Final answer:
A series of small earthquakes signaled the awakening of Mount Pinatubo, an event that clearly indicated the volcano's increasing activity and led to timely evacuations, potentially saving thousands of lives.
Step-by-step explanation:
The activity that signaled the awakening of Mount Pinatubo from a 500-year-long slumber was a series of small earthquakes. These tremors were early warnings that the volcano was becoming increasingly active after centuries of dormancy. Luzon's Mount Pinatubo is an active stratovolcano, and its awakening frightened residents and attracted the attention of volcanologists. The pre-eruption seismic activity provided critical alerts that ultimately resulted in the evacuation of thousands of people and undoubtedly saved many lives. This pattern of increasing seismic activity is not uncommon and is a well-known precursor to volcanic eruptions, as similarly witnessed with other volcanoes, such as Mount St. Helens.
When Mount Pinatubo erupted massively in 1991, it was the second-largest volcanic eruption of the twentieth century, following Alaska's 1912 Novarupta eruption. The eruption had severe environmental and economic impacts on the Philippines, including a significant drop in global temperatures, voluminous ashfall, and the coinciding damages of a powerful typhoon hitting the region at the same time, highlighting the destructive capacity of volcanic eruptions.