Outside The Interzone: Apr 26, 2015

Saturday, May 2, 2015

Geo 1095: May 2, Day 852: Joint Surveillance

Here we're looking basically perpendicularly into another pair of approximately parallel joint planes. The lens cap (52 mm diameter) is resting against the farther joint, and there a similar plane in the upper right quadrant. It should be clear, even in this shrunken format, that the rock is chock-a-block with a myriad of little holes. These are gas bubbles, referred to as vesicles, that were created as the original lava depressurized and cooled, just like bubbles forming in a carbonated drink after you pop the top on the can or bottle. The texture is called vesicular, and is quite common in volcanic rock. One might suppose that such a texture would also allow water to percolate into and through the rock, but the problem is that the bubbles (in most cases) aren't connected enough to permit flow except at very low rates- the distinction is described by "porosity" versus "permeability." Porosity refers to the amount of void space, while permeability refers to the degree that porosity permits flow-through. Vesicles increase porosity, but not permeability; joints increase both.

Photo unmodified. July 6, 2013. FlashEarth location.

Friday, May 1, 2015

Geo 1095: May 1, Day 851: Lichen the Joints

As I indicated yesterday, joints cutting through more or less solid, blocky rock are crucial to understanding how the enormous amounts of precipitation that falls in the High Cascades as rain and snow simply disappear over the many square miles that comprise this plateau. This is a surficial feature, and I'd guess the deeper buried joints don't tend to be as wide, but clearly, even at a fraction of this width, such fractures would allow rain and snow melt to easily trickle down to the water table. There's a bit of a mystery here, as it has become clear that the hydrologic divide in this area is not congruent with the topographic divide. In most environments, ridge crests mark the boundary from which water flows into one basin or another, but this is not the case here. Water is clearly flowing, hidden underground, across the Cascade crest, predominantly from west to east. I'll clarify and expand on that later.

Another interesting thing I notice in this photo is that lichens seem to be present in the shaded portions of the shot, but absent in the areas that are fully sunlit. On one hand, that may be just a fluke of this shot and the time of day I took it. On the other, it makes sense to me that the scorching heat of dry basalt in full sunlight would make it much more difficult for lichens and other colonizing organisms to become established. So areas that are shaded, at least sometimes, during the warm and dry season are easier for the lichens to live upon.

Photo unmodified. July 6, 2013. FlashEarth location.

Thursday, April 30, 2015

Geo 1095: April 30, Day 850: Aa

"Aa" (pronounced "ah-ah") lava is rubbly and often jagged on the surface. This "texture" forms as somewhat viscous, or pasty, lava flows cool and solidify on their surface, but are disrupted and broken as the still molten interior continues to flow, dragging the exterior along for the ride. This implies that the rubble we see at ground level should give way to more or less solid rock at some depth below. There are some magic asterisks after "solid rock," though. First, as the deeper lava cools and shrinks, it will develop fractures called joints to relieve the tension caused by decreasing volume. Second, lava flows like this often move like a bulldozer or tank tread: the upper surface moves forward and rolls over at the front of the flow, then stabilzes, fixed on the ground where it came to rest. That rollover process further breaks up the cool exterior and creates a bed of broken rock called a "basal breccia." Finally, in an active volcanic area like McKenzie Pass, there are flows on flows on flows, so if you were to dig a shaft or borehole here, you'd expect to see that sequence of {aa-> solid with joints-> basal breccia} repeat many times. I don't know that the aa and basal breccia would necessarily be easily distinguishable, though, so it would probably look more like {breccia->solid with joints} repeatedly.

This is key to understanding all the gushing water we saw in the upper McKenzie Valley...

Photo unmodified. July 6, 2013. FlashEarth location.

Wednesday, April 29, 2015

Geo 1095: April 29, Day 849: Anne & Chris Again

This is almost a repeat of yesterday's. I try to avoid that by poking around nearby shots before selecting the one I like best. Apparently I didn't do that yesterday, but I like this one better, and they haven't seen it. My blog, my rules. North (closer) and Middle Sisters in the distance.

Photo unmodified. July 6, 2013. FlashEarth location.

Tuesday, April 28, 2015

Geo 1095: April 28, Day 848:Anne, Chris, Dee Wright and (Two of) Three Sisters

After leaving the Deer Creek area, we headed up to McKenzie Pass to take in the views from Dee Wright Volcano Observatory. Here we see Anne and Chris on the upper platform, with North and Middle Sisters as a backdrop on the left. (See this post for all four of our merry band of geotrippers. If you can't tell, that's me on the left and Dana on the right.)

I've already posted a number of panoramas of the views from atop the observatory (See July 9, July 10, and July 12 of the Geo 365 series), so the coming posts will focus on some of the nearer features here.

Photo unmodified. July 6, 2013. FlashEarth location.

Geo 1095: April 27, Day 847: Deer Creek Bridge

Looking upstream from Deer Creek Hot Spring to the bridge, this shot gives some context for how to find the spring. There's some parking near the west (left, from this perspective) end of the bridge, and some intertwining footpaths from the road down to the riverside here. I suspect the line of cobbles in the lower left were purposely placed to divert the cold river water around the warm puddle of the hot spring.

Photo by Dana Hunter, unmodified. July 7, 2013. FlashEarth Location.

Sunday, April 26, 2015

Geo 1095: April 26, Day 846: Deer Creek Hot Spring

The other feature to visit from Deer Creek Bridge is a nearby hot spring, maybe a hundred yards downstream from that crossing. It's at the river's edge, and as far as I'm concerned, not terribly appealing in terms of bathing. It's shallow, kind of yucky looking, and smells strongly of sulfur. Now, again, IMO, this doesn't make it less interesting or worthy of inspection, just not a hot spring I'm inclined to get into, physically.

On the other hand, I know a number of people who have, and they enjoyed it. Whatever floats your boat. However, if you're the sort of person who's discomfited by having a bunch of geology types come gawking at the spring while you're soaking in it nude, you might want to think about some sort of bathing suit or rethinking your choice about where to stew. This is a well-known, easily accessible, and fully public location. Strangers may show up at any time... clothed or not.

While hot springs in and of themselves are uncommon enough to be intrinsically interesting, this one likely marks a permeable region along the western fault (system?) of the High Cascades Graben. There are a number of hot springs along this trench, some of which are developed, others, like this one, just warm puddles in the woods.

Photo by Dana Hunter, unmodified. July 7, 2013. FlashEarth Location.