Cerro San Miguel is an isolated lava dome that was constructed on the SW flank of the Tilarán Range near the mouth of the Gulf of Nicoya, NE of the port city of Puntarenas. The dacitic dome covers an area of about 9 km2 and was Potassium-Argon dated at less than 700,000 BP. A series of vertical-to-subvertical concentric fractures cuts the upper part of the dome.