Gasp! EarthCache
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Here on the front steps of the Smithsonian Institution’s National Museum of Natural History, look to the left as you face the building. You'll see a red rock sitting on a platform. It's about 3 feet tall, cut on one side, and rough on the back side. Take a good look at this rock. No need to climb up on the ledge if you don’t want. The rock consists of red and gray layers. The rock is called a banded iron formation or its acronym, a BIF. The red chert (SiO2) layers are intermixed with gray, iron-rich hematite (Fe2O3).
This rock records a time in our Earth’s history that is very unlike today. On the earliest Earth, the atmosphere was poor in oxygen and the acidic oceans were rich in dissolved iron. Starting about 2.4 billion years ago, blue-green algae (photosynthetic cyanobacteria) in the oceans started to produce significantly more oxygen in the atmosphere. On what is thought to be a seasonal basis, warm, oxygen-rich waters from the surface were mixed with deeper, colder, iron-rich waters. This mixing produced layers of iron oxides (the gray hematite layers) deposited on top of silica-rich mud (the red chert). By about 1.8 billion years ago, atmospheric oxygen reached the point that few banded iron formation deposits could form. This period of increasing oxygen is often called the great oxygenation event. Other rocks formed during this same, oxygen-poor period include ore deposits containing gold, uraninite (UO2), pyrite (FeS2), and platinum metal. These minerals are unstable in today’s oxygen-rich atmosphere.
Another unique feature of the early Earth was much higher heat flow. Typical lava flows on Earth today erupt at temperatures near 1250 Celsius (about 2300 Fahrenheit). In contrast, a type of rock called komatiite erupted on the early Earth at temperatures of nearly 1700 Celsius. Komatiite lava flows were so hot that they would have appeared blue in color!
All of these rock types are preserved in the ancient cores of the continents, areas geologists call cratons. Cratons are the building blocks of the continental masses and preserve the earliest rocks. In many ways, cratons are like the Museum itself, places that preserve the history of our planet.
In order to claim this Earthcache, e-mail answers to the following questions taken from the bronze plaque on the platform holding the rock. Answer the following questions:
1) Where was the rock found (town, county, state)?
2) How old is this rock (in billions of years)?
Feel free to post a picture of yourself with the rock, but please don't post a picture of the plaque. Also feel free to visit the Hall of Geology, Gems and Minerals on the 2nd floor of the Museum. Hours are 10 a.m. to 5:30 p.m. every day except Christmas, with extended late hours during the summer months. Admission is free. With specimens ranging from the iconic Hope Diamond to a vial of diamonds formed during the explosions of ancient stars and trapped in a meteorite, the Hall of Geology, Gems and Minerals is a world-class cache of Earth rocks – and ones from other planets!
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