Changes on the Martian surface are detected by imaging the same area more than once. In this image acquired on May 13, 2018, NASA's Mars Reconnaissance Orbiter observes several new dust avalanches on the slopes of ridges within the Olympus Mons Aureole. These changes occurred within six years. (Also see the animated GIF).
Dust avalanches create slope streaks that expose darker materials usually hidden below a lighter-toned layer. Cascading fine-grained material easily diverts around boulders or alters direction when encountering a change in slope (see the top right corner of the first close-up). The dark steak in another close-up is approximately 1 kilometer in length that not seen in a previous image. Past avalanche sites are still visible and fading slowly as dust settles out of the atmosphere and is deposited on the dark streaks over time.
More information is available at https://photojournal.jpl.nasa.gov/catalog/PIA22595