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?
In addition many models have been proposed that estimate the average composition of continental crust through time.
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 classroom
Following 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.
Asking "big questions" about how the Earth works also follows the recommendations for curriculum design presented by Wiggins and McTighe (1999) Understanding by Design who call for the development of guiding questions to help promote "enduring understanding."
Suggested Teaching Activities
Given 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.
The is a cooperative learning strategy in which small groups of students are assigned to become the "experts" on a given topic. After achieving mastery of their topics the students are reorganized such that there is one "expert" on each topic in newly formed groups. The students are then responsible for teaching each other the essential ideas on their assigned topics. A jigsaw could be developed by assigning students to find evidence from the literature for the composition and evolution of the Earth's continental crust based on:
- 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.
Assign students the task of playing the role of Armstrong Fyfe, Hurley, Veizer, and Jansen and others from the reference list by presenting the essential evidence and interpretations. What evidence is contradictory permissive (of the interpretations) or compelling? This set of references is purposefully of historical interest. What new evidence has been brought to bear in the past decade on the question of crustal genesis and evolution?
Reading From the Primary Literature
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)
Questions related to crustal genesis and evolution can be extended into a major class project using a Problem-Based Learning Approach. In PBL groups are presented with contextual situations and asked to define the problem decide what skills and resources are necessary to investigate the problem and then pose possible solutions (Duch Groh and Allen 2001). PBL exercises are student-centered yet faciliated by faculty as co-investigators. There is an emphasis on inquiry and the development of skills (e.g. critical thinking skills). PBL simultaneously develops problem solving strategies disiplinary knowledge bases and technical skills. PBL places students in the role of problem solvers confronted with authentic and purposefully ill-structured problems which mirror ongoing research in the field. PBL activities are complex in nature; requires inquiry information gathering and reflection; has no simple fixed formulaic "right" answer. What better topic to engage PBL than the origin and evolution of continental crust?
References and other Resources
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Bennett V. C. 2003 Chapter 2 - Compositional evolution of the mantle in Carlson R. W. ed. Teatise On Geochmistry: Vol. 2 The Mantle: New York Elsevier p. 493-519.
Bennett V. C. Brandon A. D. Heiss J. and Nutman A. P. 2007 Coupled 142Nd 143Nd and 176Hf isotopic data from 3.6-3.9 Ga rocks: new constraints on the timing and composition of early terrestrial chemical reservoirs 38th Lunar Science Conference: Houston TX.
Bowring S. A. and Housh T. 1995 The Earth's early evolution: Science v. 269 p. 1535-1540.
Boyet M. and Carlson R. W. 2005 142Nd evidence for early (>4.53 Ga) global differentiation of the silicate Earth: Science v. 309 p. 576-581.
Boyet M. and Carlson R. W. 2006 A new geochemical model for the Earth's mantle inferred from 146Sm-142Ndsystematics: Earth and Planetary Science Letters v. 250 p. 254-268.
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