Irazu volcano, a large composite cone, consists of interbedded lava flows, lahars, and ash beds. This rock sequence, named the Irazu Group, has been divided into four formations; from the base: Reventado Formation, Sapper Formation, Birris Formation, and Cervantes Formation. Only the Reventado and Sapper Formations crop out in the Reventado watershed. The Reventado Formation consists of at least four widespread medium-gray finely porphyritic lava flows and interbedded lahar and some ash. Where not excessively jointed, lava flows within the formation are structurally sound and generally fresh. The Sapper Formation also consists of interbedded lava, lahar, and ash; lavas are black dense and coarsely porphyritic and in the middle part of the watershed are structurally sound if not excessively jointed. In the upper part of the watershed Sapper lavas are deeply altered, pyritic and structurally unsound.
At least three major inactive and six major active landslides have been recognized In the Reventado watershed, all apparently in the Sapper Formation. Mudflows in the Reventado watershed have been supplied with debris from the caving of oversteepened stream banks and to a lesser extent from the active landslides.