Nature of Southeast Asia

The Southeast Asia region includes mainland Myanmar, Thailand, Cambodia, Laos, and Vietnam as well as the complex archipelago of tens of thousands of islands (Malaysia, Indonesia, the Philippines, and Papua New Guinea) separating Asia from Australia. 

It's a complex tropical region whose massively crumpled topography results from two colossal collisions:

1. India smashed into mainland Asia between 30 and 60 million years ago, forming gigantic mountain chains that stretch from Iran to Vietnam.  
2. Australia continues to move northward, squeezing the ocean crust and driving slabs thousands of miles wide downward into the mantle where they melt and bubble back into surface volcanos as part of the Pacific Rim of Fire.

The Himalayas and Tibetan Plateau form a huge area of cold elevated terrain inland of an area of very warm seawater known as the Warm Pool. This juxtaposition is responsible for the Asian Monsoon, a seasonally-reversing system of winds and rainfall that absolutely dwarfs every other monsoon system on Earth. Meters of rainfall in summer months runs off the complex terrain in a firehose of giant river systems that form huge river deltas such as the Mekong.

Flowing nearly 5000 km from the Tibetan Plateau, the Mekong River is among the longest in Asia and floods every summer as the mountains wring prodigious Warm-Pool water from the onshore flow. The Mekong Delta feeds hundreds of millions of people from what's sometimes called the "Rice Bowl" of Asia. At least 40% of global rice exports come from the region. Fishing and aquaculture on the delta are also critical sources of protein for Vietnam, Cambodia, and Laos. Massive hydroelectric dams in China (where the Mekong is called the Lancang) have severely restricted flows downstream and threaten agriculture in the delta.

Offshore lies the enormous Malay Archipelago, sometimes called Maritime Southeast Asia or the Maritime Continent. Innumerable islands, most small and uninhabited, sprawl across thousands of miles. The archipelago contains most of the world's most active volcanos as "island arcs" formed by the subduction of ocean crust. 

The Malay Archipelago is a complex of island arcs formed by subduction

The combination of superabundant seasonal rains,  Equatorial heat, and the isolating effect of island populations on evolution produces an incredible abundance of ecosystems both on land and in the sea. The region is among the most biodiverse in the world, with many endemic species of both plants and animals. Endemic plants found only on small islands in the region were prized as spices and aromatics which drove a thriving ancient and medieval trade in botanical products across the world. The Spice Trade led to conquest, colonization, and imperial exploitation in the modern period.

Behind the island arcs of the Maritime Continent, the ocean is so shallow that its bed is exposed as dry land during each of the many ice ages over the past couple of million years. Building polar ice sheets drains the oceans by hundreds of feet, so that Peninsular Southeast Asia extended well out into what is now the Malay Archipelago ("Sundaland"). Similarly, the continental shelf north of Australia was exposed during ice ages ("Sahul"). During these periods, flora and fauna mixed freely across each region and were then separated when waters rose again as the polar ice melted. 

Wallacea, whose islands and deep ocean water separate Maritime Southeast Asia from Australia also divides biogeography as defined by the so-called Wallace Line. South and east of the line, marsupials like kangaroos and wallabies are found. North and west of the line live placental mammals like tigers and monkeys. This biogeographic boundary between Asia and Australia is a dramatic example of the dynamic interactions among plate tectonics, climate, and ecology that make this part of the world so special.

The rugged terrain, heavy rainfall, and dense forests of Southeast Asia formed a formidable and barrier in ancient times, second to the nearly impassable Himalayan mountain barrier. These geographic barriers contributed to the separate development of major civilizations in East Asia (China) and South Asia (India) which are home to half of humanity. Much of the interplay between these two dominant cultures involved maritime trade through the islands rather than the much more difficult travel through the Himalayas and mountains of Central Asia.

Southeast Asia is home to nearly 700 million people -- about as many as all of Europe -- including many densely-populated metropolitan area surrounding Jakarta, Manila, Ho Chi Minh City, Bangkok, Kuala Lumpur, and Singapore. It's been a bustling region of international maritime trade and cultural exchange for thousands of years.