There are two known ways that such silica-rich soil – which is 90% pure silicon dioxide – could have formed. Water heated by subsurface volcanic activity, with lots of silica dissolved in it, could have percolated up into the soil, and then as it evaporated left the silica behind.The find has resulted in the area where the silica was uncovered being christened "silica valley." The New Scientist says NASA is now working to figure out the composition of the remaining portion of the soil that isn't silica in order to determine how the mineral was formed.
Or hot, highly acidic steam from a volcanic eruption – essentially concentrated sulfuric acid – could have rained down on soil that contained a variety of minerals, and leached away everything except the silica. Both mechanisms occur on Earth: the action of acidic steam is seen around fumaroles in places such as Yellowstone, and the water percolating through volcanic soil is common around volcanoes in Hawaii.
Either way, there was a lot of water involved. "Both of these involve substantial interactions of water with hot volcanic material," [rover lead scientist Steven Squyres] said.