GEOLOGY GEOLOGICAL ASPECTS OF THE COLCA VALLEY By Guido Salas The Colca Valley is located in a geologically rich and active area. It is located in the south of Peru, in and between the Andes mountains, and it belongs to the South American tectonic plate. This continental plate lies under the Nazca oceanic plate. The friction between both at a depth of over 100 kilometres causes the magma to rise to the lithosphere through large geological crevices, thus forming volcanoes. So, the Colca area is predominantly made up of volcanic rocks, with the presence of some sedimentary rock. This type of rock can be seen to the northwest of Maca. The most recent sign of vulcanism, is made up of the highest mountains in the valley: the Hualca Hualca, Ampato and Sabancayo volcanoes. The Colca valley is noticeable for the diversity of materials which cover it: the alluvial terraces made up of conglomerates, gravel and mud slime. The second structure is made up of alluvial cones. These materials have descended from the high parts due to water erosion and probably glacial activity. Maca, Lari and Madrigal stand out for the presence of great masses of rock. These frequently slide, greatly affecting agriculture. The landslides are produced by the clayey lithology of some of the sediments, the abundance of water and the fracture of the land and even by active geological faults. The volcanoes Two of the at least four active volcanoes in the south of Peru, are to be found in the Colca valley or in the tourist areas linked to the Colca. These are the Sabancaya and Misti volcanoes. Along with the Ubinas and Huaynaputina volcanoes, they both belong to the central Andean volcanic region. The snowcapped mountain, Sabancaya, by the Colca valley, is the highest of the active Peruvian volcanoes. It is located near to the ancient Hualca Hualca volcano and the most recent one, Ampato. The Sabancaya has been permanently active sine 1987. Its activity alternates between periods of relative calm and periods of increase in intensity. At present, the columns of ash from the Sabancaya rise from one to four kilometres into the air for ten minutes at a time, at irregular intervals. The ash is carried by the wind for several kilometres. The volcano started its reactivation in 1990 when the columns frequently reached over five kilometres into the sky and were seemingly full of volcanic particles. At that time llama and alpaca pasture in the area was stopped due to the ash. At present, it is impossible to foresee the future evolution of the Sabancaya. Historical narratives tell of similar activity during the XVIII century. And the volcano's name in Quechua means "he who spits", suggesting that this type of eruption did occur during the pre-Columbian era. Only a greater eruption could affect the Colca valley inhabitants, at 20 kilometres from the volcano. How to visit Sabancaya The best place to see the Sabancaya eruptions is the Pampa de Sallalli. You can reach there using a four wheel drive vehicle, along a dirt road which heads directly off the Arequipa-Chivay road, at Solarpampa. The journey lasts four or five hours from Arequipa, and approximately two hours from Chivay. The climb up to the crater is long and a guide is recommended. You must sleep the night at the mountain pass between Sabancaya and Ampato. Misti The Misti volcano is the most well known of the Peruvian volcanoes, not only for its symmetrical beauty but also, and most of all, for its proximity to Arequipa. Its activity has been known of for at least 35,000 years, when the most ancient eruption occurred. During this long period of time, up until the present, several dozen significant eruptions have occurred. The latest eruption took place 2000 years ago. This eruption started with the emission of volcanic material which fell on Arequipa. It is probable that over the last 2000 years, several minor eruptions have taken place. If a significant eruption of Misti did occur, Arequipa would be in a critical position. An eruption with an unfavourable wind, could cover the city in ash and pumice stone causing severe damage. It is possible that serious avalanches could occur. For example, the city is partly built on the remaining sediment of the collapse of the Pichu Pichu volcano. An avalanche of this nature would completely destroy the city of Arequipa in a few minutes. How to visit Misti The climb up Misti usually takes two days there and back. There is no technical difficulty in climbing it, but it is essential to be physically fit. A mountain guide is advisable. You can descend into the exterior crater, but it is dangerous to venture into the small central crater. Ashlar stone The beauty of Arequipa lies partly in the white ashlar stone used for building all of the colonial buildings in the centre. In the Colca Valley, the Lari church has also been built completely out of this stone. Ashlar is a volcanic rock which comes from a particularly violent eruption which took place over two million years ago. It is the product of monstrous streams of fire and lava which submerged the whole Arequipa basin. At the time of its emplacement and due to the high temperature, it solidified and formed the compact consistency which enables it to be used as construction material. If an eruption similar to that of the ashlar ever occurred again, the city of Arequipa and the neighbouring plains would disappear without a trace. Seismic aspects of the Colca The south of Peru is well known for its strong seismic features. The seismic activity in the Colca valley is significant. In July 1991 an earthquake of 5.2 degrees occurred and completely destroyed Maca and its church. The geyser and the hot springs The Colca valley has several hot mineral springs, probably associated with the vulcanism. Their origin is in the rain water which filters in the subsoil and is heated in the depths due to its proximity to a magmatic chamber. Thus the water is lighter than the surrounding cold water and it rises to the surface. If there are cracks in the earth, they favour a rapid rise to the surface, the water does not lose its heat. In some cases the magma transfers a certain quantity of carbonic gas to the water, changing it into high quality mineral water. To the south of Pinchollo, in the Colca river canyon, there is a constant jet of water vapour. There are also hot spring baths at Chivay and Ichupampa. Mineral resources in the Colca Valley: mines The Colca valley has two mines. The Maca and the Madrigal mines. The Maca mine contains diatomite. This contains the same typology as carbon, petroleum and natural gas. The mine is located in the Jatun Chicta region. The Madrigal mine contains metalic minerals. Up until a few years ago, copper, zinc, lead and silver were mined here. Due to economic measures, their mining ceased to be profitable and at present the mine is abandoned. The mine also contains quartz, calcite, galena, sphalerite, chalcopyrite and pyrite. THE CANYON: A DEEP AND ANCIENT GORGE High up, in the desolate places of Imata, near the Jayuchaca lagoon, the many Colca tributaries spread out over an Andean arch of over sixty kilometres. They run down the hills and, like the Chilamayo river, the headwaters of the Colca, they join the new river bed. It is here that the river begins its wilful journey which ends in the Pacific Ocean under the name of the Camana river. The torrent runs through parts of the canyon which only the midday sun touches and on its journey it lays bare the geological formations which have gathered since ancient times. HISTORY OF THE CANYON Seventy million years ago, during the Senonic period, the first Peruvian Andean upheaval, the Peruvian Uplift, ocurred. This wide-ranging orogenic movement of mountain building was followed by abundant volcanic outlets which spread all along the meridional mountain range, forming a strong platform. Much later, towards the end of the Tertiary period, the great volcanoes which stretch from the 150 parallel latitude to Tierra del Fuego, began to rise. Simultaneously, igneous (crystalline) rocks appeared which, after extensive erosive processes caused by differences in temperature, became exposed to the outside world. Thus, the Huaraz, Volcabamba and Urubamba mountain ranges had been formed. During the Oligocene period, thirty five million years ago, the Continental Mountain Range emerged with heights of up to a thousand or two thousand metres, causing new foldings of the sedimentary strata. Much later, towards the end of the Tertiary period, during the Miocene and the Pliocene periods, new eruptions occurred causing a great accumulation of volcanic layers. Tangential thrusts, particularly in the Pliocene period, produced the Quichano folding and lifted the range up to the altitude which it has today. During this period and the Pleistocene period, volcanic activity begins again and forms, among other outstanding mountains in the south of the country, the Coropuna and the Ampato. The eroding effect of the water which descends from the glaciers and from the different glaciations which occurred in the heights of the range millions of years ago, has laid bare the enormous upheavals in the geological formation. The fluvial lake terraces located above the present river bed on which the terraces are, are a clear indication of the above. That is, the break in the base level due to sudden (in geological time, naturally) and sharp upheaval of the Andes, caused the narrowness of the river and molded the terraces which stretch all along both sides of the valley.