Iron does not burn on heating but iron filings burn vigorously when sprinkled in the flame of the burner. why??

There could be two possibilities for vigorous burning of iron fillings:

• The surface area of the fillings is much more than a block of iron.
• The heat supplied to the iron fillings does not have a way to distribute itself through conduction, like in iron block, as the entire filling is surrounded by the flame, and thus, the iron fillings reach their ignition temperature but iron block does not.