Plate Tectonic Theory
The earth is composed of a series of layers. Imagine an egg as a model of earth. The yolk represents the earth’s core, the white represents the mantle layer and shell is the earth’s crust. The crust and top of the mantle from a rigid layer called the lithosphere, which is broken into a number of pieces called plates. Beneath the lithosphere the mantle rock is hot and under pressure, making it soft and pliable. As the soft mantle circulates, the overlying plates are moved.

About 95% of earth’s volcanoes and earthquakes occur at plate edges, known as plate boundaries. At parallel boundaries, the plates slide by each other causing earthquakes. At divergent boundaries, where plates move apart, magma fills the cracks making both plates grow. At convergent boundaries, where plates collide, dense oceanic plates sink under continental plates collide, dense oceanic plates sink under continental plates and are re-melted. Volcanoes along the Pacific “Rim of Fire” such as Mt. St. Helens in Washington State and Mt. Fuji in Japan are created by such “recycled” plates.

Because Hawaiian volcanoes are formed in the middle of the Pacific Plate and not on the edge , the plate tectonic theory by itself doesn’t explain their origin.

Hot Spot Theory
Hawaiian volcanoes are produced by a hot spot in the mantle. The hot spot is a rising current where the upper mantle melts, producing magma. Magma is less dense than the surrounding rock, and rises up to the surface. A volcano forms on top of the plate, so as if on a conveyor belt. The volcano becomes dormant as it moves away from the hot spot and frequent eruptions cease. A new volcano forms in its place.

The pacific plate moves toward the northwest at a rate of about 4 inches (10 cm) per year, so the oldest Hawaiian volcanoes are located to the northwest. The youngest and largest island, Hawaii, has three active volcanoes which have erupted during recorded history. An under-sea volcano named Loihi, located about 20 miles (32km) southeast of Hawaii, may form a new island some day.

Although you may think of the 8 major island as Hawaii, there are really 132 islands in the chain. These islands will eventually be submerged by erosion from rain, wind, waves and landslides, becoming underwater mountains called seamounts. A chain of seamounts extend from Midway Islands to Alaska. According to radiometric dating of seamount rocks, the Hawaiian hot spot has been creating volcanoes for at least 81 million years.