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:
- A regional basement fracture known as the 38th parallel lineament
provided a conduit for igneous activity during the Eocene.
- 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.
- 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.
- 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
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
margin during the Paleogene period likely contributed to the igneous
Surber, 2006; Tso et al., 2004). In the end, though, the exact
tectonic origin of these igneous bodies is unknown. Further study
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
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
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
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
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
|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
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
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.
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.
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