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.