(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Anyone looking at a map of the Pacific cannot help but be astounded by the feat of the first Polynesian navigators to these isolated islands of Hawaii. In contrast, their homelands in the south Pacific are dotted with tens of thousands of islands.

As the islands of the south Pacific are typically of volcanic origin, the region has the highest density of volcanoes on Earth. Many of the older, extinct volcanoes sank below the surface, while reefs grew upward, forming beautiful coral islands or atolls.

There are, however, still a lot of active volcanoes in the south Pacific, many of them so recent that they have not yet broken the surface of the ocean.

Some of the volcanic chains, such as the Marquesas, and Hawaii, trace the path of the Pacific Plate over a stationary “hot spot” — a place where magma wells up through the crust to erupt on the sea floor, building a volcanic edifice.

Other volcanic chains, such as the Mariana Islands and Vanuatu, are island arcs, associated with subduction zones — boundaries between tectonic plates where one plate, generally the older and denser of the two, thrusts down into the mantle beneath the other plate. Arc volcanoes form on the overriding plate, in this case the Pacific plate, roughly paralleling the plate boundary. This is the classic “ring of fire” volcanism.

Although Kilauea is the Pacific volcano making the news recently by sending lava flows into inhabited areas and producing land-building ocean entries, there are volcanoes in the south Pacific, as well as elsewhere, that rival Kilauea’s “most active volcano” status.

For example, Yasur volcano in Vanuatu has been erupting almost continuously since at least 1774, when Captain Cook recorded seeing ash erupting from Yasur. Eruptions tend to be strombolian, ejecting incandescent material to altitudes of hundreds of meters, though vulcanian explosions of dense, ash-laden gas also occur.

This past summer, ash clouds reached elevations of almost 2,000 m (6,500 feet), affecting flights in neighboring New Caledonia and contaminating local water supplies.

Like Kilauea, Yasur is a visitor attraction, though quite a bit farther off the beaten track. It is possible to hike to the summit area of Yasur (361 m, 1,184 ft. high) and experience the eruptive activity from the crater rim. This can be a dangerous expedition, however, as lava bombs are sometimes ejected to the crater rim and far beyond. In the mid-1990s, several people were killed by falling volcanic bombs.

The small island nation of Vanuatu consists of about 82 islands whose total area is similar to Hawaii Island. Vanuatu has many active volcanoes, including Yasur. Ambrym, the most voluminous of Vanuatu’s volcanoes, is also very active, with about 50 eruptions since 1774, most of them explosive, and two of them resulting in fatalities.

It is also a source of vog and acid rain that affect the local population. Acid rainfall in 1979 was so intense that it literally burned some of the inhabitants, as well as contaminating water supplies.

Vanuatu is not only highly active volcanically, but also seismically, as is common in subduction zones, such as the one associated with the volcanic arc of Vanuatu. Just last week, a magnitude (M) 7.5 earthquake struck offshore. Only a small local tsunami was generated, and no one was reported hurt.

The previous large earthquake in the region, a M7.1, shook the islands less than 3 months ago. And just last year, in October 2009, two tremendous quakes, M7.8 and 7.7, occurred 15 minutes apart. After these events, an M7.4 aftershock struck approximately one hour later, and M 6.6 and M 6.8 aftershocks occurred on the following day.

Indeed, the Vanuatu region experiences many large earthquakes, with almost 50 events of magnitude 7 and larger having been recorded since 1973.

Although Hawaii is also seismically active, only the 1975 earthquake had a magnitude greater than 7 in the same period. Though we have a lower occurrence of large quakes in Hawaii, they are often generated at shallower depths. This, along with the greater population density of the Hawaiian Islands, tends to make them more damaging than those in Vanuatu.