A note about Hawaii

As a somewhat brief aside, it should be mentioned that not all volcanic islands are formed from arc-trench complexes, as I described in my previous post. Perhaps the most famous counterexample is the Hawaiian islands, which were not formed near any plate boundary. In fact, the closest one is some 3,200 km (1,988 mi) distant! So how did these islands, part of a group of mostly extinct volcanoes known collectively as the Hawaiian-Emperor seamount chain, actually form?  

In 1963, the Canadian geophysicist J. Tuzo Wilson put forward a theory to explain it. His idea, later expanded upon by other researchers, was that deep below the Earth's crust, in the semisolid middle layer known as the mantle, hot plumes or superheated bubbles of caramel-like silicate slowly rise toward the asthenosphere (the uppermost region of the mantle), originating from the core-mantle boundary some 2,900 km below the planet's surface. These plumes of upwelling material rise at the same time as slabs of lithosphere are subducted or pulled under towards the lower mantle in a process called "mantle convection".  

Most of us in fact have some idea of the process of convection, even we don't happen to own, say, a convection oven: the humble lava lamp. This simple yet absorbing objet d'art is famously composed of a glass envelope containing blobs of wax whose relative density fluctuates as a result of the application of heat from the bulb at the base of the lamp:

While the analogy is only approximate, the movement of material under the Earth's crust or lithosphere follows a roughly similar pattern. When blobs of hotter material, known as diapirs, contact the crust, "hot spots" of volcanic activity on the Earth's surface often result. And because the chunks of lithosphere are contantly moving like giant conveyor belts relative to the generally slower-moving plumes rising from below, chains of volcanoes form not only next to plate boundaries but also many other places where the crust becomes softer and more ductile from the heat and pressure spreading out underneath. This is the type of process that geophysicists think produced the Hawaiian islands which, as we must remember, are really just one segment of a long chain of undersea volcanoes, or seamounts, that over time accumulated enough material to breach the surface. The chain itself, consisting of at least 80 distinct volcanoes, is almost unimaginably vast, stretching 6,200 km (3,900 mi) from the active Lōʻihi seamount southeast of the island of Hawaii, all the way across the Pacific to the Aleutian trench off the east coast of Russia: 

 

Other notable examples of geologically active areas found to be driven by hotspots are situated at Yellowstone National Park in the United States, in Iceland, and on the island of Réunion. 

Actually, it's a bit more complicated than this. It turns out that the Hawaiian-Emperor chain is not precisely a paradigmatic example of the original stationary hotspot theory, where something resembling a straight pipe or conduit supplies a single location with a continuous flow of magma. More recent research indicates that mantle plumes don't remain in fixed locations; instead they tend to wander about slightly over geologic scales of time. The arc, or in this case, the "boomerang"-like shape of the resulting volcanic chain can therefore be caused by the movement of the plume underneath as well as the motion of the plate sliding over it.  

In addition there has been an ongoing debate for some time over whether the mantle plume theory is even fundamentally correct as an explanation for volcanism. A number of interrelated competing hypotheses, generally referred to as "plate theory", have been put forward over the past couple of decades. Nevertheless, despite the relative novelty of the idea, a great deal of evidence has since accumulated in favor of the existence of mantle plumes and their instrumental role in driving volcanic activity and plate tectonics. 

It seems that for the first time, scientists are finally getting tantalizingly close to understanding the geodynamics of the planetary system as a whole: a truly remarkable achievement, considering that almost nothing was known about any of this at the turn of the previous century!