The
basalt outcropping at the crest of the hill is "dark greenish gray to grayish black, medium grained, and moderately
porphyritic. It is an
olivine-
spinel basalt with abundant large pale green
pyroxene and minor yellow-brown
olivine phenocrysts. There is also evidence for very small amounts of more evolved magma such as
trachyandesite,
trachydacite, and
rhyolite." The basalt intrudes through the
Ordovician Beekmantown Group of carbonate rocks. Mole Hill is part of a larger eruptive period during the
Eocene, which happened between 48 and 35 million years ago in
Virginia and
West Virginia . Geologists have struggled to understand the cause of these eruptions. They were small volume, episodic eruptions that happened far from any
plate boundary. While many different processes have been researched, such as a
mantle plume, or edge-driven convection, the leading theory into what caused the eruption of Mole Hill, (and other Eocene volcanoes) is thought to be a result of a large scale change in plate motion between 53.5 and 37.5 million years ago. This caused crutal extension to occur in the Valley and Ridge province of West Virginia and Virginia as the
North American Plate adjusted. Small volumes of
magma were produced in a process called
decompression melting. This allowed magma to rise through old suture zones created as
Pangea was formed and then later rifted apart 200 million years ago. Small pulses of magma reached the surface between 48 and 35 million years ago. Mole Hill was the result of one of the initial pulses of magma around 48 million years ago. ==Age==