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. Hawaiian islands undergo a systematic pattern of submarine and subaerial growth that is followed by erosion. An island's stage of development reflects its distance from the Hawaii hotspot. Background The Hawaiian–Emperor seamount chain is remarkable for its length and its number of volcanoes. The chain is split into two subsections across a break, separating the older Emperor Seamount Chain from the younger Hawaiian Ridge; the V-shaped bend of the chain is easily noticeable on maps. The volcanoes are progressively younger to the southeast; the oldest dated volcano, located at the northern end, is 81 million years old. The break between the two subchains is 43 million years; in comparison, the oldest of the principal islands, Kauaʻi, is little more than 5 million years. The "assembly line" that forms the volcanoes is driven by a hotspot, a plume of magma deep within the Earth producing lava at the surface. As the Pacific Plate moves in a west-northwest direction, each volcano moves with it away from its place of origin above the hotspot. The age and location of the volcanoes are a record of the direction, rate of movement, and orientation of the Pacific Plate. The pronounced 43-million-year-old break separating the Hawaiian Ridge from the Emperor Chain marks a dramatic change in direction of plate movement. Initial, deeper-water volcanic eruptions are characterized by pillow lava, so named for their shape, while shallow-water eruptions tend to be composed mainly of volcanic ash. Once the volcano is high enough so as to eliminate interference from water, its lava flows become those of ropey pāhoehoe and blocky ʻAʻā lava. Our current understanding of the process of evolution originates from the first half of the 20th century. The understanding of the process was advanced by frequent observation of volcanic eruptions, study of contrasting rock types, and reconnaissance mapping. More recently our understanding has been aided by geophysical studies, offshore submersible studies, the advent of radioactive