Since I grew up spelling it D-I-K-E, that is how I'll refer to it in this Description. If you like spelling it the other way, then fine. I won't hold it against you ;)
You are standing on an area known as the Atlanta Lobe of the Idaho Batholith. In a general sense, the dikes of the Atlanta Lobe trend in a NE fashion from Boise to the vicinity of Butte, Montana. These dikes intruded on the batholith (the large body of granitic rock that covers the middle part of the State), during the Eocene Epoch, some 50 million years ago. The composition of the dikes varies, depending on where you are on the batholith. Some of the rocks that make up the dikes can be dark, which is thought to be derived from basalts from the Earth's mantle. Other dikes can be lighter in color, like those in this area, which are generally similar to rhyolites.
Because dikes intrude relatively cool country rocks, they frequently display a chilled margin, with grain size becoming coarser towards the center where the rate of cooling has been slower. If the dike cooled very slowly at great depth, the large crystals of pegmatite dikes have had time to form.
Pegmatite Dikes
Pegmatite dikes are formed under conditions of a water rich fluid, and therefore exhibit large grain sizes. Most pegmatites contain quartz, alkali feldspar, micas, and tourmaline. However, some pegmatites contain minerals such as tourmaline, garnets, apatite, beryl, topaz, spodumene, magnetite, sphene , and zircon, and various other rare minerals. The occurrence of these rare minerals is due to the slow crystallization process that pegmatite dikes undergo, which are normally removed if the crystallization process occurs much faster.
Aplite Dikes
Aplite dikes tend to be light in color, and uniform in grain size. Aplites form after most of the crystallization of the host rock is complete, so aplites tend to be rich in quartz and feldspar.
The dikes on the outcrop at the posted coordinates, tend to be on the smaller side, maybe only an inch or two in width.
Logging Requirements:
- Are the dikes on the outcrop linear, or do they crisscross each other? Can this tell you anything about how the dikes were layed down during the crystallization process?
- Are the grain sizes uniform, or do they vary.
- If you continue up the trail to Boulder Lake, you may notice that the dikes get larger as you increase in elevation. Can you think of a reason for this?
- Do you think these dikes are Pegmatite or Aplite based on your observations?
References:
"Salmon River Suture Zone", Matt McKay, University of Alabama Research, 2010
Lund, Karen, 2004, Geology of the Payette National Forest and vicinity, west-central Idaho: U.S. Geological Survey Professional Paper
Digital Atlas of Idaho, Idaho State University, Dr. Paul Link
Roadside Geology of Idaho, David D. Alt, Donald W. Hyndman