Having had quite a lot of fun with the last five minute geology session, and having a rather dramatic lack of time to actually write about geology, I thought we'd do another. My Facebook friends are an endless font of prompts. This time, they sorted themselves loosely in a fire and ice theme, with a bit of iron thrown in.

Steven Raine: Maars

Maars are some of the most awesome volcanoes of all time. They basically look like meteor craters, but they're entirely earth-made. They're what happen when you get an eruption through groundwater.

I had no idea they even existed before I started traipsing through Oregon with Lockwood DeWitt and he took us to see Hole-in-the-Ground, which is an absolutely marvelous maar out in the middle of nowhere. It's just a perfect bowl-shaped crater that you'd swear was created by a meteorite. But nope. Purely local. That area of Oregon is pocked with all sorts of volcanic landforms caused by volcanoes doing really odd things in the Pleistocene.

See, back then, that part of Oregon was covered in shallow lakes. Shallow water + volcanism = neato formations!

I later found out there are maars in Arizona, which both delighted and saddened me. I haven't been back to see them since I left home. Knowing I was within easy driving distance of a whole different type of volcano, but never saw it, makes me itch to go back home. Which I will, because you all need to visit the San Francisco Volcanic Field with me at a time when I have a camera capable enough to do it justice.

If you get a chance to go see a maar, absolutely do. And report back!

Jay Sinclair: Banded Iron Formation

Banded Iron Formations, or BIFs for short, are some of the weirdest and most wonderful geological formations on earth. They're one of those wild things from our past: the conditions don't exist for them to form today. You need a lot of iron and not so much oxygen so you don't end up with mere rust.

I'm fuzzy on the specifics, but not fuzzy at all on how lovely they are. Banded iron formations created some of the most intriguing rocks on earth. I haven't been able to see those massive deposits in the field, but I have seen hand samples, and they are wonderful. The colors and the patterns aren't usually ostentatious, but they're quite beautiful.

BIFs gave Lockwood's kitty his name. You can see the resemblance!

Tommy Thompson: Gneiss. Very gneiss.

Gneiss IS very nice! I love metamorphic rocks of all sort, but gneiss is in a class of its own: it's one of the first I was able to reliably recognize in the wild.

Gneiss is a streaky rock that can form either from sedimentary rocks or igneous ones. It forms under the roots of mountains, mostly, where temperatures and pressures are mind-boggling.

My favorite gneiss is, happily, one that can be found in abundance quite nearby: orthogneiss. It's a salt-and-pepper colored rock that is unbelievably shiny on fresh surfaces. Enormous dark biotite and hornblende crystals gleam and twinkle like little shards of midnight captured in stone. It's easy to recognize in the field: in addition to being a dramatic, glittery black and white, it's got a very distinct pattern. Look at it from the side, and the black minerals are long streaks. Look at it head-on, and the streaks turn into points. This is an excellent rock to start new geologists on!

Susan Algren Van Sant: Moraine

Moraines! These are things I barely knew existed back in Arizona, although if I'd spent more time on the San Francisco Peaks, I could have gotten to know one.

Moraines are some of the best glacial features of all time, in my opinion. You may be partial to U-shaped valleys and other dramatic features carved from the ice, but I adore these gargantuan piles of rubble. Have you seen how they form? Just a bunch of debris carried along and arranged by the ice, and left behind when the ice melts.

There are a few different types of moraine, and each one can tell you things about a glacier long vanished. There are lateral moraines, which form alongside the glacier. There are medial moraines, which form down the middle. And there are terminal moraines, which are deposited at the end of the glacier. We know a lot about where Pleistocene ice sheets went and what their extent was by the moraines they left behind.

They are also immense sources of quality sand and gravel for construction purposes, so they can be commercially important. The largest ones form lovely local hills, and in generally flat areas like Illinois and Indiana, may be the only appreciable elevation for miles around. Large or small, long or short, along or across, moraines are some of the neatest glacial geology ever!