Construction of the volcanic edifice occurred in four phases, from 103–72, 72, 60–46, and 35–25 ka,[5][6] building up the large cone that reaches about 1,400 metres (4,600 ft) above the plain on which the nearby city of Quetzaltenango sits.

Following the cone-building eruptions, activity seems to have changed to a pattern of long periods of repose followed by the emission of small lava flows from vents on the mountain.

The eruption began on 24 October, and the largest explosions occurred over the following two days, ejecting an estimated 8 cubic kilometres (1.9 cu mi) of magma.

The wind then changed from the south to the east and ashes began to fall at Helvetia, a coffee plantation 10 kilometres (6 mi) southwest.

Although most of Santiaguito's eruptive activity has been relatively gentle, occasional larger explosions have occurred.

In 1929 part of the dome collapsed, generating pyroclastic flows which killed anywhere from several hundred up to 5,000 people.

[19] Occasional rockfalls have generated smaller pyroclastic flows, and vertical eruptions of ash to heights of a few kilometres above the dome are common.

[20] Santiaguito is now a tourist attraction, as minor eruptions occur with regularity and can be observed by visitors from the summit of Santa Maria.

The areas to the south of Santa María are considerably affected by volcanic activity at Santiaguito.

The town of El Palmar, 10 kilometres (6 mi) from Santiaguito, has been destroyed twice by lahars from Santiaguito forcing the town to be moved to the present Nuevo El Palmar, and infrastructure such as roads and bridges have been repeatedly damaged.

Much shorter lava flows occur during the longer periods of low volcanic activity.

The 1902 crater has left the southern flank of the mountain above Santiaguito highly over-steepened, and a large earthquake or eruption from Santiaguito could trigger a huge landslide, which might cover up to 100 square kilometres (39 sq mi).