The Anatomy (geology) of the Pachamama in Arica
 
 

 

One of the most outstanding geological features of Arica is El Morro, about 600 feet high.

 

 It represents the abrupt end of the coastal cordillera, that in northern Chile produces a very long cliff that extends for hundreds of miles down to Antofagasta. To the north, towards Peru, the land slides down gently to the sea. To the east the land rises steeply up to the altiplano, with some deep and narrow valleys running down to the sea from the first mountain ridge. Beyond the valley walls there is first a flat surface and then many rounded hills containing debris that has fallen from the Andes driven by floods. The valleys begin at the outskirts of the first mountain ridge (Huaylillas Sierra) of the Central Cordillera, formed by volcanic (magmatic) rocks. At that altitude there are some extensions of flat pampas, formed by sediments and the settling down of huge clouds of volcanic gases, small particles and ashes, liberated by the explosion of volcanoes. These are called ignimbrites.
 

An airfield in a pampa close to Zapahuira. The snow-capped mountains are the inactive pre-holocenic Taapaca volcanoes.
 
It is at the narrow corridor between the Huaylillas Sierra and the rest of the occidental Andes were most ancient settlements of coastal and altiplanic people are found, since that is where the rivers that flow down through the valleys are born. Farther to the east there are steep hills of solid rocks that become higher and higher until the altiplano is reached, at some 14,000 feet of altitude. There lies the Chungara and Titicaca lakes, the peculiar wetlands that feed the llamas (bofedales).
 

Llamas grazing at Parinacota's wetlands (bofedales).

 

and some beautiful volcanoes that are classified as holocenic, since they have had activity during the last geological period of the planet, initiated some 10,000 years ago. To the east of the altiplano lies the Oriental Cordillera (Cordillera Real) that descends towards the Amazonian lands that cover the Brazilian shield.
 
From the Chilean altiplano to the coast of Arica there is a distance of just 60 miles, but an altitude difference of 14,000 feet. To the west, the sea bottom descends 24,000 feet, making Arica the steepest place of the surface of the planet.

A transerse section of the surface of Arivc and Parinacota.

 

 

 
There is a reason for the extraordinary characteristics of this territory. Arica and its hinterland are precisely the place where the Pachamama (the sacred Mother Earth of the Andeans) has been concentrating the efforts to procure herself a new look for several million years, generating huge earthquakes in the process.
 
You will most likely travel to the altiplano in your way to Bolivia or just to visit the Lake Chungara. The trip is a feast for geologists. I will try to give a simple explanation of how the Andes and the altiplano were created.
 
Once upon a time, behind the mountains close to the coast, there was a flat central-west South America, with a few scattered volcanoes and all main rivers flowing down to the Pacific Ocean. Our land, as I will call what now extends from the altiplano to our coast, was being pushed from the west by a solid plaque of the earth crust, the Nazca plate lying under the ocean. To the west lies the Brazilian shield, which is very rigid and, since it prevents our land to recede, the Nazca plate has been forced to sink under our cortex since it began moving 200 million years ago, in a process called subduction.

 

 

Subduction of the Nazca plate.

 

Something like that occurs all over the planet, but the ancient material of our coastline is too solid to crumble down to a gentle slope, and is instead raised by the process, since what crumbles down is not carried by the plaque but accumulates under the edge of our land, therefore raising it. As it goes deeper and deeper, the material of the Nazca plate becomes heated and it melts, liberating gases that accumulate pressure until they suddenly break the crust and produce an explosion at the surface. Then the melted material ascends building the volcanoes and increasing from time to time the thickness of our hinterland with material that initially belonged to the bottom of the sea.
 
 
The volcanic explosions that shaped this territory began 20 million years ago, initiating the process of accumulation of material from the subducted Nazca plate at the surface, eventually forming the Central Cordillera. Ten million years before, the Brazilian shield began moving to the west, elevating our hinterland where now lies the Oriental Cordillera. Between both cordilleras lies the flat altiplano, a space filled by ignimbrite and debris. The melted material from the Nazca plate ascends along the line of the Central Cordillera (today’s magmatic arch), increasing the thickness of the cortex. When the mountains become too high, they crumble drown towards the coast, producing the many rocky hills. In fact, the lower mountain range (Huaylillas) is part of the slope of the Central Cordillera that cracked up and tilted to the west.
 
The first explosion, 20 million years ago, liberated a huge cloud of gases and melted rocks that added 3,000km3 of material to our hinterland. Ignimbrites behave like a fluid when they settle down, flooding valleys and spaces between the hills and in some places covering the entire surface, forming a pampa which then receives sediments from floods and/or small pieces of debris that slide down from the mountains. Then other pieces of the solid rock mountains crumble down and more ignimbrites erupt and over and over. There is still another parameter to consider: water. During the remote past there were heavy rains that produced floods of a magnitude that can hardly be imagined. The rivers carved the valleys and the flood moved down huge amounts of debris, sometimes blocking the valleys and creating lakes. Along the road that ascends the Cordillera towards the altiplano, layers of soft rocks of different pale colors will intrigue you: yellow, red, violet and green. Some of them are lacustrine sediments from ancient lakes, containing fossilized plants. Glaciers also shaped some parts of the landscape that you will see on your trip to the altiplano.
 
All these layers of material from different sources were bent, moved and tilted by big earthquakes, sometimes translating huge chunks of the surface. You will initiate your ascent through the Lluta valley, to the north of Arica. The steep walls end in a flat pampa that used to be the bottom of a lake at the outskirts of the mountains. A huge collapse of one of the layers sent the lake down to the coast, filling up that place more than 6 million years ago. That was the biggest sliding that we know about, moving 25km3 of material and blocking ancient valleys. The surface of the collapsed lake was then covered with a thin layer of sand. At the borders of the collapse, gradually uncovered as the wind blows the sand away, you can find many well-conserved snail shells,

 

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One of the many fresh-water sells found at the borders of the Lluta collapse

 

 whose thinness reveal that they lived in fresh water. I wonder how old can they be. The Lluta valley was latter carved by the river.
 
Along the 30 million years that we have described, there were at least three dramatic episodes of violent lifting of the hinterland, redefining the landscape that was then patiently reshaped by erosion and floods and, from time to time, by ignimbrites. The last one lifted the altiplano by more that 3,000 feet, producing huge slidings towards the coast. Since then there has been a period of quiescent tectonic activity, with some “minor” events, like the volcano that exploded near where today is Lake Chungara, creating the low mountains that can be seen near the lake. The two twin volcanoes by the lake appeared latter.

 

The older holocenic Pomerape volcano in the second plane and the Parincota volcano in the first pane. Lava flowing From the latter blocked the water course and created the lake.

 

The last big ignimbrite cloud, 2,6 million years ago, contributed with 1,000km3 of material that flooded  the altiplano, the base of our Cordillera and descended along the valleys to near the coast.
 
The Pachamama is certainly experimenting with us. Unlike the Himalayas and the Alps,  raised by the wrinkling of the surface provoked by the gradual collision of two continents, the formation of the Andes is the consequence of the elevation of our hinterland and, secondarily, of the volcanic activity. Much earlier, almost 500 million years ago, she decided to incorporate to our continent a big piece of land to the west of the coast: that land is now Arequipa (Peru), lying quite far from the coast.
 
I have just summarized the formation of the Andes, initiated during the middle Tertiary period of the Cenozoic era. Before that, during the Jurassic period of the Mesozoic era, the magmatic arch was located close to the coast and formed the coastal Cordillera. Thus, the rocks of El Morro are more than 100 million years old. To the north, the surface of Tacna forms a gentle slope because it was more recently shaped by rubbish sliding down from the mountains.
 
Underneath the Andes and the layers that I have described, there are more ignimbrites and sediments from the Cretaceous period of the Mesozoic and at the bottom, Precambrian rocks. Some 10 miles from the road to Chungara beyond the Huaylillas Sierra, you can see the oldest rocks of Chile behind the village of Belén, almost 2,000 million years old, al little less than half the age of the planet. But that is another story...