Vulcanism at Mauna Loa

As a young man in the 1970s, I went on a short holiday to the Big Island of Hawaii from my home in Honolulu.

Camping along the Kona coast, I looked up to the southeast to see a red glow in the sky. No, it wasn't a housing tract, an airplane, or lightning. It was the start of a Mauna Loa summit eruption, with its concomitant lava fountains of red molten rock.

So began my first experience with the capricious ways of Madame Pele, the Hawaiian volcano goddess.

Today, eruptions surrounding Mauna Loa continue. Over time, activity has slowly shifted toward the southeast. This is because the Pacific crustal plate is drifting to the northwest over a hot "mantle plume" of rising partially melted mantle rocks. This molten material, or magma, is rising through thin oceanic crustal rocks due, in part, to its relatively low density.

The surface expression has been the buildup of layer upon layer of gently sloping basaltic lava flows, which have formed a shield volcano of immense proportions. Mauna Loa may be the largest volcano on earth when considering the 16,000 feet of rock below sea level and almost 14,000 feet above, and has been building for several hundred thousand years.

"A'a" flows of rough, blocky, fragmented lava result when the volatile content is low. The smooth, ropy, "cake icing" lava called "pahoehoe" is indicative of a more fluid, volatile-rich source magma.

Flank eruptions on the southeast rift zone have been the common feature of recent activity. Puu Oo, the source vent of the current eruption, is slightly east of the cone from which the prior sequence emanated. Sulfurous fumes accompany the venting of glowing hot lava near 1,000 degrees Centigrade at Puu Oo.

From there, lava travels down slope, largely through lava tubes (crusted-over surface flows), almost 10 miles to the sea, where it contacts the crashing waves, flashing the water to steam and generating huge clouds of water, acids and silica dust.

It's an impressive show. We geologists usually see processes acting slowly, relative to the human time scale, but here the activity is startlingly rapid as well as uniquely spectacular.

n Chip Carroon of Cool, Calif., has a doctorate in geology.


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