I headed into the country west of town on my 5.2-mile loop. I was feeling okay, not especially energetic, but enjoying the sunshine and cool, dry air. I made the northward turn onto Conway Rd. and shortly after Garmy chirped for two miles, I saw it. It was lying in the middle of the road, a dark circle. I was curious. My first thought was, is that a turtle? I went over to it and picked it up. It was not a turtle. It was something so, SO much better.
And now, for a geology lesson...
Banded iron formations (BIFs) are some of the earliest evidence we have for the presence of oxygen in the Earth's atmosphere. Only with oxygen can one get the red of iron oxide, as you can see in the photo above. BIFs are extremely old rocks; most are between 1.8–2.5 Ga (that's "giga-annum," a geologist's way of saying "billions of years"). Yes, that's right: the rock I'm holding in my hand is more than a BILLION YEARS OLD. That rock was around when the only lifeform on the planet was cyanobacteria (blue-green algae).
What do algae have to do with BIFs? EVERYTHING. Algae lived in near-surface waters of the Proterozoic oceans. Algae did then what they do now: photosynthesize. Life is pretty simple for algae. Take in sunlight, produce oxygen. The waste oxygen combined with free iron ions present in the ocean water to form an iron oxide mineral, most commonly manganite (Fe3O4). The oxidized iron sank to the sea floor and formed the red layers in a BIF. The dark gray layers are iron-poor, fine-grained silica (chert), which may have formed when the algae experienced mass die-offs. As the algal biomass expanded beyond the capacity for the available iron to neutralize the waste O2, the oxygen content of the sea water rose to toxic levels, resulting in large-scale extinction of the algae population and the accumulation of the iron-poor layer of silica on the sea floor. As time passed and algae populations re-established themselves, a new iron-rich layer began to accumulate. Repeat this scenario for a few hundred million years and you have a banded iron formation.
After I recovered from my initial shock and surprise at finding such an amazing rock, I nestled it into my hand and resumed running. The rock felt heavy (I later weighed it: 4 lbs) so I shifted it from hand to hand as my arms grew tired. You wouldn't think four pounds is a lot...until you have to carry it for 3 miles. Toward the end of the run I was moving it once a minute. It felt like it weighed ten pounds. I was so glad to finish my five miles. I immediately put water on the rock for the picture, because all rocks look better when wet.
In hashing, something one picks up on trail is "trail treasure." This was trail treasure of the best kind. Better than the Michigan hat, better than the big reusable bag that holds my farm share, better than the end of an aluminum billet. This is as great a rock find as the serpentinite I dug out of a Western Ireland beach when I was at field camp. This almost makes up for my ex-husband taking my rocks with him when he moved out. (Almost.)
My BIF will go into my special rock display area (what? you mean you don't have one?) with my serpentinite from Ireland, lava from Hawaii, jasper from Lake Superior, biotite from Bancroft, Ontario, fluorite from Colorado, and sea urchin shell from Maui.
BIF...you're my new BFF.