When Did the Grand Canyon Begin to Form?

South Rim, Grand Canyon. Image courtesy of Kimberly Lightle.

This blog post draws from several news sources — washingtonpost.com, The New York Times, and Science Friday. All these sources have stories and photos related to a study published March 7, 2008, in Science by researchers Victor Polyak and Carol Hill (free registration is required to view this article). Science Friday features a 15-minute audio clip of an interview with Polyak. The research suggests that the Grand Canyon began forming 17 million years ago. However, for the past 100 years or so, geologists have agreed, based on a robust data corpus, that the Grand Canyon is probably five to six million years old, even though the rock from which it is carved is up to two billion years old. So what have Polyak and Hill done to upset this long-held theory of the Grand Canyon’s age?

To put it simply, they gathered new data and analyzed it using new technology. That is, they gathered rock samples called mammillaries from caves. These mammillaries are associated with ancient water tables and suggest previous levels of the water table. Polyak and Hill then analyzed these samples with improved rock-dating technology involving the radioactive decay of uranium to lead. The Grand Canyon began forming 17 million years ago at the western end in a west to east direction, and at a rather slow rate. Some time later, the east end of the Grand Canyon began forming from east to west, at a much more rapid rate. Eventually the two ends merged and the Colorado River emerged.

However, some scientists suggest Polyak and Hill’s methods and interpretations may be too narrow or incomplete. For example, their assumption that all the mammillaries examined originated in an ancient water table may not be a safe one. One critic noted that springs do occasionally emerge from the canyon walls and they could produce mammillaries as well. Another point of contention deals with the lack of 17-million-year-old sediment, which would be evidence of a 17-million-year-old river. Hill counter-argues that such sediment may not exist because the scale of the hypothesized 17 million-year-old, western river system would not produce sizable amounts of sediment. In addition, river erosion tends to destroy such potential evidence.

How to Turn This News Event into an Inquiry-Based, Standards-Related Science Lesson
Estimating the age of the Grand Canyon is related to the History and Nature of Science, Science as Inquiry, and the Earth and Space Science content standards of the National Science Education Standards. With respect to the first two standards, several themes emerge. The researchers proposed using improved laboratory techniques and new data sources to make an estimate of the age of the Grand Canyon. In this way, they demonstrated the idea that science advances with new technologies. Science also seeks disconfirming evidence to existing theories as a means of gaining increased certainty regarding what we know about the natural world. If scientists fail in their attempt to find disconfirming evidence, they have succeeded in strengthening the existing theory. If they find disconfirming evidence of existing theories, then they pave the way to new lines of research, which must be further investigated. Eventually, existing theories may be either supplanted or revised in light of the new evidence, or they may be strengthened should the new evidence turn out to be unreliable or invalid.

The news sources related to this research also provide “air time” for scientists who argue alternate interpretations of Polyak and Hill’s data and who point out that Polyak and Hill may be ignoring some facts that impact their conclusion. These presentations underscore the role of argumentation and evidence based logic in advancing scientific knowledge as well as the social nature of science.

Ask your students if they know how old the Grand Canyon is. Ask them if they imagine someone knows, even if they don’t. From here, the discussion is going to go in one of two directions: (1) If they imagine someone knows, how do students imagine the someone knows how old the Grand Canyon is; what kind of evidence might have been used? Entertain all student contributions and stipulate that the students provide some justification for their response. You may need to do quite a bit of guiding and scaffolding here to lead students to support only evidence-based and logical responses. (2) If students imagine no one really knows, ask why not; what prevents human beings from knowing?

Depending on your students’ background knowledge and context you can relate the discussion to a variety of instructional goals and learning objectives. Do you want to emphasize the nature of science, evidence-based argumentation, and the social aspects of doing science? Then choose excerpts from Science Friday’s interview, which highlight these aspects in the context of real scientists doing real science and devise discussion questions for your students to reflect upon in order to increase their awareness of the nature of science.

Maybe you want to highlight some methods of science like rock dating. Perhaps you can use this opportunity to illustrate how new questions can emerge from gathering evidence intended to answer another question, as is illustrated in the final paragraph of the washintonpost.com story.

Or maybe you want to give students practice with science literacy. Put students in small groups and give each group one of the three sources listed in the first paragraph of this blog. Devise two or three open-ended questions for each group to discuss and reach consensus. Have the students jigsaw into new groups and share the consensus of their first group. How does each student now understand the issue of determining the age of the Grand Canyon? How does this issue intersect with the bigger idea of the nature of science?

We Want Your Feedback
We want and need your ideas, suggestions, and observations. What would you like to know more about? What questions have your students asked? We invite you to share with us and other readers by posting your comments. Please check back often for our newest posts or download the RSS feed for this blog. Let us know what you think and tell us how we can serve you better. We appreciate your feedback on all of our Middle School Portal 2 publications. You can also email us at msp@msteacher.org. This post was originally written by Mary LeFever and published March 14, 2008 in the Connecting News to the National Science Education Standards blog. The post was updated 11/16/2011 by Kimberly Lightle.

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