Eileen Herrstrom

University of Illinois at Urbana-Champaign

Undergraduate class on introductory physical geology, geography or quantitative reasoning for non-majors

Must have knowledge of plate tectonics (plate boundaries, subduction zones) and be able to use Excel (enter data and formula, fill down). For one option, students must know how to use Google Earth (placemarks, ruler).

This is a laboratory activity that follows lectures on volcanoes and on plate tectonics. It is part of the seventh laboratory exercise of the course, which comprises a sequence of activities on subduction zone volcanoes.

Identify the segmentation of the Andean continental volcanic arc in terms of the locations of volcanoes, the depth distribution of earthquakes, and the angle of subduction

Measure the arc-trench distance for seven arcs around the world and calculate the subduction angle using the arctangent function in Excel

Interpret and explain a graph of subduction angle for various arcs, practice using Excel and Google Earth (optional)

The Andean Cordillera extends along the entire western margin of South America; it is part of a longer cordillera stretching from North America through Central America and south to Antarctica. "Cordillera" is a Spanish word adopted by geologists to refer to a large mountain range. These mountains include the highest peak in the Americas: Mt. Aconcagua in Argentina, which rises almost 7000 m (23,000 ft) above sea level. The Andes also contain the mountain peak farthest from the center of the Earth: Chimborazo in Ecuador, located on the equatorial bulge of the planet. In fact, the highest volcanoes in the world are part of the Andean Cordillera as a result of the Nazca and Antarctic Plates subducting beneath western South America.

This activity may be assigned with or without using Google Earth. Student materials for both options include the instruction sheet and an Excel spreadsheet. For the Google Earth option, a .kmz file is available containing information about earthquakes in South America and volcanic arcs around the world. For the option without Google Earth, a PDF contains images of the various arcs (screenshots from Google Earth).

The activity first presents the concept of the uneven distribution of volcanoes in South America and compares it with the distribution of earthquake epicenters. Then, students interpret this information in terms of subduction angle variations along the arc. The rest of the activity involves using Google Earth or screenshots to view other volcanic arcs and measure arc-trench distances. These data are entered into Excel and used to calculate the subduction angles for various arcs. Finally, students interpret a graph of the data.

This activity was designed as part of a longer laboratory exercise that includes more detailed work with the 1980 eruption of Mt. St. Helens, a subduction zone volcano. These other activities may also be found on the SERC website. When all three activites are combined, the laboratory exercise consists of the following:

• Part I: Mt. St. Helens Ashfall Eruption <https://serc.carleton.edu/teachearth/activities/181097.html>
• Part II: Mt. St. Helens Topographic Profiles <https://serc.carleton.edu/NAGTWorkshops/geodesy/activities/204957.html>
• Part III: Volcanoes at Subduction Zones

In both the traditional face-to-face and online versions of the course, this activity is assessed based on the answers to the questions. It is also possible to have students submit their completed spreadsheets, although this option works best in a small class.

Mt. St. Helens, 2017: US Geological Survey: Online resource – Accessed 16 June 2019
https://volcanoes.usgs.gov/volcanoes/st_helens/

Tilling, R.I., 2009, Volcanism and Associated Hazards: the Andean Perspective: Advances in Geosciences, v. 22, pp. 125-137: Online resource – Accessed 16 June 2019
https://www.adv-geosci.net/22/125/2009/adgeo-22-125-2009.pdf

Subduction Zone Volcanoes, 2017: Annenberg Foundation: Online resource – Accessed 16 June 2019