Here's a pretty cool outcrop I found as we were leaving Cotopaxi National Park in Ecuador (in early January). I've got two small photos taken laterally on different parts of the outcrop (exposed by a stream), and then I follow those with two close-up crops, showing the details. I've posted the full-size versions of the first two photos on Flickr, so you can click through if you want more details. The zoomed-in shots are displayed here at the same size you'll find on Flickr.





What's going on here? It looks like we've got a series of thinner, relatively fine-grained layers below, topped off with a massive, poorly-sorted layer. The lower layers are all ash- and lapilli-sized grains, each stratum pretty well sorted. The upper layer consists of all kinds of different-sized chunks, including some boulders, "floating" in a really fine-grained matrix. Check it out:





I interpret this as a series of volcanic ash-(& lapilli-)falls that were then buried beneath a lahar, a volcanic mudflow. The lahar's slurry-like consistency was capable of transporting really large clasts, and when it slowed down, it set up like nature's concrete.

I think this is pretty spectacular stuff.

Labels: ecuador, igneous, mass wasting, south america, stratigraphy, travel, volcano