Burial, compaction, and porosities in a subduction zone
This activity introduces students to what happens to sediments buried in subduction zones. Students calculate the rate of burial in a subduction zone and compare it to burial by sedimentation on the ocean floor. They also examine porosities of sediments, how porosity changes with depth, and what porosities can tell scientists about water pressure.
- Reinforce features of subduction zones.
- Reinforce the concepts of compaction.
- Use simple calculations to help draw conclusions.
- Synthesis of concepts relating burial, fluid escape, porosity, and water pressure.
Context for Use
This can be used in an introductory class where students have been introduced to subduction zones and the rock cycle. It can also be helpful (but not necessary) to have discussed groundwater, so the students are already familiar with porosity.
The activity could be adapted to a classroom presentation/discussion rather than a student activity.
It could also be adapted to a Groundwater class section on fluids in geologic processes.
Description and Teaching Materials
The activity is self-contained except for links to the site maps and cross section through the ODP sites. If the web will not be accessible during the activity, the teacher can download these three figures in advance and include in the assignment.
Assignment Handout (Microsoft Word 1.8MB May28 09)
Teaching Notes and Tips
Some background on seismic reflection surveys would add to the understanding, but it is also OK if students just treat it as a picture.
Students might ask how porosity is measured. For the ODP cores, a volume of sediment from a core is weighed before and after drying. The weight of the water that was in the sediment is converted to volume (using the density of water). The water volume represents the pore volume.
Students with math phobias may need encouragement and help to see that the time to move from Site 1173 to 808 can be calculated by dividing the distance by the convergence rate (4 cm/year).
Additional information on water flow can be discussed (especially if students have already covered groundwater). You could note that the sediments, which are mostly clay and silt, would be expected to have very low permeabilities, and would be classified as "aquitards".
Additional discussion after the activity can include the difficulty of measuring water pressures in sediments at the bottom of the ocean. Once the hole is drilled, if it is left open the water escapes and the pressure escapes. Scientists have sealed holes at the seafloor and put in pressure gauges with datalogger. They then have to return to the hole with a submersible to collect the data.
Assessment will depend on whether the activity is used as an in-class example or as a lab activity. Assessment of the calculation portions are straightforward. For the synthesis portion, students should discuss in groups and present their results.