The climate of Irosin belongs to the Type II of the modified Corona System, the classification of Philippine Climate. The Type II climate is characterized of having no dry season or not a single dry month, a very pronounced maximum rain period from December to February and minimum monthly rainfall occurring during the period from March to May.

Irosin’s basal soil and rock formation and composition are mainly of volcanic origin. The greater geologic make-up in the northwest and southwest sectors is of recent alluvial plain deposits. The north, south and east points is composed of undifferentiated Bulusan volcanic, debris flows and mud flow deposits, pyroclastic flow deposits, lava dome deposits, recent alluvial plain deposit and pre-Bulusan caldera deposits. The prominent peak of Mt. Jormajam and the bounding barangays of Bulusan Volcano are also made up of volcanic.

Irosin has a very irregular terrain characterized by the plain on the valley floor, secluded plains and valleys on the mountaintops and undulating hills and mountain peaks. The valley of which Irosin is famous is located inside the Irosin caldera. The Irosin caldera is estimated to be 200 meters deep and 11 kilometers in diameter and was formed during a calderagenic eruption of Bulusan Volcano that occurred about 40,000 years before present. The valley floor is a contiguous zone from the south to northeast which occupies an area of 35 sq. km. or 23% of Irosin’s land area. The Irosin valley is 5 kilometers at its widest point at southwest to northeast continue reading : Topography and Slope

The Municipality of Irosin lies at the foot of Bulusan Volcano, the tail end of the Sierra Madre Mountain Ranges at the southern tip of Luzon Island. It is located at the center of Sorsogon Province bounded by coordinates 12.7780° N, 123.9521° E and 12.6373° N, 124.0962° E. It is nestled in a caldera with a valley floor that is surrounded by mountains and peaks making the town the only landlocked inland municipality of the province.