Puyehue ("poo-yay-way") volcano in the Chilean Andes of South America started erupting last week on June 3. This area had a small eruption in 1990 but nothing major since 1960. It's part of a complex of volcanoes, called the Puyehue-Cordón Caulle Volcanic Complex (PCCVC), located almost 1,000 km (620 miles) south of Santiago (40°35'25"S, 072°07'02"W).
Due to the sharp increase in seismicity (earthquakes) in the area, geophysicists were monitoring the volcano and were able to issue a warning that it was about to erupt.
There are some amazing images over at the Boston Globe's The Big Picture and The Atlantic's In Focus with Alan Taylor. Check them out.
The ash plume reached of 10 km (6 mi) into the sky. As the ash was carried eastward, by the prevailing winds, it settled like snow up to 30 cm (1 ft) thick on the ground in places.
Airports as far away as Buenos Aires in Argentina were closed down as the ash clouds drifted out over the Atlantic Ocean. Here's the volcanic plume as seen from NASA's Aqua satellite on June 4.
The geologic details of the Puyehue-Cordón Caulle Volcanic Complex (PCCVC) are, to use the same word twice in a slightly different way, complex. I'm not familiar with the details beyond the facts that the area has had a complicated history of eruptions along a large fault zone of a wide variety of magma compositions dating back some 300,000 years (well within the Pleistocene Ice Age).