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.
Water resources are abundant in Irosin. The most prominent water resource is the extensive river system of the municipality. The sources of the creeks and streams, which are the main feeders of the river network, are springs which are located in the upland and hilly landscapes of volcanic hill landforms. This spring source of the riparian system has an elevation range from a low 35 to a high 370 meters elevation level. The total length of rivers is 68.43 kilometers, the total length of creeks is 46.51 kilometers and there is 12.72 kilometers of dry creek. The Irosin riparian system has an approximate total length of 127.66 kilometers.
Cadac-an River is the biggest and longest river and has 4 main river tributaries and 8 collateral creek feeders. This river covers a length of 16.59 in the territory of Irosin and with a total length of 37 kilometers. This river originates from the slope of Bulusan Volcano and the Mount Mara-ot Banwa ranges and almost cut Irosin in half due to its geographic location in the heart of the municipality and this finally empties in Juban before it flows to Sorsogon Bay. The average volume of water flow is measured at 2,845 liters per second.
Due to its length, the Cadacan River has the most traversed points and areas starting from Mapaso to its outfall in Bolos. It is to this fact that the orientation of the riparian system of the municipality follows the course of the Cadac-an River which has a southeasterly to northwesterly direction. Salvacion, Liang, Manangkas and Cawayan Rivers are not tributaries to Cadac-an River instead they flow to the south towards the river system of Bulan, Matnog, Sta. Magdalena and Bulusan.
There is one lake in Irosin, the Danao Lake located in barangay Patag with an approximate area of 5.05 hectares and with a depth of 4 meters. There are plenty of natural and cold springs that are presently utilized for household consumption, recreation and irrigation. The major springs are located at barangay Bolos, Gulang-Gulang, Bagsangan, Tabon-tabon, Carriedo and Monbon.
The broad-rimmed alluvial plain is the catchment basin for the mountain-sourced riparian network. With this topographical feature, Irosin is believed to have a vast aquifer and is the recipient of the water recharge capacity of its surrounding mountains. Bulusan Volcano, Mt. Jormajam and Mt. Mara-ot Banwa are the major watershed areas not only of Irosin but also the adjoining barangays of the surrounding towns.
Ground water has an average water level of 15 meters, while the average water extraction depth for wells ranges from 12 to 30 meters which is indicative of the possibility of the existence of vast underground aquifers. Surface water volume has an estimated capacity of 810,000 cubic meters.
The valley floor of Irosin with a base that starts from barangay Tabon-Tabon and extends to the barangays of Carriedo, Buenavista, San Isidro, Batang, Gumapia, Tongdol, Gabao, San Pedro, Bagsangan, Monbon, Gulang-Gulang and Bolos is akin to a very big basin. Waters that accumulate and flow down from the surrounding mountains are contained in the valley and drain-up naturally through ground absorption, evaporation and by continuously flowing downstream either gradually or in great volume through the Cadac-an River network before it integrates with sea waters of Sorsogon Bay in the municipality of Juban. During the eruptions of Bulusan Volcano, lahar deposits flow down through the gullies and water tributaries and finally settled in the catch basin along the Cogon and Gulang-Gulang areas.