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Volcano Watch — Rising to great heights: rocks in the jet stream?

November 7, 2002

Air travelers to Hawai`i are familiar with the jet stream. The strong, high-level winds typically come from the northwest and slow west-bound flights by 30 minutes or more. Residents of Hawai`i returning from the mainland are frustrated by the jet stream; it takes longer to get home than to leave!

Once you're on the ground in Hawai`i, however, the jet stream is almost always a non-issue. It rages high above our heads, and we usually have to deal only with low-level trade winds, out of the northeast, and kona winds, from the south or southeast.

But recent discoveries are leading us to a startling conclusion: powerful explosions from Kilauea threw heavy rocks up into the jet stream. The stream carried the rocks kilometers (miles) downwind from the explosion site before dropping them onto the ground. Here's the developing story.

Past "Volcano Watch" articles described our finding that a powerful explosion in about A.D. 800 threw rocks from Kilauea's caldera to the area beyond Halape, on the south coast 16 km (9.5 miles) away. At Halape, the rocks are no more than 1.5 cm (0.6 inches) in diameter, but nearer the caldera, they are much larger. One rock, weighing 4,425 grams (9.75 pounds), was found 8 km (5 miles) from the caldera. Another rock, a coarse-grained gabbro from deep within the volcano, weighs 1,292 grams (2 lbs 13 oz.) and was thrown 10 km (6 miles) from the caldera.

The rocks are distributed southeastward from the caldera. Some rocks have been found north of the caldera, but they are smaller and less numerous than those to the southeast. The distribution suggests either a vent pointed toward the southeast---like a cannon aimed toward Halape---or strong winds blowing from the northwest.

An inclined cannon is unlikely, because the explosion apparently came from deep within the volcano, as outlined below.

So, we conclude that the rocks were ejected into the jet stream, which typically blows 35-60 knots from the northwest at elevations above 12 km (7 miles).

Our earlier interpretation was that the explosion was part of a Plinian eruption, which is capable of carrying large rocks to great heights. In such an eruption, however, the rocks are lifted upward by a large mass of up-rushing pumice. The column of pumice is crucial to the physics of the process. We find, however, only a very small volume of pumice and scoria (a less bubbly form of pumice) associated with the rocks. The eruption column was not nearly dense enough to lift heavy rocks sufficiently high to allow their distribution far from their vent.

Studies of coarse-grained rocks, called gabbro, ejected by the explosion show that they formed at depths of 4-7 km (2.5-4 miles) inside Kilauea. These rocks were in the process of crystallizing from magma when the explosion took place. So, the explosion itself was probably centered deep within the volcano, at least as deep as the gabbro was forming.

We believe that a cannonlike explosion took place, and that the cannon was nearly vertical, rising from the core of the volcano to the surface. The "cannonballs" were ejected with such velocity that they could enter the jet stream, which carried them downwind as they stopped rising and started to fall. The fact that the large rocks are found so far from the site of explosion suggests that they penetrated well above the base of the jet stream-possibly to a height of 25 km (15 miles) or more.

What drove the explosion? A buildup in gas pressure is the best candidate. The gas probably was carbon dioxide, which exists as a gas deep within the volcano. Other gases remain dissolved in the magma until near the surface. The recent finding that Kilauea emits about 8,400 tons of carbon dioxide each day-nearly double the previously assumed flux-lends credence to the accusation that CO2 is to blame. We remain at a loss, however, to explain why the gas was pressurized rather than leaving the volcano passively.

If you are wandering around near Kipuka Nene or Halape and find a solitary rock on the ground surface, scratch your head in wonder; that rock may once have been in the jet stream.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. The crusted-over Mother's Day flow in the western section of the flow field is the site of the activity. Occasional breakouts from the tube system feed surface flows visible on the pali and in the coastal flats. Lava continues to enter the ocean at the Wilipe`a and West Highcastle lava deltas. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The steam clouds are highly acidic and laced with glass particles. The National Park Service has erected a rope barricade to delineate the edge of the restricted area. Do not venture beyond this rope boundary and onto the lava deltas and benches.

Two earthquakes were reported felt in the week ending November 7. A resident of Leilani Estates subdivision felt an earthquake at 8:04 a.m. on November 4. The magnitude-2.0 earthquake was located 6 km (2.4 mi) northeast of Pu`ulena Crater at a depth of 2.3 km (1.4 mi). Later that night at 11:04 p.m., residents of Hilo, Glenwood and Volcano were shaken by an earthquake. The magnitude-2.7 earthquake was located 11 km (6.6 mi) north of Volcano at a depth of 24.4 km (14.6 mi).

Mauna Loa is not erupting. The summit region continues to inflate, but no earthquakes were located in the area for the last seven days.