The fifteen volcanoes that make up the eight principal islands of Hawaii are the youngest in a chain of more than 129 volcanoes that stretch across the North Pacific Ocean, called the Hawaiian–Emperor seamount chain. Hawaii's volcanoes rise an average of to reach sea level from their base. The largest, Mauna Loa, is high. As shield volcanoes, they are built by accumulated lava flows, growing a few meters or feet at a time to form a broad and gently sloping shape.