Coulées are a cross between lava domes and lava flows: they are thick extrusions erupted on slopes steep enough for shear stresses to exceed the yield strength, thus permitting the flow to ooze down slope. One of the world's best known examples of a coulée, and the largest of its type in the world, is the Chao dacite occupying the saddle between Paniri and Léon volcanoes. 39Ar-40Ar ages indicate it is less than 100,000 years old. It is a 14.5 km long coulée, with flow fronts 350-400 m high and has a total erupted volume of about 26 km3. A characteristic feature of this body are the prominent 30 m high flow ridges or ogives on its surface, resulting from folding during emplacement of surface layers with different mechanical properties due to differential cooling of the lava. Eruption of the huge coulée appears to have occurred in three main stages. Chao is so young that its internal structure is not exposed, but in older, dissected examples, coulées display ramp structure, formed as successive increments of lava are extruded from the vent and squeeze earlier erupted increments outwards. Most of the outward movement of the flow is accommodated in a thin zone of shearing at the base. (Fig. 7.25).

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