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What's the deal with BIFs?

As a former field geologist, I am always looking for more ways to incorporate Earth Science into all my classes. My 10th-grade Integrated Science class begins the year looking at characteristics of life. They have already had biology so this is a great refresher to start the year off. See my characteristics of life post for more information. After identifying what "life" is on Earth and what conditions need to be present for life on Earth, then we dive into Earth's History and the Geologic Time Scale. The Geologic Time Scale is part of the Middle School standards but my students have never been exposed to the concept so that is where we start. The timescale opens up many avenues for teaching. Within the timescale you can cover extinctions, plate tectonics, continental drift, seafloor spreading and magnetic reversals, evolution of life, time scales, patterns, cause and effect, stability and change and the list goes on and on.


What are Banded Iron Formations?

Banded iron formations are interbedded layers of iron-rich strata with minerals such as hematite and magnetite and iron-poor layers containing chert or jasper. The formation of these geologic formations was integral to Earth's early atmosphere formation, particularly the Great Oxygenation Event. The iron formations are a record of this early and essential shift in the development of our atmosphere and ocean and photosynthesizing life.


Scientific Sketch, Banded Iron Formations, S. Erickson, 2024

The weathering of continental rocks and the off gassing of hydrothermal vents released iron ions into the newly formed oceans. Iron oxidizes in the ocean and "rusts", this turns the oceans green. Meanwhile, cyanobacteria begin to photosynthesize. Carbon Dioxide (CO₂) and Water Vapor (H₂O) from the atmosphere and ocean are drawn in and combined with the energy from the Sun produce Oxygen gas and carbohydrates the bacteria use as food [CO₂ +H₂O + sunlight yields O₂ and C₆H₁₂O₆]. The Oxygen is released into the ocean as a waste product. They form bacterial mats that trap carbonate ions and form alternating layered mounds of bacteria and calcium that solidify and form stromatolites, our first fossil in the fossil record. These stromatolites date to the Archean Eon of the Precambrian Era. The oldest known stromatolite has been found in Apex Chert samples from Western Australia which date to 3.5 billion years ago.


As the cyanobacteria replicate and continue to photosynthesize. The cyanobacteria produce oxygen at a rate greater than it can combine with other compounds or be sequestered in minerals. The ocean becomes saturated in oxygen. This is called the Great Oxygenation Event. Over time the accumulated oxygen begins to exchange in the atmosphere and react with methane. As more oxygen enters the atmosphere, more methane is displaced until oxygen is a major component of the atmosphere.


The iron ions (Fe⁺², Fe⁺³) combine with the free oxygen in the ocean and form the minerals of hematite (Fe₂O₃) and magnetite (Fe₃O₄). The hematite and magnetite fall the ocean floor to form an iron rich red layer of the banded iron formation. This continues until the iron is depleted. The cyanobacteria continue to produce oxygen create a toxic ocean tat destroyed anerobic life. The cyanobacteria begin to die off. The silica ions from weathered rocks begins to combine with the oxygen and form cherts and jaspers. The iron poor black layers of the formation form. This cycle repeats itself over millions of years until approximately 1.8 billion years ago. At this point in Earth's history the atmosphere is enriched in oxygen and iron is depleted from the oceans. The anerobic cyanobacteria that produced the oxygen to create the layers of the BIF are no more and have been replaced with minerals to become the fossilized stromatolites we find today.


Banded Iron Formations can be found in Australia, across Lake Superior and into Canada, Brazil, India, Russia, South Africa, and the Ukraine. Approximately 60% of our iron reserved and most of the iron ore we mine comes from these formations, making them a important commodity in our society.



Teaching Banded Iron Formations

After introducing the timescale, we look at the fossil record and index fossils. Our record begins with stromatolites and banded iron formations, so I wanted to dive deeper into the science of how banded iron formations were made. I have a couple of samples of Lake Superior Banded Iron that students use for a scientific study on the formation. Students measure the width of the layers, calculate the volume and density of the sample, and sketch in their science notebooks. Hands on rocks, minerals and other objects from nature, are always a great object for students to practice their qualitative and quantitative observations. Its a good change of pace in the science classroom and helps with note booking skills.


We then listen to Gary Lewis's podcast on Banded Iron Formations and complete a graphic organizer as we listen to the podcast. Gary's podcast is only 10 minutes long so its the perfect length for my high schoolers before they get super antsy. If you want something more in depth or for high level students, you can check out the GeologyBites podcast. Students are introduced to new vocabulary and new concepts while refreshing photosynthesis and cellular respiration reactions from biology class the previous year. Students then take a virtual field trip to Karijini Gorge in Australia to gather more evidence on banded iron formations in a 2x2 chart in their notes.


After we investigate the individual samples, listen to a podcast, and go on a virtual tour, we then as a class sketch the process that the BIFs form. I follow this up with a scientific writing assignment as their summative before moving into the evolution of the atmosphere. Below is my sequence in my Earth History Unit that covers BIFs.

Activity

Student Outcomes

NGSS Science and Engineering Processes

Students will be able to interpret and analyze a graph showing the abundance of banded iron formations over geological time to draw conclusions about changes in Earth's atmospheric oxygen levels and their impact on geological processes. Using evidence from the graph, students will explain how the formation patterns of BIFs correlate with shifts in oxygen levels and early biological activity, supporting their conclusions with specific data points.

Analyzing and Interpreting Data

Students will be able to collect and analyze both qualitative and quantitative data from samples of banded iron formation by using scientific sketching techniques and precise measurements. They will interpret their findings to construct explanations about the environmental and geological conditions that contributed to the rock’s formation, supporting their conclusions with observed mineral composition, layering, and texture.

Planning and Carrying Out Investigations Constructing Explanations

Students will be able to synthesize information from a podcast about banded iron formations to explain how the presence of iron and silica reflects ancient environmental conditions and geological processes. After listening, students will construct explanations connecting the geochemical cycles discussed to the formation of banded iron formations, using evidence to support their understanding of Earth’s atmospheric and oceanic evolution over time.

Obtaining, Evaluating and Communicating Information Constructing Explanations

Students will be able to analyze and interpret data by graphing the percentages of iron and silica in a sample of banded iron formation, and then identify and explain any observed patterns or correlations

Analyzing and Interpreting Data

Students will be able to use evidence from a virtual tour of Karijini Gorge to analyze the geological composition and formation of banded iron formations, identifying and explaining how natural processes, such as sedimentation and oxidation, contributed to their structure. Through this exploration, students will interpret visual and geological data to construct explanations of how Karijini Gorge’s unique landscape developed over geological time.

Analyzing and Interpreting Data

Class Model Formation of Banded Iron

Students will be able to develop and use a class model to demonstrate the formation process of banded iron formations, illustrating the role of cyanobacteria and stromatolites in the cycling of iron and oxygen. Through the modeling activity, students will explain how biological and geological processes interacted to shape the ancient Earth's environment, providing evidence of early life and its impact on atmospheric composition.

Developing and Using Models Constructing Explanations

Banded Iron Formations Prompt

Students will be able to explain the significance of banded iron formations (BIFs) in understanding Earth’s early atmospheric conditions and biological evolution by composing a scientific response that uses evidence to support their conclusions. In their writing, students will analyze how BIFs provide insights into the oxygenation of Earth’s atmosphere, the evolution of early life, and the cessation of BIF formation around 1.8 billion years ago.

Engaging in Argument from Evidence Constructing Explanations

In this sequence of activities I touch on five NGSS High School Standards.

  • HS-ESS2-7: "Construct an argument based on evidence about the simultaneous coevolution of Earth’s systems and life on Earth."

    • Explanation: Each activity involves exploring how banded iron formations (BIFs) provide evidence of Earth’s early atmosphere and biological evolution, specifically the interaction between life (cyanobacteria, stromatolites) and atmospheric changes. Students use various forms of data (graphs, models, scientific sketches, and written explanations) to understand and argue how Earth’s systems and life coevolved.

  • 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."

    • Explanation: Activities like analyzing BIF samples, scientific sketching, and model-building help students recognize how BIFs reflect ancient geological processes and the oxygenation of Earth’s oceans. The formation and locations of these rocks relate to Earth's tectonic movements and provide a geological timeline.

  • HS-ESS3-1: "Construct an explanation based on evidence for how the availability of natural resources, occurrence of natural hazards, and changes in climate have influenced human activity."

    • Explanation: Students connect the availability of iron deposits in BIFs to changes in Earth’s atmosphere and resources, understanding how these ancient resources and climate shifts shaped Earth’s history and ecosystems.

  • HS-LS4-5: "Evaluate the evidence supporting claims that changes in environmental conditions may result in: (1) increases in the number of individuals of some species, (2) the emergence of new species over time, and (3) the extinction of other species."

    • Explanation: By studying BIFs, students consider how early life forms (e.g., cyanobacteria) influenced atmospheric changes that, in turn, shaped biological evolution. This aligns with understanding how environmental shifts impacted species survival and evolution, particularly the “Great Oxygenation Event.”


Other Resources for Teaching about BIFs


Hope you can use one or more of these ideas to incorporate into your classroom.





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