Volcano Watch: Columnar jointing provides clues to cooling history of lava flows

Colonnades in a basaltic lava flow downstream of Boiling Pots on the Wailuku River.  Photo courtesy of James Anderson, UH-Hilo.

Colonnades in a basaltic lava flow downstream of Boiling Pots on the Wailuku River. Photo courtesy of James Anderson, UH-Hilo.

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

A swimmer takes a leap into the Boiling Pots area of the Wailuku River. File photo by Baron Sekiya | Hawaii 24/7

A swimmer takes a leap into the Boiling Pots area of the Wailuku River. File photo by Baron Sekiya | Hawaii 24/7

Above Hilo’s iconic Rainbow Falls on the Wailuku River is Boiling Pots. This area, named for the many “potholes” in the river bed, churns and roils after a hard rain in the mountains. There are many interesting geologic features exposed along this section of the Wailuku River, but perhaps the most impressive are the large basalt columns, most visible when the water level is low.

When thick basalt lava flows cool, they tend to form hexagonal cracks, called columnar joints. Among the world’s best-known examples of these “columnar basalts” are the Giant’s Causeway in Ireland and the Devil’s Postpile in California. No exposures of those scales exist in Hawai`i, but the columnar jointing in the Mauna Loa flow exposed at Boiling Pots, though small, provides a great opportunity to observe the result of lava’s cooling process.

Upon emplacement, basaltic lava flows immediately begin to cool from the top, bottom, and sides toward the center, where the most heat remains. While the bottom of the flow cools slowly because it is insulated by the ground below, the top cools more rapidly because it is exposed to the atmosphere’s cooler air, wind, and rain, as well as standing or running water.

The difference in cooling rate often forms two distinctive types of joint patterns. The slower cooling bottom section typically forms thick, regular-sized columns called “colonnades” by geologists. At Boiling Pots, the colonnade section is approximately 13 m (39 ft) high.

The faster cooling top section often forms thin and less regular columns. The resulting “hackly” column pattern is called “entablature.” It may be difficult to recognize entablature sections as columnar basalt because its highly fractured appearance lacks a dominant orientation of the columns.

Why do columnar joints form? As a lava flow cools from the outside edges toward its center, molten lava slowly solidifies into rock in the same direction—the solid-liquid interfaces move toward each other from all sides. The downward-growing upper rock and the upward-growing lower rock begin to fracture or crack because rock contracts as it cools. As more rock forms toward the center of the flow, an early crack will grow toward the center of the flow’s molten core.

The cracks thereby grow perpendicular to the cooling edges (top, bottom, and sides).

Several studies of the cooling Kilauea Iki lava lake clearly showed that the cooling cracks grow incrementally as discrete cracking events that were both heard by scientists and recorded by seismometers. A cracking event is marked by the discrete lengthening of a crack that occurs after sufficient thermal stress accumulates in the cooling rock. When the stress is released, the crack stops growing.

On a smaller scale, this process can be detected on a newly emplaced, still-warm lava flow just by listening to it. Discrete cracking and popping sounds are the audible signature of the formation of cracks and the flaking of the flow surface.

In some basaltic lava flow exposures, the joints form near-perfect hexagonal columns. This hexagonal symmetry is best seen in a top-down view, looking at a cross section of the column geometry. Mathematical descriptions of this cooling process predict that cooling flows should form perfect hexagonal columns with a series of 60- and 120-degree angles.

These angles are the most effective at relieving the complex thermal stresses acting on a cooling body of rock. As it turns out, however, few things in nature do exactly what they should, and the result is that we see columns of different shapes—not perfect hexagons. They look the way they do because the cracks occurred where they needed to be in order to relieve the internal thermal stresses. This is the case at Boiling Pots. Even though many of the columns are not hexagonal, they have an overall hexagonal symmetry.

Visit the HVO Web site (http://hvo.wr.usgs.gov) for detailed Kilauea and Mauna Loa activity updates, recent volcano photos, recent earthquakes, and more; call (808) 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov

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