Today HVO continuously documents

The observatory has four different monitoring networks: the seismic network, which measures earthquakes; the geodetic network, whose tiltmeters and GPS receivers measure changes as subterranean lava inflates or deflates a volcano’s surface; a network of instruments that measure gas emissions; and a collection of remote cameras, some of which can be accessed by visiting the observatory’s web site at http://hvo.wr.usgs.gov/observatory.

Geochemist and IT specialist Lopaka Lee coordinates those networks. Lee was born in Hawai‘i—as was about half the observatory’s staff—and the data he keeps flowing hold special importance to him and anyone else living in the shadow of the volcano: His mother’s family lost their home in a 1956 eruption that wiped out the village of Kapoho in Puna. “The data that come in and the information that we supply to the public and the [civil defense] decision makers—the reliability of that data has to be 100 percent,” he says. “It can never fail. It can never stop.”

Observatory scientists also study the past in order to better predict the future. Kilauea’s near-continuous lava flows have claimed few lives in recent years, but that hasn’t always been so. Kauahikaua points to one USGS study by geologist Michael Garcia, who found that in the past Kilauea had long periods in which it produced explosive eruptions. One such eruption in 1790 wiped out a large portion of the army of Ka‘u chief Keoua, Kamehameha the Great’s primary rival. Keoua’s warriors weren’t burned to death like the people of St. Pierre. They reportedly looked as if they’d fallen asleep—probably overcome by toxic gases like sulfur dioxide, which the observatory now monitors on a daily basis. Kilauea undoubtedly will have another explosive phase in the future— something Big Island residents need to keep in mind.

Kilauea gets the majority of the headlines these days, and people tend to forget the catnapping giant next door. The scientists don’t. Mauna Loa is as heavily instrumented as its little sister, and it’s long overdue to erupt. It went off fifteen times during the twentieth century—an average of once every 6.6 years. Its most recent eruption, in 1984, sent a flow to within about four miles of Hilo. Frank Trusdell, the observatory’s Mauna Loa specialist, notes that during that last eruption, Mauna Loa spewed forth as much lava in twenty minutes as Kilauea typically exudes in a whole day. And its flows are more dangerous because they can move faster: “Most of the flows in South Kona moved from their source down the slopes to the sea in less than one day,” says Trusdell. Fortunately, he says, Mauna Loa looks quiet for now: “We have no earthquakes to indicate anything imminent,” though his tiltmeters show that the mountain’s flanks are inflating very slightly.

The observatory also watches for three other dangers: earthquakes, landslides and tsunamis. A Hawaiian volcano is riddled with faults and fissures and is so heavy that it actually depresses and cracks the ocean floor beneath it—a prime spawning ground for earthquakes. And every so often a hunk of the island breaks off and slides into the sea. Usually it’s a lava shelf that builds up where a flow meets the ocean; it’s the job of HVO scientists, national park staff and civil defense to make sure no sightseers are out on such a shelf when it goes (which happened in 1993 when a visiting photographer was swept out to sea and about a dozen others were injured after one such “bench collapse”). From time to time much bigger chunks fall off. One prehistoric landslide near modern-day Kealakekua Bay generated a tsunami that left beach cobbles 365 meters above sea level on Lana‘i. If a similar slide were to happen on Kilauea, it could generate a tsunami even bigger than the one that devastated communities from Indonesia to Africa in 2004.

Today volcanoes from Iceland to Yellowstone are monitored with webs of instruments modeled on the ones Jaggar pioneered. Monitoring probably helped limit the deaths to only eighty-two when Mount St. Helens erupted in 1980. Instruments on Mount Rainier could provide evacuation time for millions in the Seattle area should that volcano awaken. GPS sensors and tiltmeters helped Icelandic volcanologists predict the 2010 Eyjafjallajokull eruption, whose ash cloud closed airports all over Europe.

Today the seismographs in the Jaggar Museum are anachronisms; where the originals had converted vibrations from a needle into lines on a drum of smoked paper, the current technology translates digital signals radioed from faraway instruments. Volcanologists might still hook gobs of molten lava with rock hammers and quench their samples in old coffee cans, but sophisticated chemical analysis can now tell them the lava’s temperature before quenching, whether it came from deep or shallow magma reservoirs and whether the flow is likely to be a short-term phenomenon or part of a long-term event.

Powering all that data is still the simple idea that Jaggar brought back from Martinique: The more we know about what the Earth is doing, the safer we’ll be.