Much of Iriga is mountainous as the skyline is dominated by the majestic Mt. Asog (aka. Mt. Sumagang, mt. Iriga). It is located about 400 kilometers south of Manila, 37 kilometers south of Naga, and about 61 kilometers north of Legazpi City. It is bounded by the town of Buhi in the east, by the municipalities of Baao, Nabua and Bato in the west, by the province of Albay in the south, and by the municipalities of Ocampo and Sangay in the north. It has several natural springs which is the source of clean water for its residents as well as neighboring towns.
Mount Iriga also known as Mt. Asog, is an old volcano in the Bicol Region. It rises 1,196 m (3,924 ft) with a base diameter of 10 kilometers. It is a small stratovolcano immediately SW of Lake Buhi in southern Luzon, Philippines. The volcano is dominantly andesitic in composition, but has several basaltic flank cones. It has a large crater breached to the SE, which formed during a massive flank failure that produced a large debris avalanche which buried several villages and formed a hilly, irregular deposit on the plain south of Lake Buhi 4 km from the summit.
Although it is outshined by the world-famous Mt. Mayon to the south and the higher, grander Mt. Isarog to the north, it possesses its own charms. For one, the views at the peaks (yes, plural) are breathtaking, offering a view of Mt. Mayon, Mt. Malinao, at Mt. Masaraga on the south; and even a distant Catanduanes southeast. Knife-edged slopes characterize the contour of this mountain, especially in the Buhi side. Mt. Asog has a colorful history. It was named after a ancient datu (chieftain) of the Agtas named Asog, who reigned over the region. Sometimes it is also called Mt. Sumagang, which means “Mountain of the Rising Sun”. In some lowland areas, the sun seemingly rises from the mountain itself. Being on the eastern side of Luzon, the sun rises majestically from the Pacific Ocean when viewed from its summit.
Iriga is part of the Bicol volcanic arc which comprises at least 12 volcanic vents and complexes and is one of the 3 historically active, but the least active centers of the arc (the other 2 being Mayon and Bulusan). Iriga has only 2 known recorded eruptions in historic times.
The catastrophic debris avalanche of Iriga volcano had been believed to have occurred during the 1628 AD eruption, but later work has now shown that the collapse and eruption occurred earlier at some unknown date in the past few thousands years. The avalanche was followed by phreatic explosions that created a small crater at the base of the scarp. The horse-shoe shape of Mt. Asog is compared to Mt. St. Helens in Washington State in the United States which had a similar massive flank failure in recent geological times.