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Physical Landscapes as the Andes. The Andes mark the western margin of both Colombia and the rest of the South American continent. They are a grand system of mountain ranges, interrupted by north- to ­ south-­trending valleys. The mountains’ geologic complexity is evident in their wavy layers of gray and pink lava, red and brown sandstone, creamy marble, and speckled granite. The rugged beauty of the Andes is a product of both past and ongoing volcanic activity, geologic uplift, and erosion. Andean uplift occurs because tectonic plates (large sections of Earth’s outer rock layer) are colliding. The collision involves a large, ­westward-­moving South American Plate and the smaller ­eastward-­moving Nazca Plate. The advancing edge of the South American Plate lifts upward as it crashes against the smaller oceanic plates. This collision causes rocks to fold and break along faults. Volcanism and earthquakes (movements along the faults) accompany the ­uplift. Three ranges make up the Colombian Andes: Western (Occidental), Central (Central), and Eastern (Oriental). These ranges open and fan out northward from the Pasto Knot, a mountainous zone in the southwestern part of the country. Between the Western and Central ranges flows the Cauca River. The Magdalena River, which the Cauca joins, separates the Central and Eastern ranges on its long journey to the Carib-­ bean Sea. The Central Range (Cordillera Central) is the highest of Colombia’s three major ranges. Several spectacular volcanic peaks rise more than 18,000 feet (about 5,500 meters). This range is notorious for its dangerous volcanoes. In 1985, the ­cone-­shaped Nevado del Ruiz volcano erupted violently, kill-­ ing more than 23,000 people. It remains the deadliest volcanic eruption ever to occur in South ­America. The Eastern Range (Cordillera Oriental) lies east of the Magdalena River. It is the broadest of the three mountain groups. There, high, snowcapped peaks frame picturesque, ­flat­bottomed basins. The basins are large and have served as the magnet for human settlement because of their size, favorable elevations, freshwater lakes, fertile volcanic soils, and grassy



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