Chile's geology is the key to understanding its wandering history and present-day geography. Fossil records paint a sketchy picture of ancient flora and fauna, revealing ancient links with now-distant continents, the ebb and flow of the sea, and patterns of climatic warming and cooling. Lava flows atop glacial deposits attest to centuries of flux, of mountain building and catastrophe. Fluvial landforms in the north hint at a cooler, wetter past, when mastodons and other immense mammals inhabited the Atacama Desert.
During the Triassic period, some 250 million years ago, Chile formed part of the supercontinent Pangea, which concentrated all the world's land mass in a single island, surrounded by a global sea. Antarctica, Africa, India and Australia were Chile's closest neighbors. Once Pangea began to split apart, these land masses together would form a second supercontinent, known as Gondwanaland. Present-day floral affinities among these now-distant landmasses date from the Gonwanaland period. This great paleocontinent was also doomed to dissolution. First to split off were Africa and India, leaving South America, Antarctica, and Australia. These, too, began to drift apart, and by 27 million years ago the Drake Passage had fully separated Antarctica and Australia from the tip of South America. South America was on its own.
Meanwhile the continent itself was undergoing vast transformations. A land bridge -- now known as Central America - in turn isolated the continent and linked it with North America, creating periods of isolation and speciation punctuated by the influx of new, more highly competitive flora and fauna. Global and regional climate change throughout the Cenozoic (65 mya) transformed tropical ecosystems to temperate ones, created deserts, and revealed landforms long hidden by the sea. The Andean cordillera, which has been continually building and rebuilding itself via active volcanoes and violent faulting for the past 65 million years, isolated Chile from the rest of the continent. Most recently, a period of glaciations, the last ending some 10-15,000 years ago, created the beautifully sculpted landforms which characterize southern Chile. Nearly all the great lakes owe their existence to glaciers which scoured great basins and left high terminal moraines, like great impermeable dams, at their leading edge. Wide, U-shaped valleys with flat bottomlands and vertical walls are another legacy of these great implacable sculptors.
Today, the Lake Region is Chile's most geothermically active area. A chain of volcanoes, most of them active, extends in a rough line just west of the continental divide, and dozens of hotsprings lie scattered through these mountain valleys. A recent history of cataclysmic eruptions is apparent in tremendous lava flows, which have created waterfalls and in places stemmed the flow of western-flowing rivers, creating high mountain lakes almost overnight.
Volcanoes are one of the few things that every region of Chile has in common. Hundreds of these are active, and high levels of geothermal activity throughout the country also give rise to hundreds of hotsprings and geysers. Generally the highest peaks in any given region, volcanoes are the target of many mountaineering expeditions and provide unique terrain for many ski areas, especially in the south. Chile's Pacific Islands are also of volcanic origin.