This was for a competition
These are the commands to be ran on a clock, in that order:
/execute @a ~ ~ ~ detect ~ ~-1 ~ dirt 0 execute @a[c=1] ~ ~ ~ detect ~ ~-2 ~ air 0 scoreboard players set @a dangerBlock 1
/execute @a[score_dangerBlock_min=1] ~ ~ ~ /fill ~ ~-1 ~ ~ ~-1 ~ air 0 replace
/execute @a[score_dangerBlock_min=1] ~ ~ ~ /summon ItemFrame ~ ~-1 ~
/execute @a[score_dangerBlock_min=1] ~ ~ ~ /execute @e[type=ItemFrame,c=1] ~ ~ ~ /summon FallingSand ~ ~ ~.5 {Block:"minecraft:dirt",Time:1}
/execute @a[score_dangerBlock_min=1] ~ ~ ~ kill @e[type=ItemFrame,c=1]
/scoreboard players set @a dangerBlock 0

You need to run this command first:
/scoreboard objectives add dangerBlock dummy