What geological feature is formed when lava fills an area and hardens?

A lava plateau is formed when lava covers a large area and subsequently hardens. This geological feature is characterized by extensive flat areas created from the accumulation of relatively fluid lava that can flow over long distances before solidifying. The process typically involves numerous eruptions, where thick layers of basaltic lava flow out from fissures or vents and spread out across the landscape, gradually building up the plateau over time.

In contrast, a volcanic island is typically formed from lava that builds up above sea level, not necessarily covering a large flat area. A caldera refers to a large depression formed when a volcano erupts and the emptied magma chamber causes the ground above to collapse inward, while a shield volcano is characterized by its broad, gentle slopes formed primarily by the flow of low-viscosity lava. While shield volcanoes are related to the flow of lava, they are different from a lava plateau in terms of their shape and formation process. Thus, the definition and characteristics of a lava plateau make it the correct answer for this question.