Geomorphologically, shield volcanoes are distinguished by their broad, low-profile shapes. Unlike steep composite volcanoes, shield volcanoes are built almost entirely of layers of thin, runny basaltic lava. Because the lava is low in silica, it has low viscosity, allowing it to travel miles from the vent. This creates a mountain that is very wide at its base but has slopes typically less than 10 degrees. The Hawaiian Islands are the classic geographical example, where shield volcanoes rise from the deep ocean floor to become the largest individual mountain masses on Earth by volume.