Summary

In this activity, students will apply their understanding of radioactive decay to analyze and interpret the meaning of Atlantic seafloor isotope data. Students will then use their results to suggest past changes that have occurred with the seafloor.

Grade Level

High School

NGSS Alignment

This activity will help prepare your students to meet the performance expectations in the following standards:

• HS-ESS1-5: Evaluate evidence of the past and current movements of continental and oceanic crust and the theory of plate tectonics to explain the ages of crustal rocks.
• HS-ESS2-1: Develop a model to illustrate how Earth’s internal and surface processes operate at different spatial and temporal scales to form continental and ocean-floor features.
• HS-PS1-8: Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy released during the processes of fission, fusion, and radioactive decay.
• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data
  • Engaging in Argument from Evidence

Objectives

By the end of this activity, students should be able to:

• Consistently use radiometric data to date rock samples and explain how radiometric dating works.
• Use radiometric data to suggest a theory about the Atlantic Ocean’s past (specifically, seafloor spreading).
• Explain how different radioactive isotopes (with different half-lives) can be used to explore the history of different objects. Specifically, students will consider the difference between C-14 dating of organic material and K-40 dating of seafloor rocks.

Chemistry Topics

This activity supports students’ understanding of:

• Radioactive decay
• Radiometric dating
• Half-life

Time

Teacher Preparation: 30 minutes
Lesson: 2-60 minute class periods (optional extensions potentially 2-4 class periods more)

Materials

• Copies of the student handout
• Colored pencils/crayons
• Internet access and digital projector (optional)

Teacher Notes

• This activity is part of a series of activities designed to guide students in arriving at an understanding of HS-ESS1-5 and allow them to develop hypotheses for HS-ESS2-1. Prior to this activity, students have already developed a basic understanding of the types of radioactive decay and also the role that time plays in that decay. Students have learned about alpha and beta decay and can write nuclear equations for each. Students have also completed activities like The Demise of Frosty to understand of a time-based phenomenon can be used to explore past events and Using Dice to Explore Radioactive Decay to understand decay rate and half-life. Lastly, students have completed the Radioactive Decay and Peat Bogs activity, where students develop an understanding of how C-14 data can be used to reliably determine the age of organic materials.
• There are a variety of ways this activity can be can be facilitated depending on the desired degree of student collaboration and support. The background and prelab questions allow students to develop initial ideas about the data they will explore and to connect their prior experiences to this new activity. Have students read the background, consider the prelab questions 1-4 and then discuss their answers with another student. They may then consider questions 5-7 individually or in pairs. They should generally find all of these questions easy to answer given their prior experiences, but question 7 can be challenging for many.
• The data set provided is very large. Students are not told about any specific outcomes beforehand. They have the skills and knowledge to analyze the data and its implications become apparent to students as the analysis proceeds. While it may seem quite varied, the data locations (latitude and longitude) are in a grid pattern. The data for this activity was created because raw seafloor data is exceedingly complex with many complicating factors that must be considered. In the calculations section, students use the graphs of K-40 to date each rock sample. The activity was originally created for a 10^th-grade, introductory chemistry course. If it were used in a more advanced course, you may choose to remove the graphs and ask students to use a more mathematical pathway to determine the age of a sample. Students could use the equation where t_1/2 is the half-life of the isotope and N is the portion of the original sample remaining, written as a decimal. For example, if a sample of K-40 (t_1/2 = 1,251 MYr) is 75% of its original concentration, then = 519 MYr old.
• For the age determination students could analyze the data individually, but this would be very time consuming and repetitive. Since the amount of data is large, it is suggested that students collaboratively analyze the data. A method the author has used is to assign pairs of students five data points to analyze. In a class of about 30 students, this results in multiple groups analyzing the same five data points. Once a student pair has dated their five samples, they find the other groups that analyzed the same five samples and compare their results. Then, once these larger groups have confirmed their analysis, they can submit their results to a full-class data table. This can be done using a Google Form or shared Google Doc of the table, by projecting the data table on a whiteboard or having students go collect the results from other groups individually.
• For question 3 in the calculations section, student need the provided map of the Atlantic Ocean. A separate document includes a larger version of the map, which can be printed on 8.5”x 14” paper. The author typically has students plot the data on the map with a partner. Alternatively, the map could be built as a whole class by projecting it on a whiteboard. This can shorten the time required for this activity, but limits individual student thinking. You may wish to remind students about the definitions of latitude and longitude before they begin. Students should find the proper location for each data point, mark that point and write the age of that sample next to it. The map was produced by The National Oceanic and Atmospheric Administration (NOAA).
• The analysis section is where the magic happens. The results from the data table do not seem to have any real patterns or trends. The patterns in the data are only revealed when viewed spatially on a map. Students often need a little direction in starting the first question in the analysis section. Show and explain that the dashed line in the example passes through points that are approximately 20 MYr old. You may also choose to remove the suggested 20 MYr intervals and allow students to decide intervals on their own. Students will arrive at a map that looks something like the Atlantic region on this map. As students see the pattern develop many will have an ‘Aha’ moment and begin discussing potential hypotheses that would explain the evidence.
• Students should develop an evidence-based argument in the Conclusion section. They can do this in pairs or individually. You may wish to show students a topographic map like this to allow them to consider how topography data fits with their radiometric data. While students’ prior experiences are varied, most have some experience with plate tectonics. There may be some variation in conclusions here, but most students will recognize that there seems to be a curve running down the middle of the Atlantic Ocean where the youngest rock is found and that the rocks are getting older in both directions as you move away from the curve and towards the continents. A key part of a complete hypothesis is being able to rewind the data and suggest that all of the seafloor samples formed along the curve (volcanic activity) and that the rock is pushed out from the curve, causing the seafloor to spread. Regardless, this response should represent students developing a hypothesis that they can support with evidence. Allowing students to share their Claim-Evidence-Reasoning with each other and critique each other’s arguments can help develop a more correct consensus understanding.
• The extension questions allow students to connect this activity to prior experiences and to apply their newfound understanding to a new situation.
• The Zircon Dating Using Uranium-Lead Decay can be used as practice, enrichment or formative assessment. The data were pulled from the USGS website. The author chose a variety of locations, but included one near where his students live. You may wish to find relevant samples. Part of the intent of this extension activity is for students to see that all of the rock samples from ‘continental crust’ are significantly older than the oceanic samples they just considered. This contrast between continental and oceanic crust brings about interesting questions about their formation and history. Here’s some background information.
• If you have the time and inclination, this activity from Rice University does an excellent job of finishing the work required for ESS1-5 and ESS2-1. The website describes the use of the activity well. This activity can be completed in two to three 60-minute periods.