Imaging Magma Under St. Helens

Magma is basically lava that is still underground; ie molten or partially molten rock.  It is really hot; over 900°C and is probably liquid or mushy, not solid like rock.  A fundamental question in volcanology and volcano hazard studies is how magma gets from its source region, thought to be 100 km or so deep, to the surface. If we can figure out the path it takes and how much is where within the earth this will help immensely with understanding the process.  So it must be really easy to detect?  Unfortunately, not so.  Unless it is moving, it has relatively subtle differences from surrounding rock when looking for it with geophysical techniques.  

It is well established that volcanoes give off all sorts of indications in the days and weeks before they begin to erupt.  Increased earthquake activity, swelling of the ground surface, changes in gas and water chemistry and possible thermal anomalies are typical signs that a volcano is becoming restless and may erupt.  All of these signals typically originate from sources that are within a few kilometers of the surface as magma starts to move.  To reach the surface the magma must break through solid rock.  Doing so generates earthquakes we can detect with seismic techniques.  Gas being released by the magma can find its way through the newly broken rock into ground water or into the air where it can be detected with chemical techniques.  Geodetic techniques such as GPS monitoring can detect the slight swelling of the ground surface as the magma rises.

But,  most of the time, when volcanoes are quiescent, magma is not moving or is moving so slowly that on a human time scale it is effectively not moving.  This is the case currently at Mount St. Helens (and all of the other Cascade volcanoes as well).   We know there must be magma below Mount St. Helens, probably in some sort of reservoir 5 or more kilometers deep.   But how much?  How big is the reservoir and how deep does it extend?  Is there a continuous "pipe" extending almost 100 km down to the source zone?  Does the magma pool at different depths where it may evolve by incorporating some nearby rock by melting it? Does magma exist in areas away from volcanoes, maybe at the sites of future new volcanoes?

 If we could produce an image outlining regions of magma in a significant part of the earth's crust in the region around a volcano we would be well under way understanding the whole system.  The figure on the home page of iMUSH, (shown here), shows estimates of where contrasts in seismic velocities determined by a previous, low-resolution imaging experiment exist.  Magma has seismic velocities lower than solid rock and will not propagate certain types of seismic waves (shear-waves) so there should be seismic anomalies easily detectable.  This figure is the type of image we hope the iMUSH experiment will produce, but at much higher resolution and with more quantitative detail.

Magma also has very different electrical properties than rock and so electrical geophysical techniques may also be used to map its extent.  Images generated by a previous electrical survey (shown here) of the area show what appears to be a large electrical anomaly descending up to 40 km below Mount St. Helens and extending to the east.  If this anomaly represents magma it is very interesting indeed.  However, other types of rocks can have electrical properties somewhat similar to magma.  Thus it is critical to use different geophysical techniques to cover the same region.

And finally, what about the rocks themselves?  Of course we can't dig down deep in the earth to see what is actually there (drilling can only reach depths of 5+ km and at those depths is incredibly expensive and only samples that one place).  But, we can let the rocks come to us.  They do so in eruptions and Mount St. Helens has brought up all sorts of rocks during the eruptions that have taken place during the past 32 years.  Even rocks from prehistoric eruptions can provide useful clues as to where they came from, how long they lived at what depths and how fast they ascended.  Part of the iMUSH project is for geologist to do special studies on some of these rocks to tease out their history.

As the project unfolds we plan to include observations, analysis results, speculations and general arm waving ideas in these pages.  Tune in from time to get an idea of how this science is done.