Given that all of the Hawaiian Islands were created by volcanic activity, it is somewhat surprising that only one of the islands possesses any active volcanoes. Why did the volcanoes that built the other islands stop erupting and why are those on the big island still active? This video segment adapted from NOVA describes the role of a relatively rare phenomenon, known as a hot spot, in the formation of these majestic islands.
The theory of plate tectonics describes the movement of plates across Earth's surface. A dozen major plates and several smaller ones make up our planet's crust, which includes both the surface of the continents and the sea floor. These plates are in perpetual — but nearly imperceptible — motion. Geologic activities that occur at the boundaries where plates meet demonstrate spectacularly the forces involved in these movements. Some of the most breathtaking of these activities are volcanic eruptions.
Nearly all volcanoes occur at or near plate boundaries. At these locations, sections of Earth's crust and upper mantle are squeezed together or ripped apart, either forcing or allowing molten magma to rise to the surface.
About 80 percent of all volcanic activity occurs at subduction zones, where plates collide and one slab of ocean crust is forced beneath the other slab and into the mantle. Here the crust melts, and the magma that is generated is forced upward through the overlying plate. Another 15 percent of volcanoes occur at divergent boundaries, primarily along mid-ocean ridges on the ocean floor. In these areas, adjacent plates move away from each other, creating a break, or rift, in the crust. As the rift expands, new magma flows in to fill the space.
The remaining 5 percent of volcanoes, such as those that created the Hawaiʻian Islands, occur away from plate boundaries as the result of an anomalous and poorly understood phenomenon called a hot spot. Hot spots are caused by high-temperature plumes of mantle material that rise from great depths to eventually melt through the crust to form volcanoes.
Scientists aren't certain where inside Earth hot spots originate, or why, but a couple of hypotheses exist. The first suggests that, as with other volcanoes, subducted plates are responsible for the formation of the volcanoes. However, in this scenario, plates are plunged unusually deep into the mantle. Here the material becomes super-heated and extremely buoyant. According to this hypothesis, a plume generated in this manner might reach the surface far from where it was subducted. Another hypothesis suggests that pockets of highly radioactive material exist deep within Earth, near the boundary between the mantle and the core. As a result of the radioactivity, the material becomes extremely hot, rises to the surface, melts through the crust, and forms volcanoes unconnected to a plate boundary.
Although hot spot plumes don't seem to move laterally, the plates above them do move. Thus, a volcano on the surface is eventually carried away from the hot spot and ceases to be active. Eventually, this results in a trail of progressively older volcanoes.
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