What purpose did the Zoroastrians build a towers of silence for?

The Zoroastrians used the Towers of Silence as open burial pits, placing their deceased relatives in rows so their bodies would be feasted upon by birds of prey, usually vultures. The doctrinal rationale for exposure is to avoid contact with Earth or Fire, both of which are considered sacred in the Zoroastrian religion. Contaminating the elements (Earth, Air, Fire and Water) with decaying matter such as a corpse is considered sacrilege. Orthodox Zoroastrians continued to maintain a tower until the 1970s when ritual exposure was prohibited by law.