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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.
n 2005, Spirit revealed this image of an area that contained sulfur and possibly a trace of water. It's most likely a volcanic deposit formed around gas vents.