Only Luzon and Mindanao have extensive lowland areas. The Pacific floor under the Philippines is unstable, leading to frequent earthquakes and volcanic activity. The 1991 eruption of Mount Pinatubo, in central Luzon, was one of the worst volcanic eruptions of the century. Rivers are generally short, swift flowing, and seasonal. Most important are the Pampanga, Agno, Cagayan, and Pasig on Luzon and the Angusan and Mindanao on Mindanao.
Temperatures vary more between day and night and high and low altitudes than between seasons.
The eastern part of the country gets most of its rainfall between October and April from the winter or northeast monsoon. By contrast, the western portion receives most rain between June and November and is primarily affected by the summer, or southwest, monsoon. The mountains affect the amount of rainfall deposited by the monsoons. Windward areas receive heavy rains, whereas the leeward areas are considerably drier. Typhoons are common.