Granite Intruding Granite
In this fifth installment of Grand Teton rocks from Cascade Canyon we have two igneous rocks, a fairly fine-grained granite and a very coarse-grained one. This means that one of these rocks intruded and cut through the other. One was solid while the other was still cutting through as liquid magma. The contact between them is quite sharp, which indicates that the older rock was fairly cold when the younger intruded. As to which one is the older and which is the intruder, the more coarse-grained rock gives a clue. Right at the margin of the contact, near my Leatherman tool, there are some crystals that appear to be growing out of the contact. This could be a coincidence, but it probably isn't. The grains in the more coarse-grained rock also get quite a bit larger away from the contact. These two observations are consistent with the coarser grained rock intruding as the magma, coming into contact with the older, colder rock, and first forming smaller crystals that grow out from the contact where the magma is cooling most quickly, then growing larger crystals just a few inches away as those early formed crystals provide a buffer for the heat transfer, allowing it to cool more slowly and grow larger crystals.
My observations and interpretations here could be wrong. In order to test this model, I would want to spend much more time inspecting the rocks for more clues, and obtain samples to look at under the microscope. And to really get a firm grip on the ages of these rocks geologists will date them using minerals that contain radioactive isotopes. Science moves forward by this process of observation, interpretation, and continued testing of models and hypotheses. A good scientist always asks him or herself "what if this idea is wrong?", and finds a way to check.
#GrandTetons #CascadeCanyon #GeologyRocks #Granite #NPS100 #mygtnp #FindYourPark #OptOutside