Volcanoes with broad bases and gentle slopes, built up by thin basic lava, are known as

The correct answer is D, Shield volcanoes. Basic lava is poor in silica, very hot and thin, so it travels a long way from the vent before it solidifies. Layer upon layer of such flows builds a wide dome with slopes of only a few degrees, shaped like a warrior's shield laid on the ground, of which Mauna Loa in Hawaii is the textbook example. Eruptions of this type are usually quiet rather than explosive.

Option A, composite or strato volcanoes such as Fujiyama and Vesuvius, are built of alternating layers of ash and viscous acidic lava and therefore rise as steep cones. Option B, cinder cones, are small steep hills built only of loose fragments thrown out around a vent. Option C, caldera volcanoes, are the most explosive kind, named after the wide hollow left when the summit collapses into an emptied magma chamber. The key idea to carry into the exam is that thin lava makes gentle slopes and thick lava makes steep ones.