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Volcano Watch — Hotspots

April 14, 1995

Many of the islands that dot the center of the Pacific Ocean are made up of active, dormant, or extinct volcanoes, whose geologic histories are characteristic of "hot spot" volcanism.

This is a map of the earth. The thick dashed line shows the ridge formed by the Hawaiian hot spot during the last 80 million years. The arrows show the direction and rate of motion of selected plates relative to the hot spots. The plate boundaries are shown by the lines of medium weight, and the "Pacific ring of fire" plate boundaries by heavier lines. The dots mark the youngest islands of some hot spot island chains. Can you name the island groups? The answers can be found at the end of this "Volcano Watch" column.

Many of the islands that dot the center of the Pacific Ocean are made up of active, dormant, or extinct volcanoes, whose geologic histories are characteristic of "hot spot" volcanism.

The active volcanism is limited to a localized region (or "spot") of the volcanic chain. Hot-spot island chains include the Hawaiian, Marquesas, Society, Pitcairn, Samoan, and Galapagos archipelagos.

Volcanoes are also common around the edge of the Pacific Ocean. Examples include the Cascade volcanoes (e.g., Mt. St. Helens), the Aleutians, the volcanoes in Japan (e.g., Mt. Fuji), the volcanoes of the Philippines (e.g., Mt. Pinatubo), and many others. These volcanoes have earned the Pacific rim the nickname "the Pacific ring of fire".

As the name "ring" implies, these volcanic islands are simultaneously active along a line, whereas hot spot volcanism is active only at small "spots." The Pacific rim volcanoes are related to plate boundary processes.

The surface of the Earth is made up of about 14 rigid plates, which move relative to each other like boards floating on a pond. Where two plates collide, one plate usually sinks beneath the other deep into the mantle of the Earth. The sinking plate melts to form magma, which rises and forms volcanoes on the overriding plate. This process forms the ring of volcanoes around most of the Pacific Ocean.

Where two plates move apart, magma rises and fills in the space. This kind of volcanism occurs deep on the ocean floor. Hot spot volcanoes are special because they can be found both within plates (e.g., Hawaii, Marquesas, and Society islands) and along or near plate boundaries (e.g., Galapagos, Iceland, and Azores islands).

The basic characteristics of hot spot volcanism can be observed using the best scientific tool there is--your eyes! Think about the different Hawaiian islands you have visited. Which one appeared to be the youngest? (The Big Island appears to be the youngest because Kīlauea is erupting now and Mauna Loa last erupted in 1984.) Which island is the oldest? (This is a trick question because there are many ways to answer it.)

Of the islands we can visit, Kauai has the deepest eroded valleys and the most coral reefs, which are indicative of age in the process of island formation and degradation. The oldest rocks outcropping on Kauai are about 5 million years old--older than the oldest hominid fossils in Africa!

The extinct volcano underlying the coral atoll of Midway is 26 million years old. If you could remove all of the water in the Pacific Ocean, you would find that the Hawaiian islands are peaks on a 10,000-foot-high ridge that extends 2,100 miles west-northwest from Hawaii, and then bends northward and extends for another 1,400 miles. The "elbow" in the ridge is about 43 million years old. The northernmost end is about 80 million years old and was formed during the time of the dinosaurs.)

The Society Islands follow the same pattern as Hawaii, with the youngest volcano at the southeast end of the island group and the oldest volcano at the northwest end. These observations led to the idea that deep within the earth, well below the "plate traffic," are some sources of magma, called "hot spots", that are fixed relative to each other. Volcanoes form on the plate above the hot spot. As the plate moves over the hot spot, new volcanoes form, and the old ones, which no longer overlie the hot spot, become extinct.

In Hawaii, the hot spot is currently under the active volcanoes of Mauna Loa, Kīlauea, and Lō‘ihi, a submarine volcano southeast of the Big Island. From the geometry of the islands and submarine ridges on the Pacific Plate, we know that the Pacific Plate has been moving west-northwest for the last 43 million years. The Pacific Plate moved directly northward from about 43 to 80 million years ago. Given the distances and times mentioned in this article, what is the average speed of the Pacific Plate relative to the Hawaii hot spot during the two time intervals?

A hot spot volcano making news recently is Fogo in the Cape Verde Islands off of the west coast of Africa. Fogo is a volcanic island that began erupting last week after lying dormant since 1951. One of our HVO scientists is part of the international team assessing the volcanic hazards of Fogo.

The age relations in the Cape Verde Islands are somewhat confusing, but a basic observation is that Fogo and the other westernmost Cape Verde islands have active or dormant volcanoes. The easternmost islands are nearly flat and their volcanoes are extinct. Which direction is the African Plate moving relative to the Cape Verde hot spot?

Now when you travel around Polynesia and other hot spot islands or just travel with a map and your imagination, you can observe the degree of erosion, reef growth, and the recency of volcanic activity and estimate which way the plates are moving relative to the hot spots. Note, this doesn't work well for Samoa, perhaps because the islands are near a place where the plates are coming together.

Answers: For the last 43 million years the Pacific Plate has been moving an average of 49 miles per million year (or 3 inches per year) to the west-northwest, relative to the Hawaiian hot spot. From 43 to 80 million years ago, the Pacific Plate moved an average of 38 miles per million year (or 2.4 inches per year) to the north, relative to the Hawaiian hot spot. The African Plate moves roughly eastward relative to the Cape Verde hot spot. Geography quiz: A, Marquesas; B, Society; C, Samoa; D, Eastern Caroline; E, Pitcairn; F, Guadalupe; G, Galapagos; H, Easter; I, Juan Fernandez; J, Iceland; K, Ascension; L, Madeira; M, Canary; N, Azores; O, Cape Verde; P, Fernando de Noronha; Q, Lord Howe; R, Martin Vaz; S, St. Helena; T, Tristan da Cunha; U, Reunion; V, Comores; W, Bouvet; and X, Kerguelen. How many do you know?

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