The view from the Cordillera Negra across the Río Santa Valley or Callejón de Huaylas to the Cordillera Blanca (see title image of this contribution and of the route overview) clearly reveals that the Cordillera Blanca is much higher than the Cordillera Negra. Indeed, the Cordillera Blanca - White mountain range in English - is one of the highest ranges of the entire Andes, reaching an elevation of 6768 m (Nevado Huascarán). Higher peaks only occur much farther south (Cerro Aconcagua, 6962 m) so that the Cordillera Blanca represents the highest tropical mountain range worldwide. It contains most of all tropical glaciers and, as a consequence, a large number of glacial lakes (882 of such lake have been counted). It is located approximately 300 km north of the Peruvian capital city Lima. But why are there so many high peaks? Let’s have a closer look at the geology, it’s worth it!
How the Cordillera Blanca evolved
Browse through the explanation of this WSW–ENE cross-section by clicking on the arrows. You can switch between animated and static profiles by clicking on the title symbol.
The Andes already formed a mountain range about eight million years ago. The geology of the area which we call Cordillera Blanca nowadays mainly consisted of three different units: the continental plate, some Jurassic phyllites, and Cretaceous sediments (from bottom to top). In the subduction area where the Nazca Plate dives under the continental plate the rock was melting, so that a magma chamber formed beneath the continent. The density of molten rock (or magma) is lower than the density of the surrounding rock, so it tends to rise. This process can be considered the beginning of the formation of the Cordillera Blanca.
This model represents a simplified and distorted representation of reality, and does not claim to be complete or correct. Its purpose is to convey some idea on how the Cordillera Blanca could have developed.
The secrets of a batholith
We have already learned that the Cordillera Blanca mainly consists of a so-called batholith. The rather soft sedimentary and volcanic rocks now forming the Cordillera Negra have been sliding off this batholith slowly as a tectonic movement rather than as a landslide. But what are the characteristics of those rocks forming the batholith itself?
Consult a geological textbook in order to learn which types of rock typically form a batholith, if these rocks are arranged in a certain way, and what are the reasons for the patterns observed.
Rocks formed through solidification of magma in the Earth's crust are called plutonic rocks or plutonites. They are generally quite hard and resistant. This is one of the reasons that the Cordillera Blanca stands out from its surroundings so clearly.
The exact type of a plutonic rock is defined by its mineral composition. The fact that different minerals have different melting points plays a major role for the formation of plutonic rocks. Quartz (SiO2) has a relatively low melting point, compared to other minerals, so that it is still liquid when most other minerals have hardened already.
Let us now have a look at the batholith: the magma with all kinds of minerals intrudes the existing rock and rises. As soon as the magma comes closer to the surface it starts to cool down. The minerals with the highest melting points solidify and remain deeply beneath the surface, whereas the future quartz is still liquid and continues to rise. This means that rocks without quartz - so-called basic rocks - can be found at the bottom of a batholith, while those with a high amount of quartz - so-called acidic rocks - are encountered at the top. The terminology of basic and acidic rocks is related to the fact that Si(OH)4 is the silicic acid, therefore those rocks which are rich in quartz are called acidic rocks. Acidic rocks are rather bright in colour, whereas basic rocks are rather dark.
Gabbro is a dark, basic rock without quartz: it develops deeply below the surface. Granite, in contrast, is bright and acidic (rich in quartz), therefore it can be found in the upper part of a batholith. Diorite can be encountered in between. The older the batholith is the more rock has already been eroded, and the higher is the chance of the rocks in the lower part to become exposed. Since the Cordillera Blanca batholith is relatively young (eight million years, more or less), only small parts have been eroded yet. This is the reason why we can still find granite or the relatively similar granodiorite in the Cordillera Blanca.
This contribution was slightly revised and extended by Martin Mergili.