April 17, 2003
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
World's Coolest Lava is in Africa
People like to hear about record-holders-the biggest, fastest, best. That's why we're frequently asked, "What's the world's most active volcano? Is it Kilauea?" To which we judiciously reply, "Kilauea is one of the world's most active volcanoes." We can't give a more definitive answer because there are several contenders for the most-active title. Sometimes we wish someone would ask an easy question, like, "What's the world's weirdest volcano?"
Most volcanologists would agree that the answer to that question is Ol Doinyo Lengai, a volcano in northern Tanzania. From a distance, this handsome volcano doesn't look unusual. The symmetrical cone rises 2,200 m (7,200 ft) above the baking plains of the East African Rift Valley. The Rift Valley, which extends 3,700 km (2,300 mi) from Mozambique through Ethiopia, is home to most of Africa's active volcanoes, but none other shares the unique properties of Ol Doinyo Lengai.
During the last century, activity at Ol Doinyo Lengai has been concentrated in its summit crater, which is currently about 400 m (1,300 ft) across. Because of its remote location, few scientists made the arduous trek to the crater until the last few decades. Those who reach the crater are usually rewarded with the sight of spatter cones, small lava flows, and sometimes lava lakes and low lava fountains. In June 1993, explosive eruptions inside the crater produced ash clouds visible from the surrounding plains and deposited ash on the upper slopes of the cone. Lava flows have gradually filled the crater, and, in the last two years, thin flows have regularly overflowed the crater rim and descended a short way down the cone.
The record that Ol Doinyo Lengai holds is that it is the only volcano in the world known to have erupted carbonatite lava in historical time. Because of its very unusual composition, carbonatite is literally the coolest lava on earth, erupting at 500-600 degrees Centigrade (930-1,100 degrees F), compared with 1,160 degrees C (2,120 degrees F) for lava from Kilauea's current eruption. An active carbonatite flow is black or brown and reminds many eyewitnesses of runny mud. Only at night do carbonatite flows glow a dull orange or red.
Freshly cooled flows in the crater of Ol Doinyo Lengai are black but soon turn white because of chemical reactions that occur as the lava absorbs water. In rainy weather, this color change can occur before the flows are cold. Within a few months of erupting, lava flows turn into a brown powder due to water absorption.
What makes this lava so different from the stuff we're used to? The chemical composition of carbonatite magma includes very little silica (silicon dioxide), the most abundant chemical constituent of the earth's crust. The Volcano Watch column of March 23 explained that the difference in the amount of silica in the magma of Kilauea versus that of Mount St. Helens-52% vs. 64%--is the main reason that those two volcanoes behave so differently.
Carbonatite magma at Ol Doinyo Lengai has less than 3% silica and is more akin to something you'd expect to find on a different planet. Cooled carbonatite lava is composed mainly of carbonate minerals. Most of us are familiar with calcium carbonate, the main component of coral, many calcium supplements, and the lime you add to your garden. Carbonate minerals are common in sedimentary rocks, such as limestone, but are rare in igneous rocks, which solidify from magma.
Small volumes of carbonatite exist in many parts of the world, always in association with other igneous rocks. Many geologists, however, had a difficult time believing that carbonatites once were magma. It wasn't until 1960, when a geologist reached the crater of Ol Doinyo Lengai and observed active carbonatite lava flows, that the magmatic origin of carbonatites was generally accepted.
The origin of carbonatite magma is still murky, but most experts agree that it is derived from a parent magma with a far less exotic composition. The question remains open as to how and why the elements that make up carbonatite become segregated to form the world's coolest lava.
Hot eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. The visible flows reported for the past two weeks on Pulama pali have crusted over but are still on the go. One flow along the eastern edge of the Mother's Day flow made it to the seacoast and entered the ocean near Lae`apuki on April 17. Scattered surface breakouts are found throughout the inflating Kohola flow, and the National Park Service has marked trails out to the closest activity. Lava continues to enter the ocean at the West Highcastle delta.
The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The steam clouds are highly acidic and laced with glass particles. The National Park Service has erected a rope barricade to delineate the edge of the restricted area. Do not venture beyond this rope boundary and onto the lava deltas and benches.
One earthquake was reported felt during the past week. A resident of Kapulena felt an earthquake at 4:46 a.m. on April 10. The magnitude-2.5 event was located 7 km (4.2 mi) southeast of Waimea at a depth of 18 km (10.8 mi).
Updated: April 21, 2003 (pnf)