Ash was the powder left of something after it had burned.
The Year Without a Summer in 1816 was blamed on volcanic ash, however, the Thirteenth Doctor pondered whether it was really something else which caused it. (TV: The Haunting of Villa Diodati)
The soil of the petrified jungle on Skaro turned to sand and ashes due to radiation. (TV: The Daleks)
Zolfa-Thura was reduced to sand and ashes after a war. (TV: Meglos)
The "snow" that fell in London on Christmas 2006 was actually ash from the remnants of the Sycorax spaceship. (TV: The Christmas Invasion)
The Third Doctor described Earth in the 30th century as "slag, ash, clinker." (TV: The Mutants)