An aerial view of Grand Falls and the dune field that we visited. Grand Falls is indicated by the marker. You can clearly see where lava blocked the previous course of the river.
(This is day 6 of a week-long field trip in
An aerial view of Grand Falls and the dune field that we visited. Grand Falls is indicated by the marker. You can clearly see where lava blocked the previous course of the river.
(This is day 6 of a week-long field trip in Arizona. Get caught up with days 1, 2, 3, 4, and 5.)
Today we visited Grand Falls and the nearby dune field. Grand Falls is especially interesting because it combines many of the processes that are active in shaping planetary surfaces. The falls are the result of a huge lava flow pouring into the ancient canyon of the Little Colorado River, filling the canyon and flowing both up and downstream for many miles. Obviously this had quite an impact on the river! It formed a dam and a lake upstream until finally the lake spilled over the top of the lava dam and began carving a new course for the river. Basalts and other lava rocks are very hard compared to the Kaibab limestone and Moenkopi siltstones of the original canyon, so the huge tongue of lava is preserved, by the river is currently working on carving a path around it in the softer rocks. The result is Grand Falls:
On the left you can see the dark, erosion resistant basalt that dammed the original canyon. On the right, Grand Falls are busy carving a new course for the river into the softer rock around the obstruction.
A mosaic of Grand Falls. Huge blocks of limestone sit at the bottom of the falls, showing that the falls are a powerful erosive force. (Click for full-resolution)
The other main point of interest at Grand Falls were the interesting patterns of cracks in the massive lava rock. These cracks, of "joints" tend to form perpendicular to surfaces in the flow that have the same temperature. In very simple flows, the joins often are vertical and the rock looks like it is made out of hexagonal columns. At Grand Falls, the joints are mostly very jumbled, which probably means that steam was percolating through the rock as it cooled. This might mean that there was water in the river when the canyon was filled with lava (the Little Colorado doesn't always have running water in it).
Columnar joints in the basalt flow at Grand Falls
This is one of the more ordered sets of joints; in many places there is no clear texture to the rock, suggesting complicated interactions with water as the lava cooled. Note that the normally-black basalt is stained tan-colored by the silt-bearing mist from the falls, yet more evidence that they are 