A caldera is the cauldron-like depression formed when a volcanic eruption empties a shallow chamber of magma and the cone collapses. If the volcano is at sea level, the result, after the passage of time, can be a fine harbor.
A fine harbor, and fertile soil from all that ash, attracts people. At the east end of New Britain Island in Papua New Guinea, the result was the port and one-time district capital, Rabaul. The capital shifted after two of the three smaller cones around the caldera, Tavurvur and Vulcan, explosively erupted in 1994. But plenty of people still live in Rabaul, and they live in harm's way.
Amid the news about an eruption in Iceland last week, you may also have heard about the latest explosive eruption of Tavurvur. After seeing an extraordinary Facebook post of a photo of the eruption, shot from the sailing vessel Obelisk, I dug in a bit.
One result, which you can read below, is a remarkable firsthand account from the underwater photographer Christopher Hamilton and his partner, Leah Sindel, who were aboard the boat when the harbor began rumbling. They were sailing in the region photographing World War II shipwrecks and a cave full of skulls. The boat's owner and skipper, pictured above in one of Hamilton's photos (and the Facebook shot), is Jesse Smith.
But first I want to draw your attention to two other views of Rabaul and the eruption.
First, another fine photographer, Eric Lafforgue, was on the ground in Rabaul after the eruption and captured some absolutely stunning pictures of daily life there, reflecting the realities facing millions of people in developing countries who live in places deeply vulnerable to geological hazards. At the top of this post is one of his images. There are many more here.
Second, there's the geological context. On The Conversation, a fast-expanding website in which scientists and scholars fill the gap left as conventional news operations shrink, there's a superb look at the Tavurvur eruption by Robin Wylie, a doctoral student in volcanology at University College London (@rwylie9 on Twitter). Here's just a snippet of his piece, but I encourage you to read the rest:
Don't be concerned if you don't know much about Rabaul. Until recently, not even volcanologists did. The eruptive history of Tavurvur and its volcanic entourage was largely a mystery until the 1970s, when an increase in seismicity beneath the region prompted the first extensive volcanological survey.
It revealed that, over the past 7,000 years, a number of huge, highly explosive eruptions had occurred. These cataclysms hollowed out Rabaul caldera. The largest of them occurred in the late 6th century, and is believed to have generated pyroclastic flows – that of hot gas, ash and rocks blown out of a volcano – reaching at least 50 kilometers from Rabaul. This massive eruption probably had a volcanic explosivity index of six, which is equivalent to that of the 1883 eruption of Krakatoa. And yet, while the former lives in infamy, relatively few people have even heard of Rabaul.
…While small eruptions are fairly common, experts also believe that a big one may be brewing. Devastating eruptions like the ones which carved out the caldera take place on average every few thousand years. And as the last one struck 1,400 years ago, the clock is well and truly ticking. The rebuilt Rabaul town now has only around 4,000 inhabitants, but tens of thousand more live within touching distance of a large blast. This far-flung island is worth watching.
I'll be helping run a blogging workshop for communicative scientists this fall at Stony Brook University and Wylie (and The Conversation) will be a prime example of what's possible.
Finally, please read Hamilton's note (written with Sindel's help) describing the scene on Aug. 29 when the bay started reverberating and ash and flaming boulders started falling (I inserted links to relevant photographs from their Flickr feed):
I have been on a sailing journey from New Zealand to Indonesia, passing through Vanuatu, the Solomon Islands, & Papua New Guinea. My time in these countries has been principally spent diving (mostly on wrecks from the second World War) and exploring.
We arrived in Rabaul on August 26th, and soon met local wreck hunting legend Rod Pearce, who was kind enough to show us around and take us to dive some of the most fascinating war wrecks in the harbor.
At 3.30 am on the morning of the 29th, we felt a light rain, and sleepily closed the hatches above our heads, but were faintly aware of an odd sound, somewhere in the background. Finally one of my sailing companions called out in a puzzled voice, "It's rainy but I am not getting wet." At this point we rose to investigate further, and were sharply jolted awake when we realized that it was ash and small pebbles of pumice raining down on us, not water.
We were at first bemused, thinking that the volcano not far from where we were anchored was sending out a small benign shower. The locals had told us that sometimes dust showers could occur. But soon the hatches above out heads become black with a thick layer of debris, and the volcanic rain seemed to be getting heavier. Shortly after this we were awakened by Rod Pearce, tied up to the dock next to us, who sharply informed us that everyone was getting out — now.
The crew sprang into action, and in two minutes we were making our way out of the harbor. By this point, sizable chunks of pumice and other debris were bombarding us as we coiled the lines and tried to find a path ahead of us.
I ducked down to the chart table to turn on our navigation software, and suddenly heard from above several great cries in unison. I came back up immediately and was greeted with one of the most extraordinary spectacles I have ever seen. The mountain — far from issuing a benign puff of ash — was spewing out a fountain of lava, flinging enormous molten rock fragments miles into the sky. The sound was deafening, and preceding every rumble, the shock waves that were sent out reverberated through your chest. At this point we were beside ourselves with elation, and fumbled through the drawers for a zip-lock bag containing the camera.
Photo The Tavurvur volcano near Rabaul, Papua New Guinea, erupted on Aug. 29.Credit Christopher HamiltonDawn soon came upon us, and the spectacle of the eruption was crowned by deep golden light rays filtering down what we could now clearly see to be a colossal pillar of smoke, ash and sulfur billowing above the bay.
As the light grew stronger we were also confronted with the heavy layer of ash and debris covering the sailboat, and the enormous task that lay ahead of us. Our departure date, it was obvious, had been pushed back awhile. We spent all of that day in a harbor along the other side of the bay (upwind of the volcano), scrubbing and hosing and prying and scraping the muddy, crunching mess from the decks and from every conceivable crevice on the boat. All the while we had a perfect view of the eruptions' death throes. Every so often a flash of light like a perfect sphere was perceptible for a split second above the caldera, and a shock wave would ripple up the cloud column.
Seconds later this would be accompanied by a stupendous crash. The lava was less abundant and certainly less visible now in daylight, but the mountain continued to throw out enormous chunks of debris, some of which were flung so far that they landed in the sea at the foot of the volcano with an explosive splash and a mist of steam.
By later afternoon the activity had significantly subsided, and we spent the evening with a cold beer in hand, peering out into the darkness of the bay to catch the final flickers of light above the peak.
There are more images on Flickr, as well as video shot by Hamilton.
Postscript | Capital Weather Gang has yet another vantage point on the eruption — before and after imagery from NASA showing the extent of the ash fall.
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