Scientists Propose New Model For Growth Of Hawaiian Volcanoes

By Cat Adams

At the summit, snow blankets the ground. The air is thin and cold at 13,000 feet above sea level, and the quiet solitude is deceiving. The largest volcano on Earth, the Big Island of Hawaii's Mauna Loa, stretches far underwater, towering a total of 17 kilometers above its base, its sheer mass helping the island to further depress the ocean floor several kilometers.

Beneath the warm tropical waters of the Pacific, the planet bleeds molten lava. Rock grinds against rock, causing abrupt shifts that manifest as frequent earthquakes. Instruments record information about these earthquakes from research stations scattered across the island. In January, a team of scientists compiled data gathered from hundreds of earthquakes and created a mechanical computer model that suggests events in the Earth's crust might be different than were previously imagined.

According to the study, simply the weight of some volcanoes such as Mauna Loa on the island of Hawaii may be enough to bend, then break the Earth's crust. This holds important implications for the surface morphology of the islands, especially their slopes. The research was a result of a collaboration of scientists from the Laboratoir de Geophysique Interne et Tectonophysique, Le Bourget-du-Lac, and the US Geological Survey Hawaiian Volcano Observatory, and was published in Nature last January.

Though most of us only ever have fantasies about living on one of the Hawaiian islands, some 1.3 million people can claim to be state residents. Around 200,000 of these people live on the Big Island, right on top of not one, not two, but five volcanoes. Mauna Loa has four other volcanoes adjacent to it: the more active Kilauea, Mauna Kea, Huala-lai, and Kohala. When it comes to further understanding the structure and stability of these volcanoes, the local residents have a lot to gain.

That's not to mention the many scientists who have devoted their lives to describing what, exactly, is happening below the surface of the islands. "We don't have an exact model of how the Earth works,” admits William Scott of the United States Geological Survey. The new model is helpful for sparking ideas, but only to a degree. "These are all approximations. There are a lot of things we don't know.”

For starters, scientists would like to know what types of rocks and materials make up the different parts of the volcanoes. This is where information from earthquakes is crucial. Anyone who has experienced an earthquake knows that they produce rolling waves, and these waves are recorded by seismometers in research stations across the island. Differences in the time it takes waves to reach the individual stations allows scientists to pinpoint the area the earthquakes generated from, or the epicenter.

In addition, the differences allude to the stiffness and density of the material that the wave had to pass through. The two types of seismic waves, P (for the faster-traveling Primary waves), and S (for the slower-traveling Secondary waves, ironically enough), are significantly slowed by rocks that are more broken up. Fractured rock can contain water, which has a slower wave speed.

The scientists have created a three-dimensional model of the island's composition using seismic P waves (the Primary ones) to create a map of the P wave speeds underground. This imaging process is called tomography.

So what does the model reveal? Most striking is that the volcanoes Mauna Loa and Kilauea each appear to be made of a definitive core surrounded by a top layer and outer flanks. Scientists define the core as "plastic,” meaning that it can flow. The top layer and outer flanks are dubbed "elastic,” and are brittle. Elastic rocks break rather than flow and the flanks may form large landslides. For the Hawaiians living on Mauna Loa and Kilauea, this translates as follows: the core is plastic, and may be dense enough to easily deform and shove aside the flanks of the volcano, which are brittle and elastic and may slide along the ocean floor. Depending on which part of the island the Hawaiians live on, they may experience earthquakes, large landslides, and other consequences of flank instability.

Cat Adams is an undergraduate majoring in biology with an emphasis on ecology and evolution.

Image:

A map of the five volcanoes that compose the big island of Hawaii.