Final answer:
The explosive eruption at Chaiten Volcano in 2008 was likely caused by a sudden magma rise, fracturing the rock and allowing steam to build up pressure until an eruption occurred.
Step-by-step explanation:
The explosive eruption at Chaiten Volcano in 2008 was most likely triggered by a combination of geological processes that are common precursors to volcanic activity. Specifically, these can include a sudden magma rise, which is often associated with pressure changes within the Earth's crust. This type of magma movement can fracture the rock above and allow water to seep in. When this water is heated by the magma, it can turn into steam, which may be trapped underneath a layer of caprock. As the pressure from the steam builds up, it can eventually lead to an explosive eruption should the rock give way.
In general, volcanic eruptions like the one at Chaiten can be triggered by factors such as earthquake shocks that destabilize the rock structure, pressure drop caused by fissures opening, or steam buildup underground as heated water turns into steam and can't escape. In the Chaiten case, while an earthquake could contribute to the instability of the volcano, it was not reported as the direct cause of the 2008 eruption.