Ringing Rocks - Diabase Ringing Rock Boulder Fields in Pennsylvania and New Jersey

Diabase Ringing Rock Boulder Fields in Pennsylvania and New Jersey

In southeastern Pennsylvania and southern New Jersey there is a series of boulder fields which contain boulders that ring when struck by a hard object. Although various rock types are known to develop the ringing ability, these boulder fields fall into a specific category of being composed of olivine diabase. The boulder fields formed from a group of diabase sills located in the geologic Newark Basin. The sills were formed when mafic magma from the upper mantle injected into the sedimentary basin 200 million years ago (Early Jurassic Period). When the diabase sills were still molten crystals of olivine and pyroxene settled to the bottom of the sill and formed a thin cumulate unit. By the Pleistocene Epoch (most recent Ice Age - 2,588,000 to 11,700 years BP) the sills had been exposed to the surface by crustal uplift and erosion. During the Pleistocene outcrops of the olivine diabase were not buried by the glacial sheets, however they were subjected to severe freezing, or periglacial, conditions. The periglacial environment caused the outcrops to be broken into extensive boulder fields.

Location of the Diabase Ringing Rock Boulder Fields

Although there have been over a dozen diabase ringing rock boulder fields identified (see Pontolillo 1993 for an extensive inventory of sites), 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 in Bridgeton Township near Upper Black Eddy; Stony Garden on the north slope of Haycock Mountain (PA State Game Lands Tract #157); and the Ringing Hill Park in Lower Pottsgrove Township. The Bridgeton site is the one most commonly identified with ringing rocks, and has been the subject of numerous nature and travel articles.

• Location of Diabase Ringing Rock Boulder Fields

Geology of the Boulder Fields

These particular ringing rock boulder fields formed from a rock type called olivine diabase. Approximately 200 million years ago stretching of the Earth’s crust allowed basaltic magma to travel up from the upper mantle and inject as sills into a series of shale formations of the Newark Basin. Phenocrysts of two minerals that had crystallized in the upper mantle, olivine and pyroxene, quickly settled out of the magma and collected along the base of the sills. When fully solidified this crystal-rich layer formed a separate rock unit 10’-15’ (3–5 m) thick (McCray 1997). 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. It is difficult to distinguish the two in hand specimens, and often requires microscopic examination to 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. Felsenmeer is a German term meaning "sea of rock". 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. Often the boulders are lifted and rotated by the accumulation of snow and ice, leaving a considerable amount of vacant space between them. The felsenmeer boulder fields form in-situ due to the low slope angles. In situations where the slope angles are steeper than 25°.gravity tends to move the boulders downhill to form scree or talus slopes. In fields where the angle is too flat the spaces between the boulders fills with soil and the boulders break down due to weathering.

Because the olivine diabase unit is relatively thin there was an additional requirement for the formation of the ringing rock boulder fields: the olivine diabase unit had to be dipping in the same direction as the slope of the ground surface. This specific dip-slope situation allowed broad expanses of olivine diabase to be exposed and provided enough material to create the fields. In all of the observed boulder fields the structural dip of the unit was approximately 8-10°, and the ground surface slope was <15° in the same direction. Typically this occurred where the olivine diabase unit dipped at right angles into a shallow drainage. Where the olivine diabase does not dip in the same direction as the ground surface there is generally numerous boulders embedded in the soil (boulder colluvium), but no boulder fields.

• Typical Cross-section of a Ringing Rock Boulder Field

• Geology of the Ringing Rocks Park

• Geology of the Stony Garden

Ringing Ability of Individual Boulders

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 influences 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 (McCray 1997) shows that iron content (as Fe2O3) ranges between 9-12% of the whole rock for various samples throughout the diabase sill. Although comparatively high for an average igneous rock type (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 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. The professor went on to make the observation that the live rocks were generally found toward the middle of the boulder fields where they did not come in contact with soil and the shade of the surrounding trees. He then theorized that the slow weathering rate in the dry 'microclimate' of the fields caused the stresses because the outside skin of the boulders would expand due to the conversion of pyroxene to montmorillonite (a clay mineral). Boulders along the periphery of the fields weather too quickly and break apart before the stresses can develop.

Although more rigorous testing needs to be done to verify these results, it does suggest strongly that the ringing ability is a direct result of internal elastic stresses. Less likely is the assertion that outside weathering conditions created the stresses. It is very improbable that the expansion of a thin skin around the outside of a large boulder could set up a balance of forces that would create the severe stresses found in the ringing rock boulders. The result of such a situation would be that the outside skin of the boulders would peel or exfolliate, a condition that is virtually non-existent in any of the ringing rocks sites. In addition, the majority of the weathering in the boulders occurs at the exposed top surfaces, not on the bottoms, thus the stresses would not be balanced. As a reality check, if slow weathering created the stresses then there would be ringing rock boulder fields in deserts throughout the world, a condition which does not occur.

An important point made in the experiment is the observation that the rock slices expanded when the stress was released. This distinction requires that the rock was under extreme compressive stress, not tension as the slow weathering theory implies.

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 2-3 kilometers beneath the surface (Sevon 1999). The overlying column of rock creates severe stresses in rocks. The effects of these stresses can be seen in deep mines (> 1 mile depth), where the sudden decompression creates rock bursts. Residual loading stresses would be evenly distributed throughout the boulders. This theory supports the observation that usually less than a third of the boulders in any given field are 'live'. The stresses are relieved in boulders that have been either mechanically broken or are severely weathered, and therefore no longer ring.

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. Likewise, a ringing rock boulder will only emit a dull thud if the boulder is de-stressed, however boulders will resonate at various frequencies depending on the level of residual stress.

The boulders continue to ring when removed from the boulder fields. Myths have been developed by authorities to discourage the theft of boulders from the fields. At the current stage, however, most fields have been picked clean of small portable 'ringers', and breaking of large boulders into smaller pieces releases the internal stresses - thus causing them to stop ringing (i.e., breaking a piece off of a large ringing rock will only gain a dead chunk of rock and destroy a natural curiosity forever.) 'Small' ringers found today weigh over a ton, and would have to be dragged out of the boulder fields using large equipment.

Weathering textures

A curious aspect of the ringing rock boulders in all of the boulder fields is the presence of odd weathering textures. Often the weathering textures take on the form of channels, grooves, 'potholes', 'mudcracks', and intense pitting. In some cases the textures are so distinctive that some geologists refer to them in terms of carbonate rock solution features (Sevon 1999). Inspection of the boulders reveals that the weathering patterns are not due to any internal variations in the rock, but have been superimposed onto the surfaces. The probable source of these textures was chemical weathering along joint surfaces at the time that the rock was still in place and before the boulders were broken out by frost heaving (Psilovikos 1982). The soft weathered portion of the outer surfaces flaked off once the boulders were exposed to the air. Small features on the boulder surfaces became exaggerated as the weathered material was removed, so that adjoining fissures became channels, dents became 'potholes', and old surfaces became intensely pitted.

• Weathering channels, Devil's Potato Patch

• Weathering 'potholes', Stony Garden

• Intense pitting, Stony Garden