Water Beds and Magma Beds

Mauna Ulu in magnificent nighttime eruption, 1969 - U.S. Geological Survey photo

In the 1960s and early 1970s love, peace, long hair, and free everything was the order of the day. The waterbed became a popular and important part of that lifestyle. Used properly, a waterbed purportedly would enhance one's love life. Today, I don't know of a single friend or acquaintance who sleeps on a waterbed, and I don't think it's because we have become an old and stodgy generation. I think that conventional beds simply are superior sleep inducers, and let's face it, people of any age spend at lot more time asleep than at play on a bed.

As creatures of the Flower Child generation, Anne and I tried a waterbed for one night. We were passing through King City, California, on vacation, not long before we moved to Hawai'i. In this case, our curiosity got the better of our common sense. It only took a few moments between the sheets for us to realize our mistake, when the fun of sloshing around in a semicontrolled surfing adventure wore off. The water in our bed was way too cold for anyone to sleep on-at rest we shivered uncontrollably. We solved this problem, much to the befuddlement of the motel manager, by placing several extra blankets under us as insulation from the cold. We also turned the heater to its maximum setting, but thermal inertia kept the water in the bed uncomfortably cool through the night.

At Hawaii Volcano Observatory (HVO), I once spent an early morning on a magma bed with Don Swanson. The fluid in this bed was uncomfortably hot, and adding a layer of insulation was not a practical solution to the problem. Management in this case was Pele, the Hawaiian goddess of fire, and she was not about to intervene on behalf of two intruding geologists. Don and I created our own solution: a hasty retreat from the surface of the restless hot monster. The magma bed story starts with the Mauna Ulu eruption.

When I arrived in Hawai'i to begin my stint at HVO, an eruption was underway on Kilauea's east rift zone. That eruption had begun three months earlier on fairly flat ground covered with rain forest, along a two-mile reach of newly formed cracks and rifts. Within days, the rift zone spewing out magma focussed its activity at about the center of a roughly equilateral triangle whose corners were the two preexisting craters, 'Alo'i and 'Alae, and the cinder cone Pu'u Huluhulu. During the following twenty-nine months, sporadic eruption at this spot built a 250-foot-tall mound of lava, called a lava shield. The eruptions also fed some flows that extended across the south flank's grand staircase and into the Pacific Ocean, about 8 miles away. As the lava shield around the vent grew to become a notable figure of the landscape, the U.S. Geological Survey appropriately named it Mauna Ulu, Hawaiian for "growing mountain."

Before Mauna Ulu appeared on the landscape, the Chain of Craters Road continued across the south flank of Kilauea to the south coast and then looped eastward near sea level to connect with a system of paved roads that gave access to the entire southeast part of the Big Island, including Hilo. Lava flows from early eruptions at Mauna Ulu, however, buried several miles of the road, greatly complicating travel between the summit of Kilauea and the parts of the National Park along the south coast. With partial burial, the Chain of Craters Road simply led to a parking lot next to 'Alo'i, beyond which the general public was excluded but HVO staff was expected.

Though stretched thin with leveling, geodimetering, and other studies related to deformation of Kilauea Volcano, the HVO staff also maintained a frequent visual monitor of activity at Mauna Ulu. For most days, this included a half-mile hike from the new abrupt end of the Chain of Craters Road, right up onto the Mauna Ulu shield.

A nighttime eruption at Mauna Ulu feeds streams of lava that spill into 'Alo'i Crater. -Wendell Duffield photo

Don and I often hiked to Mauna Ulu together. As our total number of round-trips grew substantially, but the landscape over which we hiked changed relatively little, at least on a daily basis, we became complacent about what was underfoot. This carelessness helped lead to the magma-bed incident.

One day on the hike to Mauna Ulu, as Don and I were walking and talking and perhaps not paying enough attention to our footing, we both suddenly felt the ground beneath us move in a soft and mushy way. This was not an earthquake. Simultaneously, and without a word spoken, we realized that we were literally on thin crust, Pele's magma-filled equivalent of a waterbed.

Apparently, overnight Mauna Ulu had erupted a piddling bit of lava, which had puddled in a low spot along our trail. By the time we arrived on the scene, a lava crust had formed over the puddle. But hidden beneath that skin lay a reservoir of still-molten rock. Though cooling and thickening with time, the crust was still so thin, probably only a few inches, that the weight of our bodies was pushing the crust down into the melt, just as a human body pushes the rubber bladder of a water bed into its liquid interior. We quickly backtracked to solid ground and waited for our heart rates to approach normal. I don't remember what happened next, but I never again walked cavalierly onto very new looking lava without first convincing myself that only solid rock lay underfoot.

In fact, we probably were never in imminent danger. Obviously, the crust, though thin, was sufficiently strong to hold our weight. Even if the crust had cracked, we may well have been able to walk safely away. The melt beneath the crust was thick, pasty, and viscous enough that we could have stepped from one crack-bounded piece to another before our weight pushed any one piece into the underlying hot ooze. Mind you, I wouldn't want to try this hopscotch dance, but I think it could be done. If the trapped melt was still frothy with gases, a different and more frantic dance might ensue.

The truly frightening thought is what might have happened if we had strolled onto the lava during what geologists call crustal overturn. Many direct observations at Kilauea over the past several decades verify that a pond of lava may evolve through the following series of steps as it changes from hot and restless melt to solid rock.

• Step one: As soon as the pond forms, or even as melt is still being added, a thin black crust coats the surface. This happens as the melt, which solidifies at about 2,000 degrees Fahrenheit, quickly cools when it is exposed to the earth's atmosphere, which at Kilauea is no more than 100 degrees Fahrenheit.
• Step two: With time and cooling, this crust slowly thickens. Simultaneously, gases (mainly water vapor, carbon dioxide, and sulfurous vapors) slowly and continuously escape from the underlying melt and collect under the crust.
• Step three: If enough of these gases collect, they buoyantly lift the crust until it breaks into pieces that tilt and sink into the underlying less-dense, frothy melt. This is called crustal overturn, because the process destroys the existing crust and new crust eventually takes its place.
• Step four: Depending upon the amount of gases remaining in the melt, steps one through three repeat, or step three ends with a stable crust that thickens until the entire pond of melt has solidified.

If the "crust" on your waterbed cracks and springs a leak, at the worst you will get soaked and have a wet mess to mop up. However, if the crust on your magma bed springs a leak and founders, you probably will have no mess to clean up. You will instead become completely incinerated toast. Perish the thought.