Chemical Inputs and Outputs at Subduction Zones
Summary 
In this activity the student analyzes global geochemistry data to create hypotheses regarding possible geochemical inputs at subduction zones, and the range of compositions associated with this type of convergent boundary. The exercise utilizes the EarthChem federation of online geochemical databases, GeoMapApp and Excel. The activity focuses on Central America, an NSF-MARGINS Focus Site, and Cascadia, an NSF-MARGINS allied study site.
Learning Goals
Students will examine the basic chemical inputs and outputs at subduction zones by comparing and contrasting major element chemistry of MORB and arc magmas. The data used in this activity is obtained directly from the literature via online databases and will thus contain the full and messy range of possible compositions associated with this particular convergent margin. The intent is introduce to students to the chemical similarities and differences between MORB and arc volcanism and begin to explore ways in which magmatic sources can be traced.
Context for Use
This activity is intended for use in an introductory geology course and can be used as an in-class discussion tool or as part of a laboratory assignment that could complement the topics of plate tectonics and identification of rocks and minerals.
Students should perform the task on Mac or PC computers loaded with the following software: a web browser, Excel, and GeoMapApp.
The assignment can be modified for an upper-level undergraduate petrology class by including additional sites and having the students create hypotheses regarding magma source components by examining trace element patterns, incompatible element ratios, and isotopic variation.
Students should be familiar with tectonic plate interactions, the major oceanic and continental rock types, and with the basic geometry of subduction zones.
Students should perform the task on Mac or PC computers loaded with the following software: a web browser, Excel, and GeoMapApp.
The assignment can be modified for an upper-level undergraduate petrology class by including additional sites and having the students create hypotheses regarding magma source components by examining trace element patterns, incompatible element ratios, and isotopic variation.
Students should be familiar with tectonic plate interactions, the major oceanic and continental rock types, and with the basic geometry of subduction zones.
Description and Teaching Materials
Included in this activity is an assignment sheet, along with an Excel spreadsheet containing the East Pacific Rise/Central America and Juan De Fuca/Cascadia datasets for download if there are difficulties with website traffic.
Assignment Sheet ( 221kB May30 07)
Central America Excel Spreadsheet (Excel 1023kB Apr3 07)
Cascadia Excel Spreadsheet (Excel 1.4MB Apr3 07)
Teaching Notes and Tips
Instructors should explore the EarthChem Portal and member databases PetDB, NAVDAT, and GEOROC. The instructor should be prepared to deal with any technical hang-ups that will occur as a function of students' computer expertise.
Note: Subaerial data often lack precise geospatial location. Therefore, data points may overlap each other and give the impression in map view that fewer samples are available than is actually the case.
Note: Subaerial data often lack precise geospatial location. Therefore, data points may overlap each other and give the impression in map view that fewer samples are available than is actually the case.
Assessment
Assessment will depend on whether the activity is used as an in-class example or as a lab activity. If used as part of a lab, students will be asked to discuss their answers in small groups where they will be required to produce a group report synthesizing their hypotheses on inputs and outputs at subduction zones.
References and Resources
EarthChem: http://www.earthchem.org
GeoMapApp: http://www.marine-geo.org/geomapapp/
GeoMapApp: http://www.marine-geo.org/geomapapp/
