Tuesday, January 8, 2013
But of particular interest is the lighter line running horizontally across the upper portion of the fragment. As the magma cooled, the volatile component was excluded from the crystals, and was thus more and more concentrated in the melt- effectively driving gas pressure through the roof... literally. Toward the end of the crystallization, the vapor pressure got high enough to fracture the overlying solid rock, and the very last liquid bit of magma, called aplite, was driven out into the network of fine fractures. I've read these actually extend out into the overlying Tyee, though I've not seen that myself. The example above is fairly typical for what I've seen in the area: subtle, and only a mm or two across.
This was a spur-of-the moment choice when Dana visited in July- I wanted her to see the old growth noble fir forest on the mountain top (that'll be tomorrow's photo), and I've always thought the aplite dikelets were pretty cool. However, I'd never really thought to look for them in an organized way- I'd just find them occasionally hiking up the north side trail. It turns out, if you walk down the trail from the summit parking area, and keep an eye on the cobbles and pebbles of sill weathered out onto the ground, they're not that uncommon or hard to find.
Photo unprocessed. July 8, 2012. FlashEarth Location (approximate) Lens cap is ~52 mm.