Base of the Torres
The best hike, and probably the most strenuous hike, I did while in South America was the full day trek to the base of the Torres del Paine (torres means towers, paine is an indigenous word for blue). This hike begins from the Hosteria Las Torres, crosses the Rio Ascensio by way of a short suspension bridge, then goes up, up, up. Over all the hike has an elevation gain of approximately 800 meters. Not all of the hike was up. The first third of the hike is steep, the middle third follows the Ascensio Valley and is relatively level with some up and down, and the final third is a steep ascent up a boulder field. As with many of the other hikes and areas visited, this trail had many geologic features and spectacular scenery.
Twelve million years ago granitic magma intruded into Cretaceous-aged sandstones and shales, forming what is known as a laccolith. The rock that makes up this laccolith is called the Torres del Paine granite. The overlying sandstones and shales were lifted up as the magma was injected between the rock layers. As the magma cooled, heat from the crystallizing granite changed the minerals in the surrounding rock layers in a process called contact metamorphism (so named because the surrounding rock is in contact with a hot magma). The fine-grained sediments were altered into a rock known as hornfels.
Granite which is typically white or pink colored, crystalline rock, rich in silicon, aluminum and potassium, but poor in iron and magnesium, and containing the minerals quartz, orthoclase and plagioclase, and often hornblende or biotite. Hornfels is a fine grained metamorphic rock. It is often black in color and is typically composed of a variety of equidimensional, randomy oriented mineral grains. It can often be a difficult rock to identify with out laboratory examination. These two rocks in the park form a sharp contrast based on their different colors. The contact between Torres del Paine granite, seen from a distance, appears as a sharp line separating the light colored granite from the overlying dark colored hornfels. Up close, this contact remains surprizingly sharp and can be found in individual boulders.
After the magma cooled, the area was uplifted and erosion began to remove the overlying sandstones, shales, and newly created hornfels. Further erosion and removal of material by glaciers during the last Ice Age, cut the
valleys we see today between the cuernos (horns) and surrounding the other peaks. The Torres formed as a result of this glacial erosion.
The features of this geologic story are seen throughout the park. Along the trail to the base of the Torres, good examples of these features were seen. The over all shape of the laccolith is difficult to see, as the magma body was quite large and no one view allows you to see the entire structure. A museum model from the visitors center shows what it would look like if we could slice the Earth open and see it all.
Other geologic features that were seen along the trail included:
- Basalt dikes intruded into the sandstone and shale layers