Volcanism in the Valley:  A Brief Examination of Trimble Knob, Monterey, Virginia

Mark A. Tinsley

ES 767, Global Tectonics
Spring 2008

Introduction
Location
Regional Setting and History
Lithology
Genesis
Conclusion and References


Introduction

When considering the geology of the Valley and Ridge province of Virginia (Figure 1), one is not typically led into a discussion of igneous activity.  This is because the Valley and Ridge province is predominantly composed of thrust faulted and folded Cambrian to Mississippian sedimentary rocks deposited in tidal flat and deep marine basin environments (Fichter & Baedke, 2000).  However, scattered throughout the Valley and Ridge are anomalous igneous features that have confounded geologists for years.  These include such bodies as dikes, sills, volcanic necks, and diatremes.  Since the 19th century, geologists have attempted to place these igneous intrusions into their historical context, but such has proven quite difficult (Tso et al., 2004).  One example of these elusive igneous features is a small conical-shaped hill (Webb, 1988) in Highland County, Virginia, known as Trimble Knob (Figure 2).  Popularly described as a "volcano," Trimble Knob has both fascinated and confused those who have studied it from either a scientific or aesthetic perspective. 

Physiographic Provinces
Figure 1:  Physiographic Provinces of Virginia.  Valley and Ridge shown in green. 
Taken from Fichter and Baedke, 2000.



trembleknob
Figure 2:  Photo of Trimble Knob.  Taken from Tso et al., 2004

Location

Trimble Knob is located approximately 1.1 km southwest of the small village of Monterey, Highland County, Virginia (Figure 3).  Monterey itself is located at the junction of Routes 220 and 250 in the west-central portion of the state.

location
Figure 3:  Location of Trimble Knob southwest of Monterey, Virginia. 
Taken from Tso et al., 2004.

Regional Setting and History

Early Phanerozoic to Present

The history of the Valley and Ridge province of Virginia is quite complicated.  By about 540 million years ago (mya) during the Late Cambrian period, a divergent continental margin had formed between what is now the Allegheny Plateau and Blue Ridge provinces of North America (Figure 1).  This margin resulted from rifting of the Rodinia supercontinent into its North American and African segments and subsequent opening of the Iapetus Ocean, which had begun some 100 million years earlier.  The quiescent continental margin thus formed deposited thick beds of limestone and dolostone.  Today, these sediments are preserved in the ubiquitous lower carbonate rocks of the Valley and Ridge province such as the Elbrook, Beekmantown, and New Market Formations.

However, this divergent continental margin was not to last forever.  Beginning about 450 mya during the mid-Ordivician period, an island arc terrane called the Chopawamsic/Arvonia collided with eastern North America precipitating the Taconic orogeny.  As a result of this orogeny, a deep flysch basin formed in what is now the Valley and Ridge province.  Into this basin flowed voluminous sediments from the Taconic Mountains to the east.  Formations such as the Massanutten Sandstone, Martinsburg Shale, and Oswego Sandstone testify clearly to this mountain building event. 

After an approximately 80 million year period of inter-orogenic calm during which limestone deposition predominated in the aforementioned flysch basin (Tonolaway Limestone and Helderburg Group representative), a second terrane, called Avalon, collided with eastern North America.  Known as the Acadian orogeny (Early Devonian period), this mountain building event deepened the flysch basin resulting in the deposition of green, gray, and red sandstones and shales.  Collectively, these sediments are identified today as the Catskill Clastic Wedge.  The Needmore, Millboro, Brallier, Greenland Gap, Hampshire, and Price/Pocono Formations are all members of the Catskill.

Following the Acadian orogeny was another period of inter-orogenic calm lasting roughly 30 million years.  During this calm period, the then-shallow flysch basin (having been filled with the Catskill Clastic Wedge) precipitated fossiliferous limestone sediments.  These are represented presently by the Greenbrier, Bluefield, and Princeton Formations.  This calm period, however, was interrupted by yet another mountain building event, namely, the colossal Alleghenian orogeny (Permian period).  The largest of the orogenies, the Alleghenian resulted from the collision of North America and Africa and the closing of the Iapetus Ocean.  The supercontinent thus formed is known today by the name Pangaea.  Extensive folding and thrust faulting characterized this mountain building event, and Himalaya-sized mountains were formed.  The distinctive folds of the Valley and Ridge province and the beautiful Blue Ridge Mountains are testament to the Alleghenian orogeny.

The final, major tectonic event to strike Virginia occurred in the Late Triassic to Early Jurassic periods approximately 200-230 mya.  This, of course, was the rifting of Pangaea into Africa and North America and the creation of what is now the Atlantic Ocean.   By the Cenozoic era roughly 70 mya, the east coast of North America had become a mature, divergent continental margin marked by quiet marine deposition.  This margin persists to the present day (Fichter 1997; Fichter & Baedke, 2000). 

Eocene Intrusions

As a result of the aforementioned tectonic activity, many igneous intrusions can be found throughout Virginia.  Some are associated with the three major orogenies mentioned; however, most of these igneous bodies were emplaced during the Triassic/Jurassic rifting of Pangaea.  The famous Triassic Basins of the Virginia Piedmont (Figure 1) are the largest and most studied of these intrusions.   Nevertheless, smaller, more isolated bodies abound.  In the vicinity of Highland County, Virginia, northwest-southeast trending dikes are quite common.  Augusta, Rockingham, and Pendleton Counties evidence many of these (Figure 3).  When geologists first recognized the igneous intrusions of Highland County in the 19th century, they assumed emplacement concomitant to rifting.  This was a natural assumption since these intrusions cross-cut both the sedimentary rocks described earlier and the extensive folds of the Valley and Ridge.  Until the mid-20th century there was little question that the igneous rocks of Highland County were Triassic/Jurassic in age (Tso et al., 2004; Tso & Surber, 2006). 

However, in 1969 two geologists named P. D. Fullager and M. L. Bottino came to a surprising conclusion.  Using K-Ar and Rb-Sr isotopic dating techniques, Fullager and Bottino found that most of the Highland County intrusions are not Triassic/Jurassic in age; rather, they were emplaced during the Eocene epoch approximately 47 mya, well after the divergent continental margin had formed (Tso & Surber, 2006).  Geologists considered this finding significant for two reasons.  First, it made the Highland County rocks the youngest igneous rocks in the eastern United States (Tso et al., 2004).  Secondly, and more importantly, it begged the question, "How and why did these intrusions form?"  Divergent continental margins are typically not conducive to igneous activity.  Naturally, renewed interest in these igneous intrusions ensued.  Subsequent study uncovered Eocene intrusions in Pendleton and Rockingham Counties as well, Ugly Mountain and Mole Hill being respective examples (Figure 3).  Moreover, isotopic dating has since determined that Trimble Knob is the youngest of all of these intrusions.  Dated at 35 million years old (ma), Trimble Knob stands as the youngest known igneous body east of the Mississippi River (Tso et al., 2004).

Lithology

The Eocene rocks of Highland County are bimodal in composition, containing both "mafic (basalt, picrobasalt, and basanite) and felsic (trachyte, trachydacite, and rhyolite) members" (Tso et al., 2004) (Figure 4).  These rocks locally cross-cut Lower Ordivician (Beekmantown Dolomite) to Middle Devonian (Millboro Shale) sedimentary rocks.  Trimble Knob itself is basaltic in composition and is found locally enclosed by the Millboro Shale (Rader et al., 1986).  However, Trimble Knob is unique among the Eocene intrusives with respect to its size, shape, and texture.  Roughly circular in horizontal cross-section, Trimble Knob has a diameter of approximately 150 m (500 ft), making it the largest of the Highland County instrusives.  Combined with its conical geomorphology, these facts might lead one to believe that Trimble Knob is a volcanic cinder cone similar to those found in the western United States; however, the absence of cinders, ash beds, and lava flows adjacent to the body preclude such an estimate (Tso et al., 2004). 

Presently, geologists consider Trimble Knob to be the erosional remnant of a volcanic neck or plug.  Nevertheless, complete understanding of Trimble Knob can be attained only through study of its distinctive texture.  The basalts of Trimble Knob have a porphyritic, sub-ophitic texture containing phenocrysts of olivine, augite, and hypersthene.  What is more, in the outer one-third of the intrusion are found xenoliths of shale, limestone, and sandstone derived from the surrounding country rock, principally the Millboro Shale and underlying Helderburg Group (Late Silurian/Early Devonian carbonates).  This region of the intrusion has been described as a breccia and varies in xenolith content from 50 to 90 percent of the breccia mass (Rader et al., 1986).  The breccia of Trimble Knob is key to understanding its genesis.

composition
Figure 4:  Composition of the Eocene rocks in Highland County, Virginia.
Taken from Tso et al. 2004.

Genesis

Any discussion on the formation of Trimble Knob must first grapple with the Eocene intrusives themselves.  Though geologists are unsure of the origin of these intrusives, theories nonetheless abound.  These include the following:
  1. A regional basement fracture known as the 38th parallel lineament provided a conduit for igneous activity during the Eocene.
  2. A nearby intrusive, which to this day provides heat for the famous hot springs in Bath County, Virginia, supplied magma for igneous activity in Highland County.
  3. A known global shift in plate tectonic motion during the Eocene resulted in the formation of the Bermuda Rise.  This shift was also responsible for igneous activity in Highland County.
  4. The transition between thin oceanic lithosphere and thick continental lithosphere along the divergent continental margin caused small scale downwelling to occur.  As a result, small scale upwelling flow occurred beneath Highland County to compensate for mass redistribution.  This upwelling flow drove igneous activity during the Eocene (Tso & Surber, 2006; Tso et al., 2004). 
In 1993, C. S. Southworth, K. J. Gray, and S. Sutter summarized the merits of each of the above theories and suggested an alternative theory.  They proposed that the Eocene activity in Highland County resulted from a combination of the above causes.  In other words, Southworth et al. posited that the young intrusives of Highland County were emplaced due to a reactivation of basement fractures (#1 above) as well as a directional shift in plate tectonic motion (#3 above) during the Eocene.  However, they added that continued tectonic extension of North America along the divergent continental margin during the Paleogene period likely contributed to the igneous activity (Tso & Surber, 2006; Tso et al., 2004).  In the end, though, the exact tectonic origin of these igneous bodies is unknown.  Further study is needed in this regard.

Nevertheless, geologists are reasonably certain as to the local origin of Trimble Knob.  Because of its extensive brecciation (i.e. large concentration of country rock xenoliths), circular cross-sectional shape, and basaltic composition, it is generally proffered that Trimble Knob is a basalt diatreme.  Diatremes are carrot or funnel-shaped volcanic plugs with steep walls that taper at depth and contain brecciated igneous rocks.  There are generally two models of diatreme formation.  In the first model, known as the magmatic type, entrained gases escape violently from rising magma as confining pressure is decreased near the surface.  The subsequent explosion fractures the surrounding country rock, incorporating xenoliths into the rising magma.  What is more, this explosion opens a funnel-shaped hole in the ground which continues to expand downward as successive explosions occur and/or as the brecciated country rock continues to collapse inward (Rakovan, 2006).   On the surface, around the rim of the funnel-shaped hole, are typically located deposits of volcanic tuff or other pyroclastic material derived from the original and successive explosions.  However, these tuffs are rarely preserved due to erosion (Tso & Surber, 2006). 

The second model for diatreme formation is known as the phreatomagmatic or hydrovolcanic type.  This model differs from the magmatic type in that the escaping gases that cause eruption are not derived principally from the magma itself, but from water in the surrounding country rock (Rakovan, 2006).  The associated explosions in this model are typically much more powerful than the magmatic type because they are steam-driven and most likely involve copious amounts of ground water.  Most geologists contend that the Trimble Knob diatreme is of the phreatomagmatic type.  This is because of its large horizontal cross-section (indicating a powerful explosive event) and its presence amidst ancient aquifers such as the Millboro Shale.  Figure 5 is a graphic representation of how the phreatomagmatic diatreme forms.  In general, the magmatic diatreme forms in a similar manner, save there is usually little or no groundwater involved.

diatremeformation
Figure 5:  Phreatomagmatic Diatreme Formation.  Magma moves upward and
encounters ground water (A).  Ground water vaporizes explosively causing a funnel-shaped
crater to form (B).  Funnel-shaped crater migrates downward as successive explosions
occur and/or the walls of the crater collapse inward (C).  Taken from Tso et al., 2004.


It must be noted that Trimble Knob's conical shape (Figure 2) is not derived from pyroclastic material sloping away from the exposed edge of the funnel-shaped diatreme (Figure 5c).  These pyroclastic materials have been eroded and are no longer present in the study area.  Rather, its shape is the result of differential weathering between the highly-resistant basalt diatreme and the less-resistant country rock. Over the intervening 35 million years since its emplacement, the diatreme has weathered considerably less than the enclosing sedimentary rocks.  The result is the diatreme today stands in higher relief than the adjacent shales and carbonates.  Thus, a hill is formed.  The conical shape of the hill is a morphological necessity of the circular horizontal cross-section of the original volcanic plug.

Conclusion

Igneous rocks are rare in the Valley and Ridge province of Virginia.  Therefore, when they are found, they are of intense interest to geologists.  Trimble Knob and other intrusives in the Valley and Ridge have confounded geologists for well over a century.  Only recently have scientists begun to understand these elusive igneous bodies.  However, there is still much work that needs to be done.  The question yet to be answered conclusively concerns the tectonic origin of the magma that produced Trimble Knob.  Until this is solved, Trimble Knob will remain as curious and as mystifying as ever.

References

Baldridge, W. Scott.  (2007).  Geology of the American Southwest.  Cambridge:  University Press.

Fichter, Lynn S.  (1997).  "Short Geological History of Virginia."  Obtained from http://csmres.jmu.edu/geollab/vageol/vahist/vahist.html.

Fichter, Lynn S. and Steve J. Baedke.  (2000).  "The Geological Evolution of Virginia and the Mid-Atlantic Region."  Obtained from http://csmres.jmu.edu/geollab/vageol/vahist/PhysProv.html.

Rader, Eugene K., Thomas M. Gathright II, and John D. Marr, Jr.  (1986).  "Trimble Knob Basalt Diatreme and Associated Dikes, Highland County, Virginia," Geological Society of America Centennial Field Guide--Southeastern Section, Geological Society of America.

Rakovan, John.  (2006).  "Diatreme," Rocks and Minerals (Vol. 81, March/April), Oxford, Ohio:  Miami University.

Tso, Jonathan L. and John D. Surber.  (2006).  "Eocene Igneous Rocks Near Monterey, Virginia:  A Field Study," Virginia Minerals (Vol. 49, nos. 3 & 4), Virginia Department of Mines, Minerals, and Energy, Richmond, Virginia.

Tso, Jonathan L., Ronald R. McDowell, Katharine Lee Avary, David L. Matchen, and Gerald P. Wilkes.  (2004).  "Middle Eocene Igneous Rocks in the Valley and Ridge of Virginia and West Virginia,"  Geology of the National Capital Region--Field Trip Guidebook, USGS Circular 1264.  Obtained from http://pubs.usgs.gov/circ/2004/1264/html/trip4/index.html.

Webb, Harry.  (1988).  "Scenic Landforms of Virginia," Virginia Minerals (Vol. 34, no. 3), Virginia Department of Mines, Minerals, and Energy, Richmond, Virginia.