Subhedral olivine grains altered to red iddingsite; surrounded by plagioclase grains and volcanic glass. Basaltic lava sample from Mount Bachelor Volcano.
That time of year
As sunlight begins to decrease day to day in the northern hemisphere, plants begin making less chlorophyll in their leaves, removing the source of their green color. Great Sand Dunes National Park in Colorado sits at elevations between 2000 and 4000 meters (7500 to 13000 feet). This picture captures the Aspen trees on the slopes above the sand dunes; along with Alaska and Canada these are some of the first trees on the North American continent starting to change color for the fall.
Image credit: NPS
On the geology of these dunes: