This activity is a learning experience that addresses paleomagnetism, seafloor spreading and geologic time. The activity that we have included is adapted from "A model of seafloor spreading" exercise developed by Bay Area Earth Science Institute (http://www.UCMP.Berkeley.EDU/ forsrec/Metzger3.html). This exercise is also available in The Paleontological Society Papers: Learning From the Fossil Record, Volume 2, October 1996: 96-99.

Participants/students will make a paper model illustrating the concept of seafloor spreading and the development of symmetrical magnetic "stripes" on either side of a mid-ocean spreading center.

Time Frame - 30 minutes

Materials

• Physiographic map of the ocean floor
• "A model of seafloor spreading" exercise from Bay Area Earth Science Institute
• Map of the ocean floor with magnetic reversals

Advanced Preparation

Presenters/teachers will review the origin of the mid-ocean ridges and the deep-sea trenches visible on a physiographic map of the ocean floor and review their relevance to plate tectonics.

Procedure

1. Presenter will pass out exercise for "A model of seafloor spreading" by the Bay Area Earth Science Institute, which is available on the web. During this exercise students will make a simple model that shows the evolution of oceanic crust through sea-floor spreading and subduction .
2. Teachers and the presenter will discuss as a large group the implications based on the above exercise for the age of the ocean floor at subduction zones and spreading zones.
3. Teachers will view a map of the ocean floor showing the various magnetic reversals present which define the various plate boundaries.

Formative Assessment

The presenter/teacher may lead a discussion of responses to the following questions:

1. How does seafloor-spreading account for the age of the ocean floor?

2. How are the sediments on the seafloor dated?

3. How is the oceanic crustal seafloor dated?

The presenter/teacher will review the models of seafloor spreading constructed by the teachers. He or she will explain how the geoscience community continually updates its time scale as new data become available (i.e., the paleomagnetic time scale).