Hawaiian Island Geology
Most volcanoes are created by oceanic crust being forced under continental crust as the continent moves towards the ocean. As the oceanic crust dives deeper the temperature increases until it melts; water in the sediments makes this newly melted crust lighter than the surrounding lava and it rises to the surface through fissures and emerges as a volcano. This can be seen in the Rim of Fire around the Pacific Ocean. Volcanoes can be found along the western edges of South and North America like Mount Aconcagua, Mount Rainier, and Mt. Saint Helens. This volcanic chain extends up to Alaska and down the Aleutian Islands, continues on into Japan (Mount Fuji) and the Philippines (Mount Pinatugo) and ends in New Zealand.
The volcanoes of Hawaii, however, have a different origin. The heat rises from a mantle plume, a hotspot in the mantle below the crust. The oceanic crust moves over the hotspot and over time a new island is formed; the old island, after a few million years, becomes dormant. The newest Hawaiian Islands are on the south east end. The oldest are on the north west side. If you follow that line, you will find submerged islands (seamounts) that are older and older. Midway is along that line.
Odd geological fact number 1: Mauna Loa on Hawaii is the highest mountain (33,000 feet) in the world if you count its height from the sea floor. Mount Everest is only 29,028 feet high.
Odd geological fact number 2: The string of seamounts in the Hawaiian chain has a kink in it (check out Google Earth) where the direction of continental movement changed. This was around 47 million years ago. There is some evidence that the plume may have moved also, otherwise the Hawaiian-Emperor Chain would be around 800 kilometers shorter.