When and how did continental crust form?submitted by David Mogk, Montana State University
Why is this question important?The genesis and evolution of continental crust is one of the fundamental questions that remains unresolved in the geosciences. Models that have been proposed for crustal growth include:
- early extraction of all the crust from the mantle
- long-term growth or
- episodic periods of crustal growth? Did most of the crust form by the end of the Late Archean?
The purpose of this exercise is to help students explore these questions through guided discovery of the primary scientific literature to find and critically evaluate the major lines of evidence that address these various models for crustal genesis and evolution.
What we know...To address these questions it's convenient to start with the compilation of crustal growth models from Taylor and Mclennan (1985):
The figure above presents a summary of the crustal growth models which includes the following authors' work:
- Early differentiation of virtually all of the continental crust at ca. 3.9 Ga ago and subsequent steady state recycling of this crust. Proponents of this model are:
- Fyfe W. S. (1978) Evolution of the Earth's crust: modern plate tectonics to ancient hot spot tectonics? Chemical Geology 23 89.
- Armstrong R. L. 1981 Radiogenic isotopes: the case for crustal recycling on a near steady-state no continental growth Earth. Phil. Trans. Roy. Soc. Lond. A301 443.
- Uniform growth rate or accelerated growth rate was proposed by:
- Hurley P. M. 1968 Absolute abundance and distribution of Rb K and Sr in the Earth. Geochim. Cosmochim. Acta 32 273.
- Hurley P. M. and Rand J. R. 1969 Pre-drift continental nuclei Science 164 1229.
- Episodic growth of continental crust was proposed by
- Veizer J. and Jansen S.L. 1979 Basement and sedimentary recycling and continental evolution Jour. Geol. 87 341.
- McLellan S.M. and Taylor R. S. 1982 Geochemical constraints on the growth of the continental crust. Jour. Geol. 90 342.
A good summary of the question of continental crustal evolution can be found in the review article by Taylor S. R. and McLennan S. M. 1995 The Geochemical Evolution of the Continental Crust Reviews of Geophysics 22 #2 May p. 241-265.To fully address these questions it is necessary to integrate data about:
- the composition of the mantle (depleted and undepleted)
- whole rock and trace element data of crustal rocks
- data from numerous isotopic systems: U-Pb Nd-Sm Lu-Hf Re-Os etc.
- estimates of the heat budget of the Earth and how this has been distributed throughout Earth history.
How to link this topic to the classroomFollowing the recommendations of Science for All Americans (AAAS 1990) (more info) teaching should be consistent with the nature of scientific inquiry:
- Start with questions about nature.
- Engage students actively.
- Concentrate onthe collection and use of evidence.
- Provide historical perspectives.
- Use a team approach.
- Do not separate knowing from finding out.
Suggested Teaching ActivitiesGiven the extensive literature on the composition and evolution of continental crust there are a number of teaching strategies that can be employed to encourage active learning by students. A critical reading of this collection of articles will provide students with a good opportunity to evaluate the chemical isotopic and physical evidence that has led to the development of these models of continental crustal growth.
- Whole-rock and Rare Earth Element data
- U-Pb (zircon) data and Pb-Pb whole rock and mineral data
- Nd-Sm isotope systematics
- Lu-Hf isotope systematics ... and other systems such as Re-Os.
Cam Davidson at Carleton College provides guidelines for learning exercises to help students learn how to read the primary literature learn how to formulate questions based on reading learn how to choose one or two main points from a paper and present these ideas to their peers learn to organize the ideas and questions of peers and lead a discussion based on those questions. See his exercise on Friday Forum: Reading From the Primary Literature
Here is an example of a literature-based assignment using this topic, submitted by Dr. Lindy Elkins-Stanton, Massachusetts Institute of Technology. Scientific Paper Reading: Continental Growth (Microsoft Word 42kB Jul17 07)
References and other Resources
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