As the term metamorphism indicates, metamorphic rocks form through textural and/or mineralogical changes of a preexisting rock. These changes are caused by increases in temperature and/or pressure of the surrounding environment and/or the presence of hot, chemically active fluids. Metamorphic rocks, for the most part, form at considerable depths, given the amounts of temperature and/or pressure required to cause the textural and/or mineralogical changes in the preexisting rock.
When pressure and temperature change, chemical reactions occur to cause the minerals in the rock to change to an assemblage that is stable at the new pressure and temperature conditions. But, the process is complicated by such things as how the pressure is applied, the time over which the rock is subjected to the higher pressure and temperature, and whether or not there is a fluid phase present during metamorphism. Temperature increases with depth in the Earth along the ‘geothermal gradient’. Thus, higher temperature can occur by burial of rock. Temperature can also increase due to igneous intrusion in the vicinity of magma. Pressure increases with depth of burial, thus, both pressure and temperature will vary with depth in the Earth. Pressure is defined as force acting equally from all directions. It is a type of stress, called hydrostatic stress, or uniform stress. If the stress is not equal from all directions, then the stress is called a differential stress. If differential stress is present during metamorphism, it can have a profound effect on the texture of the rock. Rounded grains can become flattened in the direction of maximum stress. Minerals that crystallize or grow in the differential stress field can have a preferred orientation. This is especially true of the sheet silicate minerals (the micas: biotite and muscovite, chlorite, talc, and serpentine). These minerals will grow with their sheets orientated perpendicular to the direction of maximum stress. Preferred orientation of sheet silicates causes rocks to be easily broken along approximately parallel sheets. Such a structure is called foliation.
Gneiss (pronounced ‘nice’), a foliated metamorphic rock, is generally caused by high-grade metamorphism of clastic sedimentary rocks or light colored igneous rocks. As metamorphic grade increases, the sheet silicates become unstable, allowing dark colored minerals like hornblende and pyroxene to grow. These dark colored minerals tend to become segregated in distinct bands through the rock, giving the rock gneissic banding. Because the dark colored minerals tend to form elongated crystals, rather than sheet-like crystals, they still have a preferred orientation with their long directions perpendicular to the maximum differential stress.
This particular gneiss formation began as granodiorite, an igneous rock with composition intermediate between granite and diorite. It has been subject to low-grade metamorphism so the gneissic banding is not well developed. The rock hiding this geocache has been broken along a quartz vein. The quartz crystals cover one face of the rock. Notice the reddish hue of the rock adjacent to the quartz. That “baked zone” or “metamorphic halo” was caused by the intrusion of hot liquid magma that eventually cooled, forming quartz crystals.
I have a goal for 2014 of hiding caches on days when the month and date match. I hid one cache on 1/1, two caches on 2/2, and three caches on 3/3. With luck and hard work I can keep it up all year. This is one of my eight August hides.
Remember, have fun and write a nice log.