Ringing Rocks - Ringing Rocks Pluton, Jefferson County, Montana

Ringing Rocks Pluton, Jefferson County, Montana

The Ringing Rocks Pluton is located in the mountains of southwest Montana between Butte and Whitehall, and is notable for a large tor of boulders which ring when struck with a hard object. The pluton is the deep-seated vent for a volcano which erupted 76 million years ago. The peculiar aspect of the pluton is that it is an excellent example of magma mixing in a conduit, specifically between olivine basalt and granitic magmas. Mixing of the magmas created a curious hybrid rock type which crystallized against the outer wall of conduit. After millions of years of uplift and erosion the thin walls of hybrid rock were exposed to the surface. During the Pleistocene Epoch periglacial freezing shattered the high standing walls to form a substantial tor.

Location

The Ringing Rocks Pluton is located on the southwestern flank of Dry Mountain in Jefferson County, 15 miles southeast of Butte in T.2 N., R.5 W., sections 4 and 9. Section 4 is included in the Deerlodge National Forest, and section 9 is under the jurisdiction of the U.S. Bureau of Land Management. A distinctive tor is marked on the U.S. Geological Survey Dry Mountain 7½' quadrangle. The NW¼ of Section 9, which includes the tor, was established as the Ringing Rocks Recreation Area by the USBLM in 1964. Access is by a gravel road leading 3 miles north of the Pipestone exit off I-90.

• Location of the Ringing Rocks Pluton

• Aerial photograph of the Ringing Rocks Pluton

Geology of the Ringing Rocks Pluton

The Ringing Rocks Pluton is a small-scale intrusive ring complex with a high degree of symmetry. The overall geometry is cylindrical, with an average diameter of one kilometer. The pluton consists of two major units: an inner core of quartz-bearing felsic units with a diameter of 600 meters; and an outer mafic unit composed of coaxial concentric layers. In map view the complex has a bulls eye configuration. The outer zone of the pluton consists of two alternating mafic hybrid rock types. In chemical analysis the rocks are nearly identical, however the early stage minerals, olivine and pyroxene, were preserved in one and not the other, giving the rocks sharply different weathering properties. The rock that retained the olivine and pyroxene crystals (OPM) is extremely resistant to weathering and is the material which forms the tors. The altered rock (AM) conversely is very weak and readily breaks down to a coarse soil. The inner felsic core is a medium grained quartz monzonite that gradually grades into granite at the center. The transitional unit between the mafic zone and the felsic core units is completely obscured, and is represented by distinctive low-lying swales. A series of radial dikes punctured the mafic units, beginning in the felsic zone and terminating at the outer border of the intrusion. The dikes are generally 2–4 meters wide where they are in contact with the felsic unit. The composition is highly variable, but for the most part is composed of fine to medium grained leucocratic monzonite. The dikes become narrower as they extend toward the outer border, and generally taper to less than 0.5 m in width.

• Geologic map of the Ringing Rocks Pluton

• Cross-section of the Ringing Rocks Pluton

Microscopic Quench Textures in the Olivine Pyroxene Monzonite

There are numerous textures in the hybrid OPM unit which indicate that the rock was rapidly quenched during crystallization.

Crystals of cryptoperthetic orthoclase up to 1 cm in length can be seen in hand specimens throughout most of the OPM units. The cryptoperthite crystals typically show as deep blue flashes, although white and yellow flashes can be seen in rocks nearer to the contacts. The flashes are due to labradorescent internal reflections, sometimes called a moonstone effect. In early reports (Butler 1983, Johannesmeyer 1999) this mineral was mistakenly identified as labradorite. Cryptoperthite textures form when orthoclase (a feldspar mineral containing both potassium and sodium cations) is quenched very rapidly. Sodium has a much larger ionic radius than potassium does, so during the quench event the sodium ions pop out of the crystal framework. These sodium ions form tiny oriented lenses of albite (sodium silicate feldspar) that are too small to be seen even using a microscope. It is the reflections from these albite laminae that produces the flash.

Apatite occurs as elongated needles . Many of the needles enclose narrow fluid inclusions, giving the crystals the appearance of being ‘hollow’.

Zircon crystals were identified in skeletal frameworks.

• Orthoclase cryptoperthite crystal with blue labradorescence

• Skeletal zircon crystal

• 'hollow' apatite crystal

Similar Intrusions

The Ringing Rocks Pluton is one of several bimodal vent complexes that are located within a 20 km belt east of the main Butte Pluton. Radiometric dating and cross-cutting relationships place most of these intrusions as post-Elkhorn Mountains Volcanics and pre-Butte Pluton. Similar intrusions can be found in the Rader Creek Pluton.

• Igneous geology of the Elkhorn Thrust Plate, southwest Montana

The Ringing Rocks Tor

The pile of loose boulders at the south end of the pluton which contains the ringing rocks is called a tor. The reason the tor developed is that the OPM units are extremely resistant to weathering, and conversely the adjoining AM and ABM units are very weak. As the Dry Creek drainage began to erode the NW corner of the pluton, the softer AM and ABM units were quickly stripped away, leaving vertical thin walls of OPM units standing high above the surrounding landscape. Intense freezing and thawing during the Pleistocene periglacial period slowly shattered the walls, much like breaking tempered glass. The remnants of the OPM walls is the material which formed the tor. At the north end of the pluton the orientation of the OPM units was at an acute angle to the Dry Creek drainage so that the tor there did not develop very well. On the south end, however, the OPM outcrop was nearly at a right angle to the drainage, producing a prominent tor.

• Tor at the south end of the Ringing Rocks Pluton

Ringing Properties of the Individual Boulders

Although the rocks are from different geologic settings, the Montana ringing rocks share significant characteristics with the Pennsylvania diabase ringing rocks. These characteristics include being composed of igneous mafic rock types with high percentages of olivine and pyroxene phenocrysts, having the individual boulders isolated from severe weathering by the formation of well-drained boulder fields, and having similar sounds and surface weathering.

The iron content of the olivine pyroxene monzonite (as FeO) is 7% of the whole rock (Butler 1983, Johannesmeyer 1999). As in the Pennsylvania diabase ringing rocks, this point suggests that iron content is not a primary factor in the ringing ability.

Despite the broad public interest in the ringing ability of the ringing rocks there has not been any actual scientific studies to identify the source of the phenomenon.

Weathering Textures in the Olivine Pyroxene Monzonite

Boulders of the olivine pyroxene monzonite develop odd surface weathering patterns, similar to the textures seen in the Pennsylvania diabase ringing rocks. These textures include channels, grooves, and 'potholes'.

• Weathering channels, OPM unit

• Weathering 'potholes', OPM unit

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