The magma feeding this smoker is born deep below, as the Philippine oceanic crust subducts beneath the Philippine continental plate (formed of a belt of continental fragments). When the plate reached a certain depth, and set of pressure/temperature conditions, the water in the altered seafloor basalt is literally baked out of the minerals it was locked in, and lowers the melting point of the mantle wedge above, causing it to partially melt and the resulting less dense magma rises towards the surface through a line of weakness such as a fault in the overlying crust.
The cone is formed of layers of welded ash from pyroclastic flows alternating with layers of lava that oozed out of the volcano over the millennia of its geological history. Its products are classified as basaltic andesite, lavas being graded according to richness in silica.
Sitting above the city of Legaspi, the mountain has several associated hazards, ranging from lava and pyroclastic flows (clouds of very hot air and ash running down the ravines carved into the cone by tropical rains from typhoons) during eruption to subsequent lahars, flows of running mud that rush down river valleys when downpours mix with the fine ash, obliterating everything before them. The rain from Typhoon Durian killed over a thousand back in 2006. Its most powerful recorded outburst buried the town of Cagsawa in 1814.
Image credit: Dacel Yazon Andes