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 orientation and 8.3 kilometers wide at southeast to northwest orientation.
The lowest point of the municipality is 19 meters above sea level (ASL) at Barangay Bolos near the Cadacan River. The highest point is located at Bulusan Volcano at 1,502 meters, followed by Mt. Agoho at 771 meters and Mt. Jormajam at 710 meters. These prominent peaks are part of the Bulusan Volcano mountain complex at the north of Irosin valley and are located at barangays Cogon, Bagsangan and Bolos respectively. The fourth highest peak is Mt. Mara-ot Banwa at 611 meters located at the east of the valley at barangays Tabon-tabon and Cawayan. From the south to the east of the Irosin valley are undulating hills which are remnants of ancient crater wall.
About 17% or 2,544 hectares of the land area has a slope range (steepness or degree of inclination) of 0 to 3% or level to moderately level. These areas are located in the valley floor and Macawayan has the most level slope with all of its area at 0 to 3% slope, followed by San Pedro and Buenavista with 98% and 86% of its lands at nearly level slope. Level to moderately level lands are also located in northeastern portions of Bulawan, Gabao, Tongdol, Gumapia, Batang, and San Isidro, to the southwest of Bolos, Gulang-gulang, Tinampo, Monbon, and Bagsangan, and north of Carriedo.
The land area with slope range 3 to 8% (gently sloping to modulating) is 1,949.4 hectares or 13% of the total land area. These lands are located in the southwest of Cogon, north of Tinampo, middle portion of Monbon, Mapaso, Patag and Sto. Domingo, in the urban barangays of San Agustin, Bacolod, and San Julian, and at the southwestern parts of Gumapia and Tongdol.
There are 2,548.9 hectares or 17% of land area with a slope range of 8 to 18% or moderately sloping to rolling. About 29% of the land area or 4,419.9 hectares has a slope range of 18 to 30% which is strongly sloping to moderately steep. Lands with 8 to 30% slope are located in the mountains and hills of barangays surrounding the Irosin valley.
Steep hills and mountains (slope range of 30 to 50%) occupies 2,600.4 hectares or 17% of the land area. Very steep hills and mountains or areas with 50% and above slope are about 7% of the land area with 1,036.2 hectares. These mountainous areas are found in Cogon, Bolos, Bagsangan, Monbon, Cawayan, Tabon-tabon, Mapaso, Patag, Salvacion, Casini, Liang and Carriedo.