Although volcanic activity is concentrated on the edge of tectonic plates, such activity can occur in the interiors of plates as well. A. Our understanding of islands comes from Daly's and Wilson's observations of the Hawaiian Islands, which was later confirmed by plate-tectonic theory. B. The hot-spot trails formed by volcanic island chains indicate the positions of tectonic plates as for back as the present ocean basins have existed. C. Whereas volcanic islands formed by mantle plumes are typically small, most of the world's largest islands are formed at the edges of tectonic plates. D. It has only recently been discovered that tectonic plates are closely fitting rather than loosely constructed, as geologist previously believed. E. Volcanic island chains such as the Hawaiian Islands form in the interior of a tectonic plate as the plate moves over a fixed volcanic source in the mantle. F. The Pacific Plate has existed for as long as the Hawaiian Islands have existed, namely for more than 80 million years.