When volcanoes wake up

You have to admit it’s cool living in a state with an erupting volcano, especially knowing all the destructive potential is confined to the eastern edge of the island chain.

Well, not so fast, Oahu. Believe it or not, scientists say it’s still possible a volcano can spew forth red-hot streams of molten rock right here.

They say Hawaii’s most populous isle — and, actually, most of the islands west of Hawaii island — may yet experience what’s known as rejuvenation-stage volcanism, the kind of eruptive activity that happens long after the active period that Kilauea is in the throes of now.

Granted, it may not happen in our lifetimes. Or your children’s lifetimes. And maybe not for hundreds of years.

But it could happen sooner rather than later. And if it does, it might not be all that cool anymore. In fact, it could get downright ugly.

University of Hawaii geology professor Michael Garcia, an expert on rejuvenation volcanism, said such an eruption could happen anywhere on Oahu with little warning.

"If the eruption comes from great depth and high velocity as anticipated, then there might be only short-term seismicity before an eruption," he said. "Warning will be quite limited and we might not recognize the signs since we have never observed such an eruption."

The main Hawaiian Islands are at the southeastern end of a long trail of volcanoes that began erupting more than 70 million years ago. The main islands are only a small part of the chain and are the youngest islands in a lengthy, mostly underwater mountain range consisting of more than 80 volcanoes.

Each island is made up of one or more volcanoes that emerged from the seafloor over many thousands of years, the result of the continued movement of the Pacific plate over the stationary Hawaiian hot spot, a geologic phenomenon that continues to thrust molten rock to the earth’s surface.

The volcano that created Oahu’s Koolau Mountains stopped erupting 2.1 million years ago, leaving a massive shield volcano that eventually eroded into the mountains we see today.

But Oahu wasn’t done. Volcanism started up again in Honolulu between 800,000 years ago and 350,000 years ago and then pulsed once more about 100,000 years ago. This series of rejuvenation-stage volcanoes left some features we know today such as Diamond Head, Koko Head and Punchbowl.

Is Oahu overdue for another eruption?

Hard to say, because it’s possible volcanism may have already experienced its last gasp here, Garcia said.

On the other hand: "Yes, the next eruption could be just like Diamond Head or even Punchbowl, or produce large lava flows that flow in valleys like in Nuuanu, Kalihi or Manoa."

Or it could happen at the bottom of an ocean channel or at any of the other islands. Even Kauai and Niihau — at the northern end of the main isles — may be vulnerable. Lava flows as young as 100,000 years old to 200,000 years old have been discovered there, Garcia said.​

Of all the sleeping volcanoes that make up the state’s landscape, Hawaii island’s other "active volcano," Mauna Loa, is most likely to go off first — and not necessarily because there’s been a recent uptick in the number of earthquakes recorded on the mountain.

Frank Trusdale, a Hawaiian Volcano Observatory geologist, said the surge in earthquake activity may or may not be a sign that another eruption is in Mauna Loa’s near future.

Some 15 earthquakes were recorded by seismometers in just the past month on the west flank of Mauna Loa, he said, but the quakes were relatively small compared to the ones felt before the volcano last erupted in 1984 and before that in 1975.

"It was just enough to say, ‘I’m an active volcano and I’m not dead yet,’" Trusdale said.

The world’s largest active volcano is certainly not dead. It has erupted hundreds of times over the last 3,000 years, producing lava flows every six years on average. Since 1843, Mauna Loa has erupted 33 times, averaging one eruption every five years.

Mauna Loa has a history of producing voluminous flows of basaltic lava that have reached the ocean eight times since 1868. In 185556 and 188081, lava from the mountain’s northeast rift zone did not reach the ocean but covered grounds now within the city limits of Hilo.

In 1984, lava crept within 4.5 miles of Hilo.

Scientists say it isn’t a matter of if it will erupt again, but when. And when it does, it could be costly.

The flanks of the volcano saw roughly $2.3 billion in new construction between 1984 and 2002 alone, the U.S. Geological Survey estimated.

If an eruption does occur along Mauna Loa’s southwest rift zone, neighborhoods in South Kona and Kau could be at significant risk. Not only will homes, highways and power lines be threatened, but there may be precious little time for evacuations, perhaps only hours, considering the steepness of the terrain and the likelihood lava would flow faster down the mountain there, Trusdale said.

Hilo, meanwhile, is relatively far from the active part of the rift zone. The lead time for response and evacuation is likely to be substantially longer — days to weeks, he said.

After Mauna Loa, Hualalai, the third youngest and third most active volcano in Hawaii, is the next likely mountain to blow. Scientists said six different vents erupted lava between the late 1700s and 1801, two of which generated flows that ran into the ocean off the island’s west coast. The Kona Airport is built on top of the larger flow.

Although Hualalai hasn’t shown signs of erupting in many decades, geologic mapping of the volcano reveals that 80 percent of Hualalai’s surface was covered by lava in the last 5,000 years. In 1929, an intense period of earthquakes over more than a month was likely caused by magma rising close to the surface, Trusdale said.

For these reasons, scientists have described Hualalai as a potentially dangerous volcano likely to erupt again in the next 100 years.

There are two other volcanoes on Hawaii island. Kohala last saw lava flows 60,000 years ago, while Mauna Kea last erupted 4,000 years ago.

"In geologic terms, it’s not that long ago. We’re not ready to throw in the towel" regarding Mauna Kea’s eruptive potential, Trusdale said.

On Maui, Haleakala hasn’t erupted for centuries.

And no activity there is imminent, said Dave Sherrod, a former Hawaiian Volcano Observatory geologist who studied Haleakala’s lava flows up close for several years.

Because of Haleakala’s lengthy repose, he said, there will likely be ample warning of a new eruption over a period of weeks or months, the tip-off being a series of small earthquakes, recorded by seismometers but most of them too small to be felt.

Sherrod, who now works at Cascades Volcano Observatory in Washington state, offered this scenario for how an eruption might occur on Maui:

In the hours or perhaps a day or two before the eruption, tiny ground cracks will appear, but only in the area that will finally become the focus of the eruption. They will not be gaping cracks but small fissures of an inch or so wide, each one perhaps a few yards or tens of yards in length.

Unlike Kilauea, with its large shallow magma chamber that erupts at the summit or feeds magma down its rift zones, Haleakala’s magma systems are deep and small. An eruption would be a one-time event, though once begun it might persist for weeks or possibly months.

"Maui residents shouldn’t expect a 30-year eruption like that ongoing at Kilauea," Sherrod said. "None of the eruptions of the past 1,500 years have produced long-lived lava fields. Eruptions that begin high on the volcano have rarely had enough ‘oomph’ to supply lava flows that reach all the way to the shoreline."

The eruptive center likely would be somewhere along the southwest rift zone, the crater floor or east rift zone.

Vast sections of Maui on the western flank of Haleakala would have an "infinitesimal likelihood" of future lava inundation, including North Kihei, Makawao and Kula, he said, while the chances of lava hazards increase in South Kihei down toward Makena. On the Hana side, the most recent eruptions were north of Hana town.

East Maui has experienced roughly 15 eruptions in the past 1,500 years. The most recent lava flows, at La Perouse Bay, occurred sometime between about 1500 and 1600, according to radiocarbon dating, or 1790, from anecdotal accounts of fresh lava, Sherrod said.

ENLARGE CHART

ENLARGE CHART

ENLARGE CHART