Beneath the surface, there is vertical pressure from the weight of the overlying rock. There are also
horizontal pressures in all directions pressing on the Marcellus Shale. These pressures are the result of
plate tectonic forces interacting. The Earth’s outer shell is made of very large plates, like puzzle pieces, which
slowly jostle each other. The interactions of these plates can build mountains and cause earthquakes, but in
the Marcellus Shale region, they are weaker than the vertical pressure caused by the weight of the overlying rocks. Hydraulic fractures grow perpendicular to the direction (the plane) of minimum principle stress. In the Marcellus Shale region, fractures made by hydraulic fracturing at depth will have a vertical orientationand will separate against the minimum horizontal stress.