By Mary Drew at Pasty Central (Mdrew) on Friday, August 21, 2020 - 11:40 pm:
Geology Lesson #3, with Capt. Paul and Dr. Nat (also known as Paul and Nathalie Brandes), this one near the Dead River Storage Basin, north of Ishpeming. I’m not going to pretend that I know anything about this subject, so today’s notes are directly quoted from Capt. Paul:
“The first three photos, show a kind of rock called greenstone. The name comes from the green colour these rocks take on during metamorphism of the minerals within the rock. Greenstones tend to form in belts which are zones of metamorphosed mafic to ultramafic volcanic rocks and occur within Archean and Proterozoic cratons. They tend to be associated with ancient ocean spreading centers (think Mid-Atlantic Ridge) and island arc terranes.
Another interesting feature on the Greenstone rock are noticeable “scratches” on the surface in two different directions. These scratches were created as glacial ice scraped across the surface, leaving these marks. What is interesting is that I’m not sure which glaciation caused these, as not only was there the most recent ice age that ended 10,000 years ago, but also another glaciation that occurred here some 2.1 billion years ago. I don’t know the answer.
Along the north side of the Basin near Smith Creek are these “pillows”. Pillows are created when molten lava is erupted underwater and cools very quickly, forming these teardrop shaped formations. The last three photos show these pillows in detail.”
Dr. Nat is going to give us more details on the Archaean pillow basalts in the Dead River Basin. Pay attention now, there may be a test next week! Just kidding...but do stay tuned for more Geology lessons next week.
