The Fascinating World of Minerals

Volcanologist

Minerals and volcanoes

My name is Julie Roberge, I am from Quebec, but presently I am finishing my PhD in volcanology at University of Oregon, USA. One spring day in 1980, my father made a big deal about washing the car. Why? He said, “A volcano is erupting in the United States, and tomorrow morning I will show you the volcano’s ashes. This is why the car has to be sparkling clean.” We were in Quebec, Canada, and the volcano (Mount St. Helens) was erupting on the west coast, so I was amazed the next morning when I saw the thin layer of ash on my father’s car.  I was 8 years old at the time, and I became so intrigued and curious about volcanoes that it turned into a passion. My parents never stopped feeding my interest with books and magazines about volcanoes.  So, I became a geologist and then specialized in volcanology.

I work on the Long Valley caldera volcano in California, which is a rhyolite caldera complex formed by a catastrophic eruption about 730,000 years ago, producing 600 cubic km of rhyolitic magma to the surface.  A caldera is a large volcanic depression that is more or less circular in form.  You may think that this doesn’t sound like a volcano, but it turns out that caldera are produced by the most explosive of Earth's volcanoes.  In fact they are usually so explosive when they erupt, they end up collapsing in on themselves rather than building a tall structure.  These caldera-forming events indicate that the magma chambers associated with the eruptions are huge.  In fact, layers of ash often extend over thousands of square kilometers in all directions from these calderas. Fortunately we haven't had to live through one of these mega eruptions although the eruption of Mt Pinatubo in 1991 did produce a small caldera.  Beside ash falls and ash flows these eruptions are also accompanied by high-speed flows of turbulent mixture of hot gases and unsorted pyroclastic material (volcanic fragments, crystals, ash, pumice, and glass shards) called pyroclastic flows.

 

For my thesis, I collected pumice samples from the Bishop Tuff eruption in the Long Valley caldera.  Pumice is a light-colored, frothy volcanic rock, formed by the expansion of gas in erupting lava.  It can be found as pieces or larger fragments, but it also can occur abundantly as ash-sized particles.  If you think you have a peace of pumice at home put it in water: if it floats you possibly have a nice pumice sample!  After collecting my pumice samples I crushed them and hand picked quartz and sanidine crystals.

Now, what do minerals have to do with volcanoes?

Quartz and sanidine crystals in pumice often contain melt inclusions, which are small (~0.001-0.3 mm) blebs of silicate melt trapped within the crystal.  Melt inclusions can be used to assess different properties of the melt before it erupts. Because they can form at high pressures and are contained within relatively incompressible crystal hosts, they may retain high concentrations of volatile (gas like H₂O and CO₂) that normally escape from magmas during degassing. As such, analysis of these inclusions provides direct information on the volatile contents of magmatic systems before the eruption. 

I use Fourier transform infrared (FTIR) spectroscopy to measure the H₂O and CO₂ contents of the melt inclusions.  To prepare the samples for FTIR, the quartz crystals containing the melt inclusions must be doubly ground and polished into wafers with two parallel sides.  The melt inclusions to be analyzed must be intersected on both sides of the wafer. 

Finally, since the gas content in the magma controls the explosivity of an eruption, crystals are useful to understanding the formation, evolution and configuration of large crustal magma chambers.

Build your own volcano!

Follow the link to learn how to build you own volcano out of paper and cardboard and than simulate and eruption using vinegar and baking soda.

Have fun!