Lonely 1790 ballistic being ignored by tourists visiting the Kilauea Overlook at Hawaii Volcanoes National Park. (Photo courtesy of Hawaii Volcanoes Observatory)

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Many visitors to Hawaii Volcanoes National Park stop at Kilauea Overlook and walk to the rim of the caldera, passing a huge rock along the way. Probably few think much about this rock, sitting on the surface, looking lonely, with no sign to attract attention.

Nonetheless, it is a notable example — the most accessible to the public at present — of a block that was thrown out of the caldera during a powerful eruption, probably in 1790.

Such large blocks are so heavy that their trajectories in the air are not influenced by wind. As such, they are like cannonballs or other heavy projectiles—or like a fly ball hit on a windless day. All such missiles follow a geometric pathway, closely approximating a parabola, that is controlled by the ballistic laws of physics.

The brain of an outfielder detects and calculates the parabolic path of the fly ball, enabling it to be caught. A line drive to the outfield is harder to judge than a towering fly, because its arc is so low that the parabola appears poorly defined to the brain.

Many ballistic blocks were ejected from the caldera in 1790. Some of these blocks may have been natural equivalents of towering fly balls, others, of line drives.

Some of the largest may have been ejected at the optimum angle of 45 degrees, which (other things being equal) leads to the maximum distance a block of a given size can be thrown.

Another sports analogy is shot putting. Every shot putter knows that the faster the heavy shot leaves the hand at a given angle, the farther it will go. At volcanoes, that speed is known as the ejection velocity; in baseball, bat speed is the equivalent.

Combining a 45-degree ejection angle with the highest possible ejection velocity leads to record shot puts and home runs and to heavy rocks thrown maximum distances.

In the past few months, most of the large ballistic blocks west of the caldera have been found and measured. Added to similar work done in 2006 in the southeastern caldera, many of the 1790 blocks not covered by younger lava flows have now been found and described.

In January, a student volunteer, Scott Zolkos from Middlebury College, is arriving to help put the finishing touches on the ballistic inventory.

The size of a block is tabulated by measuring its three perpendicular diameters (length, width, height), summing them, and dividing by three to get a nominal diameter. For example, a block 100 cm by 80 cm by 30 cm has a nominal diameter of 70 cm. This calculation can be refined to account for the detailed shape of each block.

Generally a block is partly buried, so its height and nominal diameter are minimum estimates.

To date, 11 blocks have been measured with nominal diameters of 150 cm or more, the largest 195 cm (6.4 feet), and 127 blocks have nominal diameters of 100 cm (3.3 feet) or more. The block at Kilauea Overlook has a nominal diameter of 144 cm.

Some of these blocks occur between HVO and KMC, whereas others are lower, on the surface of fault blocks south of HVO or west of Keanakakoi. Large blocks occur more than 1 km from the caldera rim. Probably the caldera was about 2,000 feet deep when the ballistics were ejected. Imagine rocks 1-2 m in diameter and weighing several tons thrown that high and far!

Many of the 1790 blocks are dense and hard, and some were used to make stone tools. Such blocks occur in the Keanakakoi area, though they are now covered by fallout from the 1959 Kilauea Iki eruption. William R. Castle wrote the following in the Volcano House register on May 23, 1891:

“To the E. of Keanakakoi is a lovely picnic ground…. Around this crater are found the very hard and tough rocks from which the ancient stone axes of the Hawaiians were made.”

Keanakakoi means “adze cave” and apparently reflects the 1790 ballistic blocks that fell around and into the crater.

Kiauea may have gone ballistic in 1790, but the blocks were soon put to use by toolmakers and now intrigue volcanologists attempting to learn more about that fateful eruption.