The volcanically active Hawaiian islands, however, are located in the middle of the Pacific plate. They are part of a long chain of seamounts that extends westward and northward to the Aleutian trench. The seamounts of the Hawaiian Ridge and Emperor Seamount Chain are volcanoes that are no longer active and that have sunk below sea level. See the map on page 492-493 of the textbook for a larger view of the Pacific Ocean sea floor. Hot spots are places in the asthenosphere that are hotter than usual and that produce large volumes of melted rock called magma.
Hot spots are features that are fixed in the asthenosphere.
Magma produced at a hot spot punctures the overlying lithospheric plate and forms a volcano on the sea floor. The lithosphere, however, is not fixed and as it moves over the asthenosphere, away from the hot spot, the volcano stops erupting and a new one is formed in its place. With time, the extinct volcanoes keep moving northwestward and new active volcanoes form over the hot spot. As the extinct volcanoes age, the crust upon which they sit cools and subsides. This, combined with erosion once active volcanism stops, leads to a shrinking of the islands with age and their eventual submergence below the ocean surface.