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November 25, 1998

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


A Glimpse Underneath Mauna Loa

Hawai`i is one of the most extensively studied volcanic regions in the world. However, there is still very little known about the processes that take place deep underneath a volcano in what geologists casually call the "plumbing system." Geologists must rely on data gathered from rocks at the surface of the earth, combined with geophysical information. From these data they construct a 3-D model of the many magma chambers and conduits that extend several kilometers (miles) beneath the surface of the earth. As one might imagine, it takes a bit of creativity to interpret seemingly unimportant clues gathered from the surface of the volcano in order to understand more about the complex processes that occur underneath and within the volcano.

An important clue that geologists use is xenoliths. These are pieces of rock that are enclosed in a lava flow but do not "belong" to the lava. Flows from volcanoes which are in their final stages of activity, such as Hualalai, commonly contain xenoliths. However, they are extremely rare on volcanoes such as Mauna Loa or Kilauea, which are in their middle stages of growth. During the 1980s, many volcanologists believed that xenoliths shouldn't exist at all in lavas from Mauna Loa or Kilauea. Recently, however, xenolith-bearing flows have been discovered on Mauna Loa. This exciting new finding is helping geologists understand the plumbing system of this giant volcano.

Xenoliths provide answers to the question that geologists so often ask: "Where do the rocks come from and how did they get from there to here?" The answers are often complex and unclear. Xenoliths can be picked up by the magma anywhere along its journey from the depths of the earth to the surface. Thus they provide a sort of cross section through the different layers of rock which extend for tens of kilometers (many miles) beneath the volcano. Geologists look at the xenoliths in order to interpret the path through the earth that the magma took as it rose, as well as the different conditions that it may have encountered along the way. It seems a daunting task to try to pinpoint the exact source of the xenoliths, but clues, such as chemical composition and texture of the minerals in the xenolith, help geologists determine the most plausible scenario.

The recently discovered Mauna Loa xenoliths show a range of chemical compositions and textures that reveal the complex history of the magma. Some of the xenoliths are composed of minerals which are rich in magnesium and iron, whereas others are composed of minerals which are rich in calcium. This indicates that the magma must have encountered at least two distinct chemical environments on its journey to the surface. The xenoliths also show a variety of textures that illustrate different physical environments. Some are composed of crystals that are packed closely together. Others are made of crystals which are beginning to melt and come apart from each other. Each xenolith studied tells a slightly different story.

It is the job of the geologist to consider the history of each xenolith and construct a model of the plumbing system consistent with all the available data. A good model will help other volcanologists study different aspects of the volcano. For example, if a model can predict the path that magma from Mauna Loa takes as it rises to the surface, then that model can help volcanologists anticipate eruption patterns and focus their monitoring efforts. A well-developed model can also be used as an analogy for similar studies on other active volcanoes such as Kilauea. Ultimately, in order for geologists to gain a clear understanding of magmatic and eruptive cycles, they must understand the processes that occur underneath the volcano, as well as those on its surface.

Eruption and Earthquake Update

There is no change in the eruptive activity of Kilauea Volcano. Lava continues to erupt from Pu`u `O`o and flow through a network of tubes from the vent to the seacoast. No surface flows from breakouts of the tube system were observed in the coastal flats. Lava is entering the ocean near Kamokuna and is forming a small promontory. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

A magnitude-4.3 earthquake was reported felt by residents of Maui and the Big Island at 5:54 a.m. on Sunday, November 22. The earthquake was located 20 km (12 mi) northwest of Upolu Point at a depth of 27 km (16.2 mi).

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Updated: 08 Dec 1998