The Santa Maria volcano awoke with a loud bang in the year 1902 with a large Plinian eruption that left a large crater in it's place. The eruption of 1902 billed as a possible VEI 6 eruption possibly killed as many as 5000 people.
Photo: Lee Siebert. Santiaguito erupting beneath Santa Maria.
Now, however the activity is located at the Santiaguito Volcanic Dome Complex at the base of Santa Maria and within the crater created by the 1902 eruption. Eruptions at this location started in 1922 and have been continuing to the current day. This complex currently consists of four dacitic volcanic domes with only one that is currently active the Caliente Dome. Santiaguito is famous for its consistent one to two hour vulcanian events. Other activity which occurs are Pyroclastic Density Currents, Lahars and an occasional larger event than its usual vulcanisn bursts. The most recent of these larger events occurred in 2010 and created a large ash column with associated Pyroclastic Density Currents and crater collapsing. The Santa Maria/Santiaguito complex recieved its Decade Volcano status because of the large hazard it poses to the population in Guatemala and the lack of funds locally to monitor and study the volcano sufficiently.
For full details on Santiaguito and some interesting posts I thoroughly recommend the Magma Cum Laude blog.
I have posted a video below from youtube of a typical event at Sanitaguito, the video itself was taken from the slopes of Santa Maria.