Two weeks ago, I returned from a field trip in the Big Horn and Seminoe mountains of Wyoming. Two days from now, I’ll be in the air on my way to Cuzco, Peru. The city functions as a gateway to the ancient site of Machu Picchu and as a bunkroom for hikers on the Inca Trail. But for me and for Assistant Professor Maureen Long, the city is the starting point for an even more unusual exploration.
Thanks to Professor Long and Professor Bercovici, I am going as a field assistant on another Yale Geology and Geophysics field trip. This time, I’ll be spending three weeks installing seismometers—machines used to gather data on the seismic waves that move through the earth. The goal of the project is to use this data to understand a Peruvian subduction zone.
In a typical case, subduction means that one plate is sliding under another and eventually sinking back into the mantle. Volcanoes spring up at these sites, because the decrease in pressure when the plate sinks causes water that has been baked into the rock to come out, lowering the melting point of the rock. Melting also creates bubbles of gas that rise and pressurize volcanic eruptions. In Peru, however, the bottom plate only sinks for approximately 100 km before flattening out for 500 km. This flat slab presents a host of questions.
For example, how did a slab like this one form in the first place? What does this kind of plate movement do to the crust above it? And what keeps the slab from sinking?
Though geologists have looked at other sites in Chile and Argentina where the buoyancy of the sea floor may be causing a similar phenomenon, that explanation doesn’t work for Peru. And since the slab prevents volcanic activity that typically accompanies a subduction zone, where does the water go when the plate begins to melt?
For Professor Long, the answers to these questions lie in examining seismic waves as the fingerprints of mantle flow. She can trace these fingerprints because crystals in the mantle rock point in the same direction the mantle is flowing. Since waves move fastest in the direction the crystals are pointing, wave speed can provide an indication of mantle flow direction. In looking at patterns of seismic waves, Long can determine whether they are moving quickly or slowly through a certain part of the mantle—and from there she can gather information about the mantle flow around this particular slab.
Long can’t come to Peru herself since she’s expecting a baby in mid-December, but her collaborators on the project, Lara Wagner from the University of North Carolina and Susan Beck from the University of Arizona, will be leading the trip. This May, Long will get a chance to go back and extract the data collected by her seismometers.
In the meantime, I’ll be helping to dig holes and install these machines in the areas around Cuzco and Lima. The arrays will stretch almost to the Amazon, and many of them will be planted in the backyards of cooperating Peruvians.
After the nurse at the immunizations clinic put a live Yellow Fever vaccine into my arm, she asked me how I felt. I smiled and replied, “Like I’m really going to Peru!”