Thursday, December 4, 2008

Secret Tunnels

This is the entrance to the ice caves north of Rexburg; they're basaltic lava flow tubes. As lava flows, the outer layer exposed to air cools more rapidly than the molten material in the flow's interior. A basaltic rind forms, successive flows add layers to the flow tube walls, and the lava in the tube shallows as the flow decreases to a stop. This flow tube has since been percolated by water which, in Idaho's climate, turns to ice. Most of the cave floor is covered in thick ice layers. The combination of ice and basalt flow textures offered a double-helping of fascinating geology. The ice formed stalactites, stalagmites, and dendritic "feathery" snowflake-like crystals. There were also some neat flow layers and fractures in the ice. Here's a shot of ice crystals covering the ceiling:
Beautiful basaltic textures:
Me and the roomies (Kyle, Parley, me, Tom, and Trevik). At this point, the tube was segmented into an upper and lower passage; as the flow level in the tube dropped, the surface cooled and formed a shelf across the larger tube. That shelf is where we're perched for the photo.
Here's a shot of the ice-covered floor. Half the time, we were on our backs sliding along, pushing with our hands or feet. I came away from this cave with some bruised knees and elbows and a few scrapes on my fingers -totally worth it.

Monday, December 1, 2008

Over the Thanksgiving Break

I travelled with a friend back to Arizona for the Break. She was nice enough to stop for her rock-geek passenger to see some ash fall and tuff units. This road cut is on the stretch of Highway 20 that connects I-15 with Highway 89, just southwest of Beaver, Utah. The geological map I looked at labeled volcanics of the region as Bullion Canyon volcanics. The lighter-colored units are ash deposits and the massive darker units are rhyolitic tuff.

The ash units contain concretions.

A real treat on the way back north was the slight detour to see horseshoe bend just outside of Page, Arizona. It's a very photogenic meander in the Colorado River. We got there just as light was fading after the sun had set. No, this picture isn't mine. But the one below it is. The meander is so deep that it's difficult to get the whole thing into one shot. I may try to piece my photos into a panorama.

Igneous Rock Exam

The pictures are cruddy. Fluorescent lighting will do it almost every time.
Last week's Igneous Rock ID exam went well. I went into the exam feeling pretty comfortable, and left it feeling like I had done well.
Today, I got my ID portion of the exam back; 98%.
I once again give credit to David Morris for a job well-done. What would school be like without that firm groundwork of understanding from David?I shudder to think.

Some of the samples from the exam