The prevailing wisdom is that Mars has long been a stable planet. Without tectonic plates, it didn't have the violent upheavals that lead to earthquakes and explosive volcanoes here on Earth... did it? Not so fast. Scientists combing over data from NASA's Curiosity rover have found evidence suggesting that explosive volcanoes were a reality in the past. Samples from the planet's Gale crater include high concentrations of the mineral tridymite, which you tend to see around explosive volcanoes here on Earth. If so, the material was most likely carried from volcanoes to the crater by ancient water streams.