volcano erupted without warning at around 6 pm local time with 1,500 people forced to leave their homes .
residents described people crying in the streets as they fled in the aftermath of the ` apocalypse-like ' eruption .
it is the first time the volcano has been active since 1972 , and the first major eruption there since 1961 .
the plume of ash and smoke blanketed the sky and was visible in towns up to 100 miles away in argentina .