Banded Iron - A Record of Ancient Seas
As you reach the posted coordinates, you'll find yourself standing on the sidewalk of the Smithsonian Natural History Museum. This museum is the nation's premier collection of rocks, fossils, minerals, fauna, flora, and all things natural - the Smithsonian Museum of Natural History is a must see while visiting the Nation's Capitol! For more information on hours of admission, feel free to visit their website.
As you gaze up the steps you'll notice two displays out in front of the building. On your right you'll find a set of petrified tree trunks dating back nearly 200 million years while on the left you'll see a significantly older formation. For more information on the petrified wood, I recommend completing GC2HZW7 - A Head and a Piece of Wood. Today we'll be focusing our lesson on the much older formation on the left - The Banded Iron Formation, or BIF for short.
Please do not climb or touch either exhibit! Thank you!
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. To the right you'll see an example of unpolished banded iron from Africa. The iconic banding make this an easily recognizable deposit. The paler layers of the rock indicate a silty chert (fine grained) run-off rock while the darker hued layers are hemetite and magneitite - two crystaline forms of Iron; hence the name "Banded Iron."
An Indicator for Ancient Oceans
The lighter and darker layers of sediment were deposited approximately 2.25 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 email 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, however if answers to the logging questions are not recieved in a timely manner, your log may be subject to deletion. I will always reach out to you before deleting your log to provide a warning. Thank you for your cooperation!
- 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 Natural History Museum. Do not include the banded iron in your photo. (Please note this is a mandatory requirement and is permitted under the update to the earthcaching guidelines of 10 JUL 19)
Resources
A note to finders: From 2013 to 2018 the Smithsonian operated a GeoTour which included an earthcache on this topic in this location. In order to claim a find on this cache you MUST visit the location again. As noted in the logging section, a photo of you (or a personal item) in front of the Smithsonian is a requirement for logging this earthcache. Thank you for your cooperation.
Note: This cache has been placed with permission of Smithsonian Staff. Always ensure you have permission before placing an Earthcache!