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,
Nova Iguaçu Volcano underlain by pyroclastic rock. This rock originated from rock fragments created by a volcanic explosion. The valley is made up of trachyte, another igneous rock with massive texture. The pyroclastic rock was the main justification of the Nova Iguaçu Volcano hypothesis. The existence of a volcano at Nova Iguaçu was proposed by geologists Viktor Carvalho Klein and André Calixto Vieira in the 1980s. Klein and Vieira based their discovery on the presence of pyroclastic rocks (including volcanic bombs) and the existence of a volcanic crater. They believed the pyroclastic rock to be pyroclastic flow deposits, constituting an “extraordinary well-preserved complete