The Philippines are composed of three major islands known as Luzon, Visayas and Mindanao. The largest island is Luzon, followed by Mindanao and the Visayas group. The Visayan region is composed of about 6,000 islands, including Panay, Samar, Cebu, Leyte and Bohol. Mindanao encompasses about 400 islands.
The islands extend about 1,850 kilometers from north to south and almost 1,127 kilometers from east to west. The coastlines of all the islands are extremely irregular, measuring about 36,289 kilometers in length.
The larger islands have a more diversified landscape including fertile river valleys in the interior. Only about nineteen percent of all the land in the Philippines is arable and today fifteen percent is cultivated. This includes the important rice-growing region of the Central Luzon Valley as well as the mountainside rice terraces that have been cultivated for hundreds of years. The inland plains and valleys are the most densely populated areas. Formed by volcanic action in the past, these islands contain about twenty active volcanoes. While most of the islands are clustered in a north-south direction, the long narrow island of Palawan and the nine hundread small islands of the Sulu archipelago extend southwest toward Borneo . It is believed that Palawan Island was the first Philippine island to be inhabited by Southeast Asian migrants during prehistoric times.
Of volcanic origin, the Philippines is generally mountainous. Mountain ranges extend north to south, running parallel to the coasts and, in many places, bordering them.
The mountains in Luzon include the Sierra Madre, Cordillera Central, the Caraballo Mountains and the Zambales Mountains. In the second largest island, Mindanao, are the Diwata Mountains and the mountain ranges in southern Mindanao including Mount Apo (a volcano) which, at 2,954 meters, is the highest point in the Philippines.
The Philippine Islands lie in the Pacific Ring of Fire, an area encircling the Pacific Ocean which is the most volcanically active region on earth. The Philippines has twenty-two active volcanoes and another twenty-seven potentially active volcanoes. In addition, the 34,578 ft deep Philippine Trench to the east of the Islands is the place where one tectonic plate is being forced beneath another (subduction); this creates over one hundred seismic faults between Luzon and Mindanao . Thus, the islands experience frequent earthquakes, tidal waves (tsunamis) and volcanic eruptions. The most active volcano is Mayon Volcano which had major eruptions in 1993, 2000, and 2001. While rural buildings are made of light wood and thatch and are thus easy to replace when destroyed, the government has not been completely successful at creating earthquake proof structures in the cities.
The larger islands have a more diversified topography, with rivers, broad plains and level, fertile valleys in the interior. Luzon has the Cagayan Valley (a plain about 80 kilometers wide, the Central Plain (extending from Lingayen Gulf to Manila Bay), the Cagayan River (longest river in Luzon), Abra River, Chico River, the large Laguna de Bay, Agno River, Pampanga River and Bicol River. Mindanao contains the Mindanao Valley (the largest lowland area), the Agusan River the Rio Grande de Mindanao (known in its upper course as the Pulangi).