The Irazú Volcano sub-unit is located to the east of the canton from Villa Mata de Plátano. It consists of slopes of all kinds. The contours of the hills are rounded due to the thickness of recent ash deposits that cover almost the whole surface mass. The sub-unit is composed of volcanic rocks displaying gaps, lavas, tuffs, agglomerates, ignimbrites, ash and also many streams of mud and lahars. The rocks' degree of weathering is highly variable, with its origin being due to the accumulation of different type of volcanic rocks.