We will answer this main question by addressing the following section questions:
What is the relation between the process of mountain building and plate tectonics? (5.1) What do rocks tell us about the way mountains are formed? (5.2)
What is the relation between force, stress and deformation? (5.3)
What happens when deformation takes place quickly (brittle behaviour)? (5.4) What happens when deformation takes place slowly? (ductile behaviour) (5.5)
What does this tell us about the way mountains are formed and about plate tectonics? (5.6) Objective: To be able to understand and describe the (small scale) processes that are important for the formation of mountains, and to discover how these processes work. For this you need some knowledge from mathematics, physics and chemistry.
5.1
The relation between mountain formation and plate tectonics
Mountains are found only in certain places. Well-known examples of mountain ranges are the Alps, the Himalayas and the Rockies. But mountains play an important role for the geography everywhere in the world. Soils all over the world originate as rocks and particles eroded from mountain slopes, and without this erosion and transport, many areas in the world would lie below sea level. To complete the cycle, current basins could form the source material for future mountain ranges which are yet to form. To determine how this exactly works, we will first take a look at how mountains are formed and how this relates to plate tectonics.
In this section, we will discuss the relation between mountains and plate tectonics. Next, we will see what kind of information we can obtain from rocks and what they tell us about the processes that are important for mountain formation.
Exercise 5-1*: Mountains and plate tectonics A, I
When plates of the same type collide, a contact is formed between two plates of the same weight. The clearest example is India, which was once connected to the South Pole. After migrating to the north, it connected to Asia. Everything that once lay between the two continents is now heavily deformed and folded, and currently makes up the Himalayas, the highest mountain range in the world. Proof for this theory is, amongst others, the presence of fossil sea shells that are found in the rocks.
a. Why will you find no news in the papers about the event described in the above text?
b. Draw the mountain range which is described in the text on your empty world map. Also draw the Alps, the Ardennes, the Andes, the Rocky Mountains and the Ethiopian Highlands, and any other mountain range you might have been to.
c. Surf to www.earthweek.com to see whether any earthquakes or volcano eruptions have occurred anywhere near these mountain ranges.
d. What do you conclude? Is there a relationship between mountain ranges, volcanoes and earthquakes?
e. Now take a look at GB 192B. What do you notice about the locations of the Himalayas and the Andes compared to the type of plate boundaries that are indicated on the map?
To form a mountain range some source material must be present. Delta areas such as The Netherlands or the Po basin could be the start of a new mountain range, because a lot of sediment is present (and is still being deposited) in these regions. This deposition or accumulation of large amounts of sediments generally takes place in coastal areas where rivers carry a lot of new material to the sea. The basin (in the ideal case one which subsides slowly) supplies the rivers with space for deposition. As you can see, basically two things are necessary: firstly, the supply of a lot of sediment and secondly, room for the sediment to be deposited: usually some basin. This is step one in the process of building a mountain range.
Earth Scientists call the process of mountain building orogenesis. It takes place at convergent plate boundaries, and a distinction between two types of orogenesis can be made:
(1) Orogenesis as a result of convergence of an oceanic and a continental plate: in this case, orogenesis takes place inside the continental plate - the oceanic plate only subducts. An example of such an orogenic belt is the Andes. Sedimentary, volcanic and other (magmatic) rocks are pressed together as a result of the compressional force of the subducting plate pushing against the continental plate. This results in the formation of faults and folds, and part of the rock is pushed up. In this case, the processes of accumulation and orogenesis take place simultaneously: the material is pushed together and up because of the compressional forces which are caused by the orogenesis.
(2) Orogenesis as a result of convergence of two continental plates: in this case, the rocks of both plates are deformed. Both plates have the same weight and neither subducts, so that there is no way out but up. An example of such an orogenic belt is the Himalayas, formed when the continent India collided against the Eurasian plate. In fact, this process is still happening today.
(Convergence of two oceanic plates will always result in subduction: it never happens that both plates have exactly the same weight. This is because one of the plates is always older and therefore heavier than the other. The older an oceanic plate gets, the more it cools and the higher its density becomes. The older plate of the two will therefore always subduct beneath the younger one.
Orogenic belts are formed through uplift. However, as soon as relief is formed, the process of erosion begins. In the case of strong uplift (like in the Himalayas), the mountains will become higher and higher because the uplift outweighs the erosion. As soon as the erosion starts compensating for the uplift, the mountains stop growing.
So mountains are formed when plates converge. However, if the convergent motion stops, this doesn’t mean that vertical motion stops as well. This vertical motion continues until isostasy is reached: the gravitational equilibrium between the Earth’s hard crust and the underlying mantle material which deforms much more easily.
You can visualise isostasy by thinking of the crust ‘floating’ on the mantle (see chapter 6 for a more elaborate discussion). When the crust becomes heavier, as is the case when mountains are built, it must sink in order to compensate for this extra weight. Think of a boat lying deeper in the water when it is filled with cargo.
This is how it works: equilibrium is achieved when the mass of the crust lying on the mantle is equal to the mass of the mantle material which is displaced. In other words, if an amount of crust material of mass A is added to the crust, an amount of mantle material of the same mass A must be displaced.
Think of the following simplified situation:
A 2 cm thick piece of wood floats in a bucket of water. About 1 cm of the wood is submerged in the water. Think of the water as the mantle material and of the wood as the crust. Now suppose you take a piece of the same type of wood, but this one is 4 cm thick. As you may have guessed, it sinks deeper in the water, so that 2 cm will be beneath the surface: half of its height just like for the other piece of wood.
Exactly the same is happening inside the Earth, only on a bigger scale.
The crust will move vertically until it is in isostatic equilibrium with the mantle. If the crust becomes heavier, it will sink deeper into the mantle to achieve equilibrium. As a result, mountain ranges have a so-called mountain belt root (see Figure 5.1).
Figure 5-1: Mountain building by convergence of the Oceanic and continental plate. On the continental plate, both vulcanism and mountain building can be noticed.http://plaattektoniek.htmlplanet.com/plaattektoniek/botsingzones.h tm