Boulder Garden - Banded Iron Formation (BIF)
You are standing in the Carpenter Lake Boulder Garden, an exhibit that showcases some of the geological wonders of the Great Lakes region. This eartchache will provide a short lesson on Banded Iron Formations and how this upper peninsula rock was formed.
There are a ton of geological exhibits in this section of the park and if you enjoy geology (and since you're reading this Earthcache I will assume that you do), you'll love this area!
Banded Iron
If we all recall our grade school science classes, there are three types of rocks - igneous, metamorphic and sedimentary. Igenous rocks are formed when superheated magma or lava cools. Sedimentary rocks are created by the deposition of rock particles that are transported by wind or water and solidified by either a chemical or biological process, 
and Metamorphic rocks are rocks that have experienced a mineralogical composition shift due to significant heat or pressure changes after their intial formation.
Banded Iron was formed during a period of the earth's history when the earth's atmosphere was composed of significantly less oxygen and the oceans were highly acidic. Examples of Banded Iron Formations can be found on virtually every continent in the world. In front of you you'll see an example from the local Marquette Iron Range in the UP. This example is composed of jaspilite, which consists of alternating bands of jasper (a cryptocrystalline quartz), red hematite, and a silvery-gray specular hematitie. The iconic banding make this an easily recognizable deposit hence the name "Banded Iron."
An Indicator for Ancient Oceans
The lighter and darker layers of sediment were deposited approximately 1.8 billion years ago in a world that looked starkly different than the one we know today. Back then, there was very little, if any, free oxygen in the atmosphere and the oceans were highly acidic. Because iron is soluble (dissolves) in acidic waters, more iron particles were suspended within the solution of the ocean than is possible under our current conditions. Between 2.5 and 1.8 billions of years ago, photosynthetic cyanobacteria or “Blue-Green Algae” began producing oxygen as a byproduct of the photosynthetic process. In layman’s terms, newly formed algae started putting massive amounts of oxygen into the water while simultaneously using up the Carbon Dioxide. This caused the acidity of the oceans to drop substantially and oxygen combined with the iron particles in the water to produce iron oxide particles, which sank to the ocean floor. If it weren’t for the blue green algae there would never have been enough oxygen to form the insoluble iron oxides and lay down the darker iron layers.
What does this tell us about the earth’s oceans in the Precambrian Era? If you notice the striations, it tells us that the deposition process was cyclical which means that free oxygen would periodically become available, allow for iron deposition, and then the oceans would recede back to a more acidic solution while the normal red silicate layers deposited. There are several possible explanations for the cycles. One possible reason may be seasonal mixing of the water column due to temperature variations of the surface waters causing oxygen to periodically disappear from the system. Alternatively the blue-green algae may have produced so much excess oxygen that it poisoned it’s own environment causing periodic mass extinctions of the algae and oxygenation would stop. Regardless of the reason, during the oxygen-less intervals, red silica would settle the ocean floors, then once the blue-green algae would repopulate the water and begin to produce free oxygen iron depositions would once again fall slowly to the ocean bottom, depositing the thick gray layers. Through this cyclical process, banded iron was formed!
Logging Requirements
Please send your answers to questions 1-3 to the CO via the messaging sytem or email. You do not have to wait for a response in order to claim your find.
- Based on the information provided in the description, do you think the banded iron in front of you is an example of a sedimentary, metamorphic or igneous rock? Why?
- Compare the size of the banding on the rock in front of you to the one depicted on the cache page. Are the bands significantly smaller, or larger? What does this tell you about the periodicity of cyclical oxygen changes in the ocean where this speciman formed?
- In your own words, describe why Iron bands could not form in today's oceans.
- Take a photo of yourself (or a personal object) in front of the park. Do not include the banded iron in your photo.
Resources