Mono Lake is a freshwater lake in the Eastern Sierras. The ground water enters the lake from below, through certain spots. As the mineral-rich ground water enters the lake, it deposits the minerals at the opening. These deposits end up forming a tube. As freshwater was taken from the lake for human uses and the water level dropped, these mineral structures, called "tufa", came to the surface.
A note about the sand: Having been to Maui, I was no stranger to black sand. After all, sand is just ground up rock, and if you start with lava (black), the sand will end up the same color (black). I was, however, surprised to see patches of black sand here, in a place not particularly known for volcanic eruptions. I was even more surprised when the patch moved as I approached it. Yes, the "black sand patch" was a carpet of mosquitoes enjoying the sun and awaiting the sunset. Fortunately for me, they were as laid back as the average Californian hippie. I was briefly chased by a swarm near the sunset (made it to the car), but mostly they seemed content with chilling on the beach.