What are the markings on this rock?

January 16, 2019 10:09 PM

What are the black drip markings on this rock? (Sorry no scale, 3-4 ft or so across) Rock is in Southern California, a few miles from the ocean in the hills north of San Diego.
posted by lemonade to Science & Nature (5 answers total)

Best answer: It looks like manganese oxide (MnO2) weathering out of the rock. The purplish color seen on some of the surfaces would also be that. These and these are also the same mineral, although a much finer pattern.
posted by Gneisskate at 10:30 PM on January 16, 2019 [9 favorites]

Best answer: Hello, I am a geologist, but I am not your geologist.

Agree with Genisskate that this is a weathering pattern - water reacting with the minerals to create stained patches following the grain of the stone (hence right angles to the edge rather than straight down from gravity).

+1 for geological handles :)
posted by sedimentary_deer at 1:23 AM on January 17, 2019 [8 favorites]

Best answer: Manganese oxide.

If you want to sound cool you can refer to that pattern as 'dendritic'.
posted by Green Eyed Monster at 5:47 AM on January 17, 2019 [1 favorite]

Response by poster: This is very cool, thank you! There are some other rocks with a more branching pattern like Gneisskate's second pic, so it makes more sense now. Follow on question...how do I learn about these things? Would it be in a geology-for-beginners type of book? :)
posted by lemonade at 1:00 PM on January 17, 2019

Best answer: Late reply, but I didn't see your second question until now. I would look for a book on rocks and minerals, as a basic geology book wouldn't go into enough depth on identification.
posted by Gneisskate at 1:23 PM on March 28, 2019