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One of the more interesting phenomenoms encountered on Mars by Spirit is the existance of mini dust storms called "dust devils." Here's a movie of one captured in 2007.
The "Home Plate" area investigated by Spirit showed evidence of volcanos as it showed coarse grains that were likely spread by an explosion falling on top of finer material in this enhanced image.
Bedrock shown in this 2004 image shows a complex geological history. Some of them show signs of water erosion.