A matter of mountains

Whatever your interests, any programme, article, or information about the landscape, history, or wild life of the Andes reveals just what an exciting and mysterious range of mountains they are. In this new book by Simon Lamb many of the questions about the geological history of their formation have begun to be unravelled.

Simon Lamb is now at Oxford University and since 1990 has spent many seasons working mainly in Bolivia which happens to lie in the area of the widest section of the range or on the "knee cap" as it is dubbed later in the book.

Throughout the book there are notes about logistics, politics, and the very considerable problems of working in remote hostile terranes. Finding reliable transport, guides, physical demands of high altitudes as well as getting data and samples when equipment isn't designed to work in freezing temperatures make hard field seasons.

The main objects were to find answers to the many outstanding questions of mountain building. How rapid is the uplift? What mechanism keeps the rocks at high altitudes? Is the adjacent subduction zone activity sufficient to create the range of mountains we have today? Many more problems remain to be solved but already some of the results obtained can be applied to the study of other orogenic belts around the world where past theories have not provided adequate answers.

To put the research into historical context Simon Lamb gives an outline of work done in the past by geologists on areas which have now become classic examples of mountain building.  North West Scotland in the 1880's saw Peach and Horne struggling to explain the contorted landscape and Simon Lamb uses the area now to introduce today's students to the theories of collisions and thrust tectonics. Darwin was on his travels in the 1830's when he experienced a large earthquake along the coast in Chile. Up to 3m of movement on that occasion gave him some ideas when he came to observe marine fossils way above sea level in the mountains.

In the previous century a group of French surveyors were making land measurements in the high Andes and Equador to try to fix the length of one degree of latitude on the earth's surface. George Airy and John Pratt presented their “Iceberg” and “Rising dough” explanations for mountain creation and Henry Cadell clamped and crushed his assorted layers of clay producing miniature folds and thrusts reminiscent of Highland Scotland.

Finally English geologist Harold Wellman in New Zealand on the fault zones, strike slip motion and the uplift of the Southern Alps is described. The discoveries made here showed how the Australian and Pacific Plate movements are shaping the islands and building mountains at a fair rate.

The first visits to Bolivia were spent surveying as much as possible to decide which locations would repay study to obtain the information required to produce answers to the mountain building history. North of La Paz the Zongo gorge cuts down from 6000M on ice capped rocks through granite cliffs to tropical jungle with thick humid air in a very short distance so this provided one very good site.  

In the polished rocks exposed along the river at the lowest point in the gorge a blue grey slate contained white chiastolite and shining cubes of pyrite. The river cutting down and the mountains rising had started to reveal some clues to their origins.  

Exploring the Altiplano, largely flat, almost bare, scoured with dried rivers and salt flats, was hard going. An average height of 4000M means very thin air and freezing nights. A large copper deposit here had been mined from what geologists had found were up to 8KM thick red bed sediments. These were on top of Cretaceous rocks where dinosaur tracks had been found previously. How were they now at such a height and where had the rivers carrying all this material originated?

West of the Altiplano are the volcanic ranges most people associate with the Andes.

 At the end of the Cretaceous before 65MY the volcanoes were an island chain just off the coast and the red beds began to form between them and a second range of hills on the mainland. The creation of the eastern cordillera had started and was simultaneously eroded down again to the west as the rainfall patterns were changed by the rising mass of rock. 

In the process to the east the rocks comprising sandstones and shale and had been folded, crumpled and raised along thrusts. Fossil trilobites reveal the age of these to be Ordovician and Silurian and a feeling of being in a "time warp" was almost inevitable. The Cochabamba valley at about 2500M trends nearer east/west to the main mountain range and has late Cretaceous Limestone beds now tilted and the whole area seemed to suggest extension between fault zones not compression.

The book covers many theories as it unravels the rock record showing the limestones being buried under the red beds as the rivers criss-crossed the rising plateau. Simon Lamb gives very clear explanations of all the methods used in the research from paleomagnetism, seismic data, volcanic gas analysis, fossil leaf climatic zones and many more. The techniques for collecting the data and the science involved are discussed and in many cases line drawings tailored to the topic are very clear and concise.

 Dating the rocks was done using minerals from the volcanic eruptions and some at the base of the central plains were around 12MY and those at the top 9MY. Among these were the beds with the fossil leaves. Using a technique which records the shape and size of the leaves, which when growing reflect the climate and altitude, the fossils found are now much higher than they could have existed originally.  Another problem with these deposits was that they showed none of the disruption of the lower sequences further east which had obviously been extensively faulted and folded.

Over the years the team have joined with other groups from USA and Europe, mining and oil companies working in the area to pool ideas and data.  Seismic measurements revealed that the crust was thinner in places than would have been predicted from the height of the ranges. Helium 3/4 readings from edge to edge of the western cordillera to the east range showed that the mantle was melting under a much larger area than was expected and that the accompanying high percentage of water vapour in the gas had the signature of sea water.

To reach their conclusions about the building of the Andes the group had to refer to work done over the whole world in places like the Himalayas and the Tibetan Plateau learning from both their similarities and differences. The former has resulted from the particular tectonic setting where the Pacific plate is subducting under the western coast and the "moveable" eastern ranges have over-ridden the edges of the ancient Brazilian craton a very rigid "immovable object"! The Cochabamba valley where the trend was apparently east/west is one example of extension round the "kneecap" in the chain as

the mountains spread out under their own weight. The edge of the ancient Brazilian craton deflecting this movement  and at the same time preserving the height of the eastern ranges.

Many new ideas about the effectiveness of subducting plates in raising mountain belts, the likely slowing down of global plate movement and its effect on the future rate of mountain building, evolution of crustal rocks and a "Fudge Cake" model to add to the geological cookbook can be found and explored in the text.

It was a very enjoyable book to read and with the very good accessible level of explanation of the technical details throughout any one could follow the discussion and learn a good deal. I hope some people will do just that. 

 

"Devil in the Mountain"  Simon Lamb.  Princeton University Press.  ISBN 0-691-11596-6      

 Joyce Ali