10 August 2010

Metamorphosed graded bed

Posted by Callan Bentley

This is the coolest thing I’ve seen this week: a graded bed metamorphosed via Acadian mountain building:

The graded bed starts at the Swiss army knife at left, where you see an abrupt transition between coarse grained metamorphic porphyroblasts (“pseudo-andalusites”) and finer grained quartzite. This was once a mud to sand transition when these were loose sediments in the Kronos Sea, but with elevated temperature and pressure, the clay minerals in the mud reacted to grow elegant porphyroblasts of andalusite and sillimanite. The sand was made of quartz: less reactive stuff, and all it did was fuse together when metamorphosed. I love this: the coarse to fine relationship in the original graded bed is flipped on it’s head by metamorphism. Follow the bed “up” (to the right), and you will see the quartzite grade into andalusite-dominated former mudstone. Pretty sweet, eh?

This example is from the summit area of Mount Washington, New Hampshire. More fun stuff when I get back to my home computer next Monday.