The less eroded lava flows have surface features typical of aa lava and pahoehoe lava. Many lavas were pahoehoe flows laced with a'a, evidence of subaerial eruptions. The lava within this unit consists of pahoehoe lava flows of olivine-rich basalt. Lava tubes are typically associated with extremely fluid pahoehoe lava. There are primarily two kinds of basaltic lava flows&colon; pahoehoe and 'a'a. Amboy Crater is a 6,000-year-old cinder cone volcano, made largely of pahoehoe lava. It was formed by basaltic pahoehoe emanating from vents throughout the area. Long, thin basalt flows with pahoehoe surfaces are common. It formed in an active area of volcanism on top of pahoehoe and aa lavas. A'a lava flows are denser and more viscous than pahoehoe, and tend to move slower. The initial flows were pahoehoe lava, although their extent was apparently small. The area is covered with pahoehoe lava flows dated between 700 and 500 years old. The pahoehoe flows buried the submerged barrier reef, enlarging the island by 8 square miles. Aa is formed when pahoehoe cools and loses some of its gases. Pahoehoe lavas typically have a temperature of .