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 hot, low-silica basalt
magma penetrated a stable Reservoir 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
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 analysed. 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:
X-rays (formed by bombarding a sample with electrons.)
(2) the secondary electrons that are used to see the sample
(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
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
analyser 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 colour coded by a
scanning electron microscope to show the range of calcium content from
high (white) to low (green).
Analysing a single particle of smoke
Because of their similarities, EMPs and SEMs overlap in their
capabilities. The modern EMP has become a true hybrid that combines the
viewing capability of the SEM with the analytical power of the electron
microprobe. Both EMPs and SEMs are capable of obtaining images at
magnifications over 100,000 times. These instruments can see and then
analyse something that wouldn't show up with a light microscope, such as
the following single particle of volcano smoke in this picture.
After seeing the invisible, the next question is "wonder what that's made
of?" "Is it bad for our health?" Small samples like this particle of
volcanic smoke, the size of a single human red blood cell, can be analysed
by a scanning electron microscope in 4 minutes with errors of less than 1