The islands and the mainland of Southeast Asia include a wide array of physical and cultural landscapes. The entire realm is located in the tropics except for the northernmost region of Burma (Myanmar), which extends north of the Tropic of Cancer. A tropical Type A climate dominates the region, and rainfall is generally abundant. The tropical waters of the region help moderate the climate. Southeast Asia is located between the Indian Ocean on the west and the Pacific Ocean on the east. Bordering the many islands and peninsulas are various seas, bays, straits, and gulfs that help create the complex maritime boundaries of the realm. The South China Sea is a significant body of water that acts as a separator between the mainland and the insular region. The thousands of islands that make up the various countries or lie along their coastal waters create a matrix of passageways and unique physical geography.
The three longest rivers of the realm, Mekong, Red, and Irrawaddy, are located on the mainland and have their headwaters in the high elevations of the Himalayan ranges of China. The Mekong River makes its way from the high Himalayas in China and helps form the political borders of Laos and Thailand on its way through Cambodia to Vietnam, where it creates a large delta near Ho Chi Minh City (Saigon). The Red River flows out of China and through Hanoi to the Red River delta on the Gulf of Tonkin. The Irrawaddy River flows through the length of Burma, providing for the core area of the country. Another major river of the mainland is the Chao Phraya of Thailand. With its many tributaries, the Chao Phraya creates a desirable core area that is home to the largest population of the country. Many other rivers can be found on both the mainland and the insular region. The rivers transport water and sediments from the interior to the coasts, often creating large deltas with fertile soils that are major agricultural areas. Multiple crops of rice and food products can be grown in the fertile river valleys and deltas. The agricultural abundance is needed to support the ever-increasing populations of the realm.
Tectonic plate activity has been responsible for the existence of the many islands and has created the mountainous terrain of the various countries. High mountain ranges can have peaks that reach elevations of over fifteen thousand feet. The high-elevation ranges of New Guinea, which are along the equator, actually have glaciers, ice, and snow that remain year-round. The island of Borneo, in the center of the insular region, is a segment of ancient rock that has been pushed upward by tectonic forces to form a mountainous landmass. The mountains on Borneo have been worn down over time by erosion. Mountains and high-lands stretch across the northern border of the realm along the borders with India and China. The interior nature of this border makes it less accessible. Similar dynamics can be found in the interior of the islands of the insular region, where the isolation and remoteness have helped create the environmental conditions for unique flora and fauna. In the highland areas, the human cultural landscape can be diverse. Time and isolation have worked together to form the traditions and cultural ways that give local groups their identity and heritage.
The tectonic activity makes the region vulnerable to earthquakes and volcanic eruptions. The volcanic peak of Mt. Pinatubo, in the Philippines, erupted in 1991, spewing ash and smoke into the atmosphere and impacting much of the planet. An earthquake of 9.0 magnitude occurred off the coast of the Indonesian island of Sumatra in 2004 and caused widespread disaster throughout the broader region of the Indian Ocean. As many as one hundred fifty thousand deaths were reported, mainly from flooding. A thirty-five-foot-high wall of water from the tsunami devastated many coastal areas from Thailand to India.