When the Andes were formed, sedimentary rocks were folded and bent into long ridges, called sierras. In some places huge cracks allowed molten granite and other igneous rock to well up from the depths. Wherever this rock reached the surface, it built volcanic cones. Most of the highest peaks in the Andes, including Aconcagua, are volcanoes. The mountains are still settling.
Severe earthquakes occur and volcanoes occasionally erupt with much destruction.
The Andes affect the climate by influencing precipitation. In northern Colombia three ranges spread out to catch and hold the moisture of the northeastern trade winds, making this a region of heavy rainfall. On the Pacific side, from the Isthmus of Panama to the equator, the Colombian Andes catch the southwesterly winds, and rains fall almost daily. From the Gulf of Guayaquil through northern Chile, the west coast is extremely dry. This stretch lies in the trade wind belt.