Sea Floor Magnetism
This earth science activity allows students to simulate the collection of magnetic polarity data across two mid-ocean ridges. Prior to class, the instructor arranges bar magnets in a symmetric pattern to simulate changes in polarity in the sea floor and covers the magnets so students must use a compass to remotely gather the data without directly observing the magnets themselves.
Students then graph the data and identify regions of normal and reversed polarity. The data are also plotted onto a profile of each mid-ocean ridge. Finally, students use the data they collected to draw inferences about sea floor spreading at mid-ocean ridges and calculate the average spreading rate for the last 780,000 years.
This lesson has goals related to geoscience content, the nature of science, and quantitative analysis.Students understand that the sea floor basement rocks contain remnant magnetic data that can be used to infer events in the geologic pastStudents understand that the patterns of magnetic reversals are symmetric about the axis of the mid-ocean ridge
Students understand that these magnetic polarity data can be used to determine rates of plate movement
Nature of ScienceStudents understand how geophysical techniques allow geoscientists to collect data from the sea floor without physically visiting the sea floorStudents understand that claims about events that happened in the geologic past (like magnetic reversals) are reliably based on present-day observationsStudents understand that not all scientific investigations require a controlled experiment
Students use mathematics to describe their observations and provide evidence to support their claims.
Methods of Geoscience By simulating a well-known series of studies, students will experience on inductive approach to science rather than a linear, deductive methodology.
Context for Use
This activity is designed for an Earth science content course for pre-service elementary education majors. This lab-based courses uses minimal lecture to teach the target concepts. This activity would be part of the plate tectonics unit. This activity could be adapted to a variety of educational settings including major's courses in tectonics or oceanography, introductory geology and Earth science courses, or possibly high school Earth science courses.
Description and Teaching Materials
A materials list and description of the activity are available in the following attachments.
Teaching Notes and Tips
Unless you are specifically teaching about the geomagnetic polarity timescale, the actual pattern of normal and reversed polarities does not matter as long as the pattern is symmetric about the axis of the mid-ocean ridge. The pattern presented here has the axes near the center of the profile. For added difficulty, one can offset the axis so it is not in the center of the profile.
Students may also have difficulty understanding that azimuth directions greater than 180 degrees are the same as directions less than 180 degrees. (That is readings of East and West are the same for these purposes.)
Do NOT show the students the actual pattern of the magnets. This simulates actual science in that no one can actually see the stripes on the seafloor.
Students answer questions on the attached worksheet and discuss the material in class. A later exam contains questions on seafloor magnetic polarities and divergent plate boundaries.
As an alternate assessment, students could write up their findings as a scientific report and then compare their reports to those of papers from the mid 1960s.
References and Resources
The seafloor profiles were taken from Google Earth. For an extension activity, the students could look up the locations of these two ridges and note general observations regarding the topography of the seafloor. They could also generate their own seafloor profiles.