Kīlauea has been active throughout its history. Since 1918, Kīlauea's only prolonged period of rest was an 18-year pause between 1934 and 1952. The bulk of Kīlauea consists of solidified lava flows, intermittent with scattered volcanic ash and tephra sourced from relatively lower-volume explosive eruptions. Much of the volcano is covered in historical flows, and 90 percent of its surface dates from the last 1,100 years. Kīlauea built itself up from the seafloor over time, and thus much of its bulk remains underwater; its subaerial surface is in the form of a gently sloping, elongate, decentralized shield with a surface area of approximately , making up 13.7 percent of the island's total surface area.