Geologic processes

Volcanoes 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.
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.
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.
Low silica magma, typical of Hawaiiain volcanoes, produces lava flows that move slowly and can rarely overtake a human who wants to escape.
Because buildings and structures can not easily be moved out of harms way, even slow moving lava flows can cause significant property damage.
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.
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.
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.
To analyze samples by X-ray fluorescence spectrometry, samples are fused at 1120xC with a flux; the chemist then pours the molten mixture into special molds to produce solid glass discs with a precise analytical surface.
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. Return to this point in index.
Application of instrumental neutron activation analysis
Some of our understanding of the source of molten magma has been obtained by analyzing rocks for a group of 15 elements called the rare-earth elements (REE). In a type of rock called basalt, the total amount of all the REE s is often less than 100 parts per million (ppm).
One well proven analytical technique used to determine the concentrations of REE in rocks and minerals is instrumental neutron activation analysis (INAA). In this technique, a rock or a single mineral that the rock contains is irradiated using a nuclear reactor. This causes the elements to become radioactive and to emit gamma rays with distinct energies. The sample is then placed on a detector that measures how many gamma-rays of these energies are emitted. The number of distinct gamma rays emitted is proportional to the abundance of that particular element.
To get better sensitivity necessary to measure rare-earth elements in specific rocks, samples can be irradiated in a low-power reactor. It turns some of the element into an unstable isotope whose decay can then be detected and counted to determine the quantity of the element in the sample.
To understand what the REE can tell us about how magmas are formed, scientists have developed mathematical formulas. These formulas suggest that when certain minerals interact with molten rock, there can be appreciable effects on the rock s REE contents. In a process called partial melting, for example, if a source rock contains minerals (such as garnet) that can hold high concentrations of certain REE, then these elements tend to be prevented from entering the molten rock. Because Hawaiian basalts have low concentrations of the heavier REE, and garnet has high concentrations of heavy REE, some Earth scientists conclude that the magmas have formed by partial melting of a source rock that contains garnet, and the garnet held back the heavy REE. Return to this point in index.

The smallest clues
To understand more about the causes of eruptions, geologists have to look more closely into the fine details of the solidified magma samples to find a record of the conditions before and during eruption. Mineral crystals within magmas vary in composition depending on the surrounding magma and the temperature at which they are formed.
Why do some volcanoes explode catatrophically with rapid, life-threatening devastation? Recent research indicates that magma does not necessarily move directly from its source to an eruption. A magma chamber may contain stable reservoirs or layers of one composition with a lower temperature. Subsequent influx and mixing of a second higher temperature lava overheats the mixture, triggering an explosion. The 1991 Pinatubo eruption appears to have been triggered because a hot, low-silica basalt magma penetrated a stable resevoir of cooler, high-silica type, forming an explosive mixture. The explosion forced the closing of Clark Naval Air Station and interrupted numerous air flights because of ash clouds that damaged engines.
Mineral compositions from the 1991 eruption of Mt. Pinatubo indicate that low-silica magma at a temperature of about 1,250 C mixed with high-silica magma (780 C) just before the eruption. Based on this information, volcanic rocks produced in previous eruptions were analyzed. The results suggest that the 1991 eruption is the latest in a series of eruptions that were triggered by the mixing of magmas. Magma mixing has also triggered eruptions at a number of other volcanoes.
Shortly after World War II, physicists in the United States, England, Germany, and Japan began to perfect a new analytical instrument called the electron microscope. Instead of producing a visually magnified image, this new instrument accelerated and focused electrons through a column of magnetic lenses onto a small spot on the sample. The ability to magnify objects is limited by the energy or wavelength of the radiation that is used to observe the object. Because the accelerated electrons from the column have a much shorter wavelength than light, it is possible to produce images at much higher magnifications than can be obtained using an optical microscope. Today, the most powerful electron microscopes can produce images at magnifications as high as 1 million times.
When electrons are accelerated into an object, they interact with the atoms in that object and produce three important types of radiation: (1) X-rays (you may scroll back to the picture of the Early 1900\'s Laboratory where a description of how X-rays are formed was presented for the related technique of X-ray Fluorescence), (2) the secondary electrons that are used to see the sample, and (3) back-scattered electrons, which are bounced back as a function of the mass of the sample.
In the 1950 s, the French physicists, Castaing and Guinier, developed an instrument based on the characteristic X-rays produced by the electron bombardment of the sample. This instrument can measure the number of X-rays emitted from the small spot irradiated on the sample. By counting the X-rays produced, Castaing determined the chemical composition of a portion of a sample no larger than the size of a human blood cell. This new instrument was called the electron microprobe (EMP).
During the same period of time, another instrument was brought into production the Scanning Electron Microscope (SEM). Like the electron microscope, it uses the secondary electrons created from the sample s surface to record an enlarged image of the object. Its principal advantage is that it deflects the electron beam and scans it back and forth over the sample surface (called rastering) in a pattern similar to that in which wallpaper covers a wall.
In order to see objects smaller than what normal light allows, scientists have developed an instrument that accelerates electrons. The Scanning Electron Microscope uses electromagnetic lenses to focus the electrons, since glass lenses cannot.
The secondary electrons are continuously detected, and the signal is directed to a television monitor where the image is displayed. Zooming in or backing out by changing the size of the raster area (hence changing the magnification), the scientist can use the enlarged image to aim the scanning electron microscope. At the same time, X-rays characteristic of the composition are generated. These X- rays can be detected by an X-ray analyzer and used to create a map of the element\'s abundance.
In this example, calcium X-rays produced from a pinhead-size sample from the 1991 eruption of Mt. Pinatubo are mapped and color coded by a scanning electron microscope to show the range of calcium content from high (white) to low (green).