The north-northeastern part of the island contains tholeittic basaltic rocks and is characterized by tension faulting, lava flows and fragmental deposits of igneous rocks. The volcano of Mount Okmok, characterized by its wide circular caldera, it located in the northwestern part of the island. This generally flat central basin has an average elevation of 370 m above sea level, with the rim of the caldera reaching a height of . Following the formation of the caldera, numerous satellite cones and lava domes have formed on the flanks of the volcano. They include Mount Tulik () on the caldera's southeastern slope, Mount Idak () to the northeast, and Jag Peak. A crater lake once filled much of the caldera to a depth of over , but the lake ultimately drained through a notch eroded in the northeast rim. The prehistoric lake attained a maximum depth of about and the upper surface reached an elevation of about , at which point it over topped the low point of the caldera rim. Small, shallow remnants of the lake remained north of Cone D at an altitude of about : a small shallow lake located between the caldera rim and Cone D; a smaller lake (named Cone B Lake) farther north near the caldera's gate. After the 2008 eruption, the hydrogeology of the caldera was greatly changed with five separate sizable lakes now emplaced. In addition to the caldera lakes, Cone A, Cone E, Cone G and the new 2008 vent on Cone D contain small crater lakes. The volcano is currently rated by the Alaska Volcano Observatory as Aviation Alert Level Green and Volcanic-alert Level Normal.