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Understanding Our Planet Through Chemistry

IIb. Geologic Processes


Volcanoes erupt when molten rock (magma) deep in the Earth's interior makes its way to the surface. On average, for every cubic kilometer of magma erupted from a volcano, 3 to 10 cubic kilometers are stored beneath the surface in shallow reservoirs called magma chambers.

We can see what these magma chambers look like by studying ancient reservoirs that have solidified and been exposed by erosion. One of these is Half Dome in Yosemite National Park.

Photo of Half Dome, Yosemite National Park.Half Dome, in Yosemite Park, is the remains of a magma chamber that cooled slowly and crystallized beneath the Earth's surface. The solidified magma chamber was then exposed and cut in half by erosion. Similar, still molten magma chambers are thought to underlie many active volcanoes. [118k]

The degree of violence of an eruption depends principally on the chemical composition of the magma. Of major importance is the interplay between the proportion of silicon dioxide (SiO2 or silica ), which controls the viscosity of the magma, and volatile components, such as water, carbon dioxide, and sulfur dioxide. Magmas that are poor in silica usually release their gases non- explosively and produce slow-moving lava flows, like those commonly seen in Hawaii. Although such eruptions can be destructive, humans can usually avoid the lava flow and are rarely threatened by such volcanic activity.

Photo of two girls sitting in park watching neaby lava flow.Low silica magma, typical of Hawaiiain volcanoes, produces lava flows that move slowly and can rarely overtake a human who wants to escape. [187k]

Photo of lava-caused fire burning building. Because buildings and structures can not easily be moved out of harms way, even slow moving lava flows can cause significant property damage. [160k]

Under certain conditions, however, the magma and surrounding rocks are blown apart by the release of volatiles, resulting in a dangerous explosive eruption, as happened on May 18, 1980 at Mount St. Helens, near Portland, Oregon. With only about 0.5 cubic km of erupted magma, however, this was by no means considered a large volcanic eruption. The 1991 eruption of the Pinatubo Volcano, near Manila in the Philippines, was approximately 14 times larger, involving about 7 cubic km of magma. But even the Pinatubo eruption is relatively small compared to infrequent giant eruptions of volatile- and silica-rich magma that have occurred throughout the history of the Earth.

Photo of early 1900's scientists performing analyses. In the early 1900's a chemist could analyze about 200 samples per year for the major rock-forming elements. Today, using X-ray fluorescence spectrometry, two chemists can perform the same type of analyses on 7,000 samples per year. [271k]

Major-element chemical analysis is a front-line tool in the study of volcanoes and volcanic hazards. The analysis of a volcanic rock provides a fundamental common ground for comparing the styles and violence of previous eruptions of similar composition. During the first half of the 20th century, these analyses were performed exclusively by classical wet chemical analyses chemically separating each element of interest from the other elements in the sample. This procedure was extremely laborious. A good analytical chemist could analyze only a couple of hundred rocks per year for their complete major element chemistry. U.S. Geological Survey scientists now use technology called X-ray Fluorescence Spectrometry (XRF) to perform the same type of analyses.

XRF Spectrometry starts at the atomic level. Atoms consist of protons and neutrons in a central nucleus with electrons in different orbitals around that nucleus. If an electron from an inner orbital is knocked out, the vacancy created is filled by an electron previously residing in a higher orbit. The excess energy resulting from this transition is dissipated as an X-ray photon with a characteristic wavelength. In X-ray fluorescence analyses, the electron vacancies are created by bombarding the sample with a source of X-rays or gamma rays most frequently from an X-ray tube or a radioactive isotope. By detecting the characteristic X-rays that are fluoresced, the element of interest is shown to be present in the sample. The more abundant the X-rays are, the more of that element is present in the sample.

Bombarding the sample with X-radiation does not require a liquid sample. In fact, because solid samples are more stable than liquids, virtually all samples presented to X-ray spectrometers are solids. Furthermore, there is almost no permanent change that takes place in a solid sample analyzed by XRF, allowing it to be saved and reanalyzed. This is especially important for the repeated analysis of the same calibration standards over periods of years, permitting the use of the same analysis protocol. Homogeneity requirements are frequently solved by dissolving a portion of the pulverized sample in molten flux that is then poured into a mold and cooled to form a solid glass disc with a precise, flat, analytical surface.

Photo of USGS scientist working in lab.To analyze samples by X-ray fluorescence spectrometry, samples are fused at 1120°C with a flux; the chemist then pours the molten mixture into special molds to produce solid glass discs with a precise analytical surface. [77k]

A team of two analysts, using this method, can analyze over 7,000 samples a year. Because so many more analyses are now available, geologists can answer more difficult types of questions such as what changes are happening in the magma chamber during an eruptive cycle.

At a number of frequently active volcanoes, such as Mount St. Helens (which has erupted about every 100 years), a thick and complex sequence of volcanic rocks has been deposited. Geochemists and geologists can reconstruct the eruptive history of the volcano through field studies and analyses of these rocks. They conclude that the eruptive activity at Mount St. Helens is separated by longer periods of repose. Like many other volcanoes, there are systematic changes in major- and trace-element composition through time. The 1980 eruption appears to be at the end of a chemical cycle that began about 500 years ago.

With this information we can predict the style, frequency, and warning signs of future eruptions. Newly erupted lava, pumice, or ash may then be evaluated in a historical context. In some instances, XRF analyses can be rapidly completed in less than 24 hours by express delivery of the samples to the lab and electronic transmission of data back to the volcano being examined. This is something that would have been impossible for the classic chemist.

While systematic changes in overall chemistry contribute a great deal of information about a volcano, there is still a desire to understand more about what happens deep within the Earth's crust how the magma forms and what triggers the volcano into eruption.

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