversely, there has been an almost complete lack of testing to support the conjectures. Conditions such as size and shape of the boulders and the way that the boulders are supported or stacked certainly influence the sounds that the boulders make but do not in themselves impart the ringing ability.
Although the sound is often described as metallic, it is most likely due to a combination of the density of the rock and a high degree of internal stress. The sound can be duplicated on a small scale by tapping the handle of a ceramic coffee cup.
The iron content of the diabase is often identified as the source of the ringing ability. Actual chemical analysis of the Coffman Hill diabase shows that iron content (as ferric oxide) of the rock ranges from 9% and 12%. Although comparatively high for an average igneous rock (3% is typical for granite), it is within the normal range for a basalt. This point suggests that the iron content is not a primary factor in the ringing ability.
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 where specimens of "live" and "dead" ringing rock boulders from the Bucks County park site were sawed into thin slices and then measured for changes in shape. The rock slices were measured with delicate foil strain gauges, which could measure minute changes in size.
A more plausible theory is that the elastic stresses remained in the rock when the boulder fields formed, and the slow weathering rate keeps the stresses from dissipating. A possible source of the stresses would likely be the loading stresses from the time when the rock crystallized. The diabase sill formed at roughly 1.2–1.9 miles (2–3 km) beneath the surface.
This "relict stress" theory implies that the ringing rock boulders act much like a guitar string. When a guitar string is limp it does not resonate, but a plucked string will provide a range of sounds depending on the level of applied tension.
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