Volcanoes - Island Builders, Mountain Builders, And Time Bombs

The Hawaiian Islands would not be here today if it were not for the volcanoes of the mid-Pacific. There would be no Kauai, no Lanai, no Molokai, no Oahu, no Maui, and no Big Island. They were all formed by shield volcanoes fed by a constant flow of magma from a superheated pocket in the lower mantle called a hotspot.

Shield volcanoes fueled by hotspots are different from the stratovolcanoes of the Pacific Rim. The magma pouring into shield volcanoes is mainly basalt, lighter and less viscous than the silica-based magma fueling the Pacific Rim volcanoes produced by the collision of tectonic plates.

The hotspot theory might well be demonstrated by the way in which the Hawaiian Islands were formed. The theory suggests that there is an area in the mid-Pacific hundreds of miles below the ocean floor in the earth's mantle, called The Hawaiian Hotspot. The hotspot size is estimated at 50 miles in diameter. The temperature in the hotspot is higher than that of the surrounding mantle. This intense heat melts the material from the lower part of the overriding tectonic plate and converts it into magma. The hotspot magma is lighter than the material in the rest of the mantle and rises to the earth's crust in a narrow stream called a mantle plume. The mantle plume brings magma up to the ocean floor, and a volcanic cone begins to build.

Over time, repeated eruptions produce more and more lava that continue to build the cone higher and higher, until it finally emerges above the surface of the ocean as an island many thousands of years later.

The tectonic plate on which the new Hawaiian island sits is called the Pacific Plate. The Pacific Plate moves over the stationary hotspot in a northwesterly direction at about 1 inch per year. Moving at that rate, it takes several million years for the island to clear the hotspot. Island building progresses during this long period of time, and large islands with tall volcanic mountains are created, as is the case with the island of Maui with its 10,000 ft. Haleakala volcano, and the Big Island of Hawaii with its chain of volcanoes in 13,700 ft. Mauna Kea, 13,600 ft. Mauna Loa, and currently active Kilauea.

Once the moving plate carries an island away from the hotspot plume, the volcanoes on that island go dormant and start to erode, and the plume proceeds to build the next island. Niihau and Kauai, the easternmost islands of the Hawaiian chain, were the first to be built up from the hotspot activity. The Pacific Plate continued its northwesterly movement over the hotspot and the islands of Oahu, Molokai, Lanai, and Maui were created in succession. The last island over the hotspot is the still volcanically active Big Island of Hawaii.

Just a few miles off the Big Island, a new island is starting to build. It is called Loihi, and has pushed up two miles from the ocean floor, but is still a mile beneath the surface of the ocean. It is estimated that Loihi will appear above the ocean surface in 220,000 years.

One source suggests there are 50 active hotspots throughout the world. Some, such as the Hawaiian Hotspot, lie under oceanic plates, and others under continental plates. Among the volcanic islands created by oceanic hotspots are Iceland, the Azores, the Galapagos, Samoa, the Marquesas Islands, the Society Islands, and Reunion.

The best example of continental plate hotspot activity is Yellowstone National Park. The heart of the park is an active caldera that powers hot springs, fumaroles, mud pots, and geysers such as Old Faithful. The thermal energy derives from a large hotspot underlying the caldera. This same hotspot has produced many other caldera and lava bed areas throughout the western states, as the North American Plate moves southwesterly. The Yellowstone caldera was created by a massive volcanic eruption 600,000 years ago.

In recent years, a number of scientists have challenged the idea of a stationary hotspot and of a mantle plume being the source of hotspot magma. Several other explanations have been put forth. Since hotspots are located deep in the earth's mantle, no one has ever seen one. Their existence and the way they work can only be assumed.

In contrast to hotspot island building, the big mountain ranges of the Pacific Rim, such as the Cascades in North America and the Andes in South America, were formed by stratovolcanic action, earthquakes, and uplifting of the continental plate boundaries. Earthquakes, stratovolcanoes, and uplifting are products of a process called subduction, in which the oceanic plate slides under, or subducts, the continental plate.

Stratovolcanoes tend to erupt less frequently but far more violently than hotspot volcanoes. Subduction produces magma rich in silica. This heavy and sticky material seals the heat and gasses inside the volcano and permits the pressure to build up over hundreds of years until it finally erupts, sometimes in a devastating blast.

Uplifting can be caused by the fault slippage that produces earthquakes and frees the continental plate to rise, and by the compression of one tectonic plate pushing against another. For millions of years, along the west coast of the U.S., the Pacific Plate has not only been sliding under the North American Plate, but also pressing against it. The compression has been a factor in the uplifting process.

Although most spectacular volcanic blowouts are produced by stratovolcanoes, there have also been some historic hotspot volcano blasts. The Kilauea volcano on the Big Island of Hawaii had a major blowout in 1790 that killed 80 people.

Any active volcano can be considered a time bomb. There are warning signs such as swarms of tremors that sometimes precede eruptions, but no one really knows in advance exactly when a volcano will blow or how big the eruption will be.