From the air the Philippines is seen as a series of mountain slopes softened by tropical growth and interrupted by cultivated plains and terraces, as well as by inland seas and channels. Except in Luzon and Mindanao, no portion of the archipelago is much more than 50 miles (80 km) from the water. It is a land whose mineral resources are inadequately mapped, but whose volcanic soil is constantly being eroded because of reckless farming methods and deforestation.
The Philippine Islands are the tops of underwater mountains formed by outpourings of molten materials from the earth's interior. Consequently, igneous rock appears throughout most of the archipelago. Submergence of the entire area, after formation of these mountains, resulted in the deposit of various marine sediments over the lava underlay. The process of mountain forming has not stopped, as is indicated by recurring earth tremors and volcanic action. One of the most unstable parts of the earth's crust, the Philippines lies between the continental periphery of Southeast Asia and the Philippine (Mindanao) Trench. Descending 34,440 feet (10,497 meters) below sea level, the Philippine Trench is one of the deepest parts of any ocean.
Large expanses of level land are rare, although it has been estimated that with terracing nearly half of the Philippines is potentially arable. The largest lowlands are on Luzon and Mindanao. Foremost is the Central Plain in western Luzon, which extends over 100 miles (160 km) from Lingayen Gulf south to Manila Bay. The plain continues southward into the volcanic hills beyond Batangas and Laguna provinces and averages 40 miles (65 km) in width. Cagayan Valley in the extreme northeastern portion of Luzon, between the Cordillera Central and the Sierra Madre, is equally fertile and an important rice-growing area. In Mindanao, inside the coastal highlands on the east, an alluvial plain extends from the Agusan River in the north to Davao in the south. The Cotabato Valley occupies southwestern Mindanao, and north of this valley at an elevation of 2,000 to 3,000 feet (600–900 meters) lies the fertile Bukidnon Plateau.
Of the seven largest Visayan islands—Samar, Negros, Panay, Leyte, Cebu, Bohol, and Masbate—only Bohol and Masbate, which are plateaus, escape the presence of dividing, high, central mountains. The largest stretches of flatlands run from the northern coast of Panay to the city of Iloilo, and along the western part of Negros Occidental province. Among outlying islands Palawan is undeveloped, largely because of its mountainous terrain. The islands of the Sulu group are uncultivated because arable land is scarce.
The small area of the islands precludes extensive river systems. Most rivers are less than 20 miles (30 km) long, turbulent, and seasonal in their flow. The largest, the Cagayan, is only 200 miles (320 km) long. Other major rivers are the Agno, Pampanga, Pasig, and Bicol in Luzon, and the Rio Grande de Mindanao and Agusan in Mindanao. The short Pasig River is important because of the shipping it carries through Metropolitan Manila.
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In the 880 arable islands three broad groupings of soil are recognized. The most fertile soils are those that developed from the weathering of relatively recent volcanic materials. But areas with this type of soil tend to be poorly drained. Artificial drainage is often necessary for the growing of crops such as sugarcane. The second most fertile type resulted from the weathering of deep deposits of marine or river sediment. Such soil is high in lime content, as is most of Cebu island, for example. But because limestone is porous, it loses some of its fertility through rapid water drainage. The third major soil group occurs on floodplains and highland deltas. These alluvial deposits are deep, and most lowland rice is planted in them.
While the soil resource is adequate, soil conservation is not. Fertilization, contour plowing, and crop rotation are all minimal. Three fourths of the farmland shows serious soil erosion, caused mainly by the widely practiced slash-and-burn method of the kainginero (forest-burner), who clears trees recklessly in order to plant temporary crops.
At sea level throughout the islands, the temperature averages about 80° F (27° C). The humidity is high, and for every 300-foot (90-meter) rise in elevation, the temperature decreases approximately 1 Fahrenheit degree (0.55 Celsius degree). Thus Baguio, on Luzon, is usually 15 to 20 Fahrenheit degrees (8–11 Celsius degrees) cooler than the lowlands; Bukidnon and Lanao, on Mindanao, are 5 to 10 Fahrenheit degrees (3–6 Celsius degrees) cooler than the coast. The islands consequently can supply themselves with both tropical and temperate-zone fruits and vegetables.
Rainfall is heavy and varies according to area and season. One area may receive less than 40 inches (1,000 mm) of rain per year, while another may receive more than 200 inches (5,000 mm). Generally, the east receives abundant rain all year, with the greatest amount falling from October to April. Air from the Indian Ocean condenses into monsoon rains, which fall on the west and southwest slopes of the mountains from June to November. In addition, approximately 15 to 20 typhoons, or baguios, approaching from the vicinity of the Mariana Islands to the southeast, lash the Philippines during summer and autumn. While the annual rainfall of Baguio, for example, is between 43 and 170 inches (1,100–4,300 mm), during a typhoon the city can receive as much as 72 inches (1,800 mm) in 72 hours. Much damage is caused by the winds and heavy downpours and frequent flooding, as in central Luzon in 1972.