(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)
What do 10 plastic buckets, a 1.5-inch paint brush, a small funnel, two restaurant heat lamps, and a simple weighing scale have in common?
These common gadgets are all scientific tools used to measure how much volcanic ash is coming out of Halemaumau every day.
Measuring the amount of sulfur dioxide emitted by Kilauea has become a fairly straightforward procedure, and past Volcano Watches describe how this is done.
But the ongoing Halemaumau eruption presents a new challenge. How do we measure the amount of volcanic ash (or its coarser cousin, volcanic lapilli) given off during the eruption?
Since an eruption is defined as the addition of solid or liquid material to the ground surface, knowing the amount of volcanic ash erupted daily is a fundamental part of tracking and understanding the Halemaumau activity.
It’s easier said than done, though. Sophisticated laboratory equipment can measure the solid content of small jets or streams, but modifying it for field use to estimate the solid contents of a volcanic plume has not been done, at least not successfully.
We have taken a simple and inexpensive approach for estimating the ash output. We catch some of the ash that falls onto the ground and, from it, calculate the total weight of ash erupted. Here’s how.
In early April 2008, we installed an array of 10 plastic buckets from a site near the visitor overlook to the Halemaumau parking lot. This configuration catches ash blown by trade winds from the plume, though it misses most of the ash carried by Kona winds.
Every weekday, someone empties each bucket of all the ash that fell into it since the previous collection. The ash is brushed out of each bucket through a funnel into a sampling bag.
Back at HVO, the ash from each bag is dried under heat lamps and weighed to a hundredth of a gram on a simple scale. Those steps use the five scientific tools mentioned at the start: bucket, brush, funnel, heat lamp, and scale.
With this information, the daily accumulation rate of ash that fell into the 10 buckets can be calculated, measured in grams per square meter per hour. This rate provides us a good means to track ash output on a daily basis.
But it really gives us only part of what we would like to know—the total weight of ash erupted from the vent, not just the ash in the buckets.
Now comes the only sophisticated step in the entire operation. There is a way, developed more than 20 years ago, to estimate the total weight of ash erupted from a volcano simply by measuring the weight that accumulated at certain places downwind from the vent.
We used this procedure for the 8 relatively large explosive eruptions of last year. But the procedure is time-consuming, too much to do on a daily basis, so we compromised.
We applied the sophisticated approach to several daily collections, estimating the total weight of ash erupted from the amount caught by the 10 buckets. We then compared those large weights with the total amount that the buckets had actually caught.
We discovered that, in each case, we could multiply the total bucket catch by the same number (“fudge factor”) to get the total weight estimated by the sophisticated approach.
This is not precision science, but it does allow us to estimate the weight erupted daily simply by multiplying the daily catch by the “fudge factor.” We’ve found that, on a daily basis, a few tens or hundreds of kilograms of volcanic ash are erupted, with values of several thousand kilograms fairly often.
Here are some round numbers for the entire eruption:
* Total weight ejected during the eruption: 2,200 tons
* Total weight ejected during the 8 big explosive eruptions: 1,800 tons
* Total weight ejected between big explosive eruptions: 400 tons
The weight of volcanic ash erupted daily between the eight big explosive eruptions is about 18 percent of the total for the entire eruption.
This is the first time we know that daily ash production has been measured at any volcano during a prolonged eruption. Simple tools, flavored with a pinch of sophistication, can produce valuable results.