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Volcanowatch

August 5, 2004

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Lava and seawater interactions: a hotbed of diverse research

If you walk along the coast to where the Banana flow has been entering the sea, you will see a steam plume rising from the surface of the water. The steam is often known by an informal term, laze, or lava haze. Walking through this steam plume can be unpleasant, giving you a burning sensation at the back of your throat. This is caused by hydrochloric acid, formed through the heating of seawater. The laze also contains tiny bits of glass, derived from shattering of the lava as it enters the ocean.

When hot flows of any type enter the sea, many interesting interactions occur: physical, chemical, and biological.

Physical interactions include hydrovolcanic explosions. When lava is heavily fractured, the interaction between seawater and lava is increased due to the greater surface area of the lava. This is why, in Hawai`i, `a`a flows are more often associated with explosive activity upon reaching the sea than are pahoehoe flows.

Hot flows of lava blocks and ash, called pyroclastic flows, can also cause intense hydrovolcanic activity when they hit water. Pyroclastic flows generally occur at volcanoes that erupt more viscous, sticky lava than do Hawaiian volcanoes. Pyroclastic flows are typically a few hundred degrees cooler than Hawaiian lava flows, but they are made of fine-grained particles, so that the transfer of heat from the ash to the water is much more efficient than from a lava flow.

A year ago, on July 13, 2003, a large explosion of this type occurred at Soufri?re Hills Volcano, Montserrat, an andesite volcano in the Caribbean. A large pyroclastic flow hit the sea, traveling at over 100 km/hr (60 miles per hour). The fine hot ash contained within the flow immediately flashed the seawater to steam and caused it to expand over 300 times in volume.

The rapid expansion of gas from the seawater pushed still-hot rock fragments and ash radially outward, at speeds of up to 160 km/hr (100 miles per hour), faster than the original flow! This is called a base surge, a ground-hugging flow of explosion debris. The base surge flowed inland up to 300 m (1,000 feet) above sea level, killing 50-60 cattle and burning vegetation. In places, the flow was so dense and thick that charcoal was formed, which requires heat in the absence of oxygen.

Primitive life is also found in places where molten lava comes into contact with seawater. Evidence for the earliest microbes on earth is found in the glassy rims of pillow basalt erupted on the ocean floor. Black smokers--hydrothermal vents at mid-ocean ridges, where seawater circulates in hot lava--provide a wealth of metals and sulfides leached from the lava. Such material has a profound influence on the composition of the ocean.

These minerals support a wide range of unique and diverse organisms, some recently discovered. Many of them do not need sunlight to survive and are very primitive in form. Scientists believe that these environments may hold the key to the beginnings of life on earth.

The interface between lava and seawater can be a beautiful place, as visitors to any active lava delta will attest. It can also be a dangerous place, because of the laze and the potential for explosions and even delta collapses. Above all, it can be an exciting place, both viscerally and intellectually. The next time you watch lava and seawater meet, think about what is happening. You will come away with an enhanced appreciation for one of nature's most exquisite events.

Activity update

Eruptive activity at Pu`u `O`o continues. Lava in the Banana flow, which breaks out of the Mother's Day lava tube a short distance above Pulama pali, has been visible between the pali and Paliuli for the past several weeks. A new rootless shield was built about 1.5 km (1 mile) south of Pu`u `O`o late last week, as lava escaped from one of the lava tubes. One important change is that lava stopped entering the ocean on August 4 for the first time since May 31, and the lava delta remained quiet on August 5. We can't tell if this is a temporary stop or the end of the line for the Banana delta. Eruptive activity in Pu`u `O`o's crater is weak, with sporadic minor spattering.

No earthquake were reported felt on the island during the week ending August 4.

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity was notably high during the past week, with 21 earthquakes recorded in the summit area for the second straight week. This is the highest number in some time, even more than in early May 2002, when inflation of Mauna Loa resumed. Most of the earthquakes are of long-period type and deep, about 40 km (23 miles) or more.

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Updated: August 11, 2004 (pnf)