INVESTIGATING THE STRUCTURE OF MOLE HILL USING RESISTIVITY AND MAGNETICS

The Eastern North American Margin (ENAM) is a passive margin that has been characterized by two complete Wilson cycles of activity in its history. The last rifting activity occurred approximately 200 Mya. However, there are magmatic events within the ENAM that are much younger and therefore not directly associated with the rifting events. These magmatic events include Jurassic (~150 Mya) and Eocene (~48 Mya) events, leading to a regional geology is that far more complex than might be typically assumed for a passive margin.

The Central Atlantic Magmatic Province (CAMP) was responsible for the Pangea rifting event and is preserved within the ENAM. Pangea rifting and CAMP magmatism initiated the ENAM into a tectonic passive margin however, recent expressions of volcanic activity, Eocene in age (~48Mya), indicate that the margin has recently been internally dynamic. The expressions of Eocene age volcanism are observed in a number of locations within Virginia including Trimble knob, a felsic volcanic plug located in Highland county, Sounding Knob, a mafic volcanic neck also located in highland county, and Mole Hill, a mafic volcanic plug located west of Harrisonburg, VA.

For this study we will solely on Mole Hill. In order to constrain the subsurface structure of Mole Hill a combination resistivity and magnetic geophysical surveys will be conducted on and around the volcano. To figure out the origin of the Eocene volcanoes, the use of a magnetic and resistivity survey will be conducted on and around the volcano. Resistivity is a material property which measures how much material within the subsurface resists or conducts electrical currents. Massive rocks tend to be far more resistive than weathered rocks and soil generated from the same source rock, so we would expect that the volcanic intrusion will show up more resistive. The magnetic survey will also serve to constrain the extent of basalt in the subsurface, as the bulk basalt shows up as a negative anomaly (the intrusion occurred during a negative anomaly). By combining these two methods, we are able to constrain the distribution of massive igneous rocks in the subsurface, leading to a better understanding of the processes by which Mole Hill formed, and thereby giving insights that should develop into a deeper understanding of Eocene volcanism in general.