On the eastern side the volcano exhibits a large depression called the "Valle del Bove", a large incision measuring about 7 by 5 Km with slopes reaching heights of 1,000 metres. The valley seems to have originated from the collapse of parts of ancient volcanic edifices due to explosive events. Most of the lava flows that occur on Etna are made up of scoriaceous blocks and fragments, with a rough, jagged surface (lava called "aa"); in other, less frequent, instances, the lava has a smoother, often billowy surface, forming large slabs of coiled, ropelike masses ("pahoehoe" lava). A frequent occurrence in the latter kind of lava flow is the formation of a system of outflows enclosed by a sheath of cold, solidified lava; towards the end of an eruptive event these channels empty out, creating caves and tunnels. The duration of eruptions from the mouths of the volcano varies greatly, from a few hours to months or even years. The length, breadth and extent of lava flows essentially depends on the duration and extent of the eruption, as well as other factors linked to the morphology of the soil over which they flow.