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9.2 Tectonic impacts: 2. Mountain building
| Syllabus reference (October 2002 version) | ||
|---|---|---|
2. The movement of plates results in mountain building |
Students learn to: | Students: |
Extract from Earth and Environmental Science Stage 6
Syllabus (Amended October 2002). © Board of Studies,
NSW
[Edit: 1Mar05]
Prior Learning: Preliminary modules 8.2 (subsection 2), 8.5 (subsections 1, 2 and 3); Stage 5, Outcome 5.9.
Background: Mountains are formed as a result of the interactions between plates. The types of mountains formed depend on the type of plate interaction.
gather, process and present information from secondary sources which compares formation, general rock type and structure of mountain belts formed as a result of thermal uplift and rifting with those resulting from different types of plate convergence
- A good way to compare information is to structure the
information in a table.
- A table like the one below is an effective tool to assist you
to gather, process and present information.
Well-designed tables assist you to identify useful information
and will assist you to notice trends and patterns. Try using a
table to sort out the information from the notes provided after
the table.
| Mountain belts formed by: | Mountain belt features | ||
|---|---|---|---|
| Formation | General rock type | Structure of mountain belts | |
| thermal uplift and rifting | |||
| ocean/ocean boundaries | |||
| ocean/continent boundaries | |||
| continent/continent boundaries | |||
distinguish between mountain belts formed at divergent and convergent plate boundaries in terms of general rock types and structures, including folding and faulting
Divergent boundaries
Mountain belts formed from the action of thermal uplift and rifting are of two main types:
- Mid-ocean ridges form a near-continuous underwater mountain
chain that extends for 60 000 kilometres right around the globe.
Mid-ocean ridges rise to over 2.4 kilometres above the floor of
the 5 kilometres deep ocean basins. A mid-ocean ridge can be a
wide a 2000 kilometres.
Mid-ocean ridges result from convective upwelling of mantle beneath thin oceanic lithosphere. They are formed along structurally weak zones created where the ocean floor is being pulled apart lengthwise along the ridge crest. New magma from deep within the Earth rises easily through these weak zones and eventually erupts along the crest of the ridges to create new oceanic crust. This process is called seafloor spreading.
At the top of the oceanic crust at mid-ocean ridges are basalt lavas. The lavas often form as pillow lavas. Beneath are numerous basaltic dykes and deeper down are gabbros. The topography near the ridge axis is very rough and mountainous. At the centre of each ridge there are steep-sided troughs, several kilometres wide, which are similar to rift valleys that occur on continents. Mid-ocean ridges are offset by transform faults that run perpendicular to the ridge axis. The faults are only active between the spreading centres.
- Young rift zones occur within continental landmasses and are caused by convective upwelling of mantle beneath weak continental lithosphere. When continental crust stretches beyond its limits, tension cracks begin to appear on the Earth's surface. Magma rises and squeezes through the widening cracks, sometimes to erupt and form volcanoes. Rift zones generally have intensive basaltic igneous activity. The rising magma, whether or not it erupts, puts more pressure on the crust to produce additional fractures and, ultimately, the rift zone. The uplift produces plateaus adjacent to the rift. These plateaus generally slope upwards towards the rift valley. Escarpments in the rift valley are formed from normal faulting into the rift. Such features are seen in Africa along the East African Rift Zone.
Convergent boundaries
The three types of convergent boundaries result in the following mountain types:
Ocean/ocean boundaries:
Mountains formed at ocean/ocean boundaries are of the volcanic island arc type. They form on an oceanic plate that has another oceanic plate subducting under it. There are two types of mountains that can form at ocean/ocean boundaries.
- Those that comprise elongated mounds of ocean floor sediments that have been tightly folded and chaotically mixed in the trench by the faulting and folding caused as they are scraped from the down-going oceanic plate. The southern line of islands of the Indonesian Archipelago is a good example of this type.
- Those formed of chains of explosive volcanoes. These volcanoes form from andesitic magmas that are generated as the subducted plate partially melts when it comes in contact with the hot asthenosphere. Steam and other volatile substances find paths upwards, creating vents for magma to reach the surface to create the volcanoes. The northern line of islands of the Indonesian Archipelago is a good example of this type. Island arcs can be deformed by strike-slip faults and folds.
Ocean/continent boundaries:
As an oceanic plate is subducted beneath a continent, the sediments on the upper surface of the lower plate will be scraped off to produce a wedge of sediment called an accretionary wedge. Where the accretionary wedge is forced directly against the leading edge of continental crust, the subducting plate will be forced down steeply into the asthenosphere where the plate will be partially melted. Steam produced in the process also partially melts the upper mantle. Andesitic magmas are produced from these processes. Mountains will be produced in the continental plate from the compression and uplift of the low density wedge sediments and the sediments and rocks of the continent, and from the intrusion of magma produced from the partial melting in the subduction zone. These mountains rise to very high altitudes and contain highly folded and faulted sedimentary rocks produced from the compressional forces. The upper sections of sedimentary mountain ranges remain poorly consolidated and quickly erode, producing large amounts of sediment for the rivers that drain from them. The intrusions of magma are in the form of large granitic batholiths beneath the volcanic belt. The mountains contain explosive andesitic volcanoes. The explosive volcanoes produce much pyroclastic sediment that is deposited in the mountain areas. The explosive volcanoes frequently form calderas where they develop from eruptions from large, shallow magma chambers. The Andes Mountain chain in South America is a good example of this ocean/continent type of mountain building.
Continent/continent boundaries:
When two continents collide, the ocean between them has been subducted under one of them. The continents will have been flanked by accreted sediment from the ocean floor that was scraped off from the subduction. This sediment forms into a huge wedge as it is folded, compressed and uplifted. Rocks from old oceanic plate, called ophiolites, can also be squeezed between the two continents and be uplifted as part of the mountain range formed. Ophiolites are very mafic and are composed of rocks like basalt and gabbro. Eventually the two older sections of colliding continents meet. These older sections of the continents are called cratons. Cratons are made up of crystalline igneous and high-grade metamorphic rocks. They are old and incompressible. The rocks of the craton splinter and fault at low angles, stacking on each other as they are compressed to form mountains. The Himalayas are an example of a mountain range that has been formed from compressed ocean floor sediments and fractured cratons. Low-angle thrust faults are common.
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