The shape of the arch is a result of mechanical weathering. what agent would be be responsible

Since igneous rocks form at high temperatures, and under pressure conditions ranging from one to several atmospheres.  However, the conditions at the Earth's surface are somewhat different than the conditions at which most rocks and minerals form.  Therefore, the materials are no longer at equilibrium when they are exposed to surface conditions.  Under these conditions, there is a tendency for all ordered systems to seek lower levels of energy or order.  This is all done through weathering.