Shield volcanoes have gentle slopes and large diameters. They are formed by the accumulation of low viscosity basalt lava flows. Because the flows have low viscousity they flow a long way. Lava tubes form which enable molten lava to flow long distances from the vent. They form where large volumes of basalt lava are erupted over along period of time from one location such as over a hot spot/mantle plume. They are called shield volcanoes because they look like a warriors shield. Hawaii s Mauna Loa and Mauna Kea are the worlds largest volcanoes and are shield volcanoes composedof 1000s of feet of lava built up from the ocean floor.