April 26, 1996

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

How Calderas Form

Visitors to Hawaii Volcanoes National Park are treated to spectacular views of Kilauea caldera and Halema'uma'u pit crater from the many summit overlooks. The caldera, the most imposing geologic feature on the summit, has a diameter of 2 by 3 miles and is about 400 feet deep. Others who make the long trek to the summit of Mauna Loa can peer into a caldera called Moku'aweoweo that is similar in size and depth to Kilauea caldera. Large craters such as these are often found at the summits of volcanoes. The smaller and more numerous pit craters pock the summits and rift zones. Calderas and pit craters form above a collapsed, or partially collapsed, magma chamber after a large eruption. The surface area and volume of the crater corresponds to the cross sectional area and volume loss of the collapsed magma chamber. The process that creates a caldera or pit crater can be demonstrated with a simple experiment.

In the experiment, we inflate and deflate a balloon buried in a box of flour. The flour represents the Earth and the balloon a magma chamber. As the balloon deflates, a crater will form in the flour. (Instructions are given in the illustration.)

Carefully examine the flour crater and the surrounding "ground." Like a real caldera, it has steep cliffs. The "ground" slopes away from the crater and is riddled with ring fractures. Try making different-sized calderas and calderas within calderas. What happens to your crater when you shake the box?

Real calderas are constantly changing. In the 1800s, Kilauea caldera was several hundred feet deeper than it is now. Summit eruptions have filled it to its current level. Since 1983, the center of the caldera has been slowly subsiding at a rate of about one-third of a foot per year because the ongoing eruption at Pu'u O'o is draining the summit magma chamber. The walls of the calderas are steep and unstable, and landslides can occur at any time. Numerous landslides often accompany strong earthquakes. If you try this experiment, I am sure that the next time you see a caldera, you will understand how it was created.

Volcano Update

The current eruption of Kilauea continues unabated, with ocean entries at Kamokuna, Kamoamoa and Lae'apuki. The new observation area opened by the National Park Service at the end of the Chain of Craters road allows visitors a better and closer view of the spectacular lava flow activity.

One earthquake was felt during the past week. A magnitude 2.6 temblor located 4 miles south of Honokaa was reported felt at 8:07 p.m. on April 20 by residents of Pa'auilo and Honoka'a. It originated from a depth of 20 miles. No damage was reported as a result of the earthquake.