Shield volcano are cave-like volcanic straights formed by the hardening of overlaying lava. These structures help further the propagation of lava, as the walls of the tube insulates the lava within. Lava tubes can account for a large portion of shield volcano activity; for example, an estimated 58% of the lava forming Kīlauea comes from lava tubes. In some shield volcano eruptions, basaltic lava pours out of a long fissure instead of a central vent, and shrouds the countryside with a long band of volcanic material in the form of a broad plateau. Plateaus of this type exist in Iceland, Washington, Oregon,
Under the Volcano 1984 it served as the basis of a film of the same name. In 1998 the Modern Library ranked "Under the Volcano" at number 11 on its list of the 100 best English-language novels of the 20th century. Lowry had already published one novel, "Ultramarine" (1933), by the time he was working on "Under the Volcano", and in 1936 wrote a short story called "Under the Volcano" containing the kernel of the future novel. That story was not published until the 1960s; passages of it are found also in the account of Sigbjorn Wilderness, found in "Dark as the Grave