Geographic Processes operating in mountain environments
Geomorphic processes
Mountain building
The Earth’s surface is constantly changing, with tectonic activity building it up and erosion wearing it down. In this way, mountains are formed and shaped by a combination of forces. See the diagram below.
Forces of uplift: The primary force of uplift comes from tectonic activity known as plate tectonics. Plate tectonics are a consequence of the structure of the earth and the convection (heat) currents that affect the crust of the planet. The tectonic plates are like giant jigsaw puzzle pieces in the Earth’s crust. They are forced to move because of the convection currents that move from the core to the mantle of Earth.See the diagram below.
Geomorphic processes : Mountain building (continued)
The Earth’s crust is made up of a series of “plates” that “float” on the more dense almost semi-liquid mantle below. Convection currents (mostly of molten rock) in the mantle move the tectonic plates around. The heat generating these convection currents comes from the core of the planet, spreading outwards toward through the mantle toward the crust of the Earth. Some plates are forced to bump into each other (like the Indo-Australian and Asia plates) creating “ridges”, some are pushed apart (like the Indo Australian and Antarctic plates, causing “trenches”, while others “slide” (or grind) by each other, like the Middle Eastern and Eurasian plates. The movement of the tectonic plates is very gradual. For example, the Indo Australian plate (a fast moving plate) is moving northward at about 6 - 8 cm / year. This plate has been moving for the past 40 million years. How far has the Indo Australian plate drifted (at 6 cm / yr)? 240 000 000cm
Places or areas close to the edges of the plates have a greater likelihood of experiencing:
- earthquakes or minor tremors;
-volcanic eruptions;
Places or areas far from the plate’s edges generally experience more stable conditions, unless local forces are active.
Look at the map on page 110 of the text.
a. What do you notice about the location of most of the world’s volcanoes and sites of major earthquakes, relative to the plates?
All the major earthquakes and volcanoes are on the edges of the tectonic plates.
b. Mark in on the map below, the mountain ranges that look like they formed along or near the edges of plates.
c. Why do you think Australia’s nearest neighbours – Papua New Guinea and Indonesia – experience so much tectonic activity while most of the Australian continent is so stable?
Because Australia is in the middle of a tectonic plate while Papua New Guinea and Indonesia are right on the edges of that plate.
d. Look at the area where the Indo-Australian plate meets and appears to bump into the Eurasian plate just north of India. Use an atlas to find the mountainous areas in this area. What does this tell us about tectonic plates and mountain building?
The more tectonic activity there is the more and the higher mountains grow.
There are many more dormant volcanoes – currently inactive but still able to erupt.
Extinct volcanoes were active once but are unlikely to ever erupt again.
Australia has no active volcanoes but there is plenty of evidence of dormant volcanoes, such as Mt Warning in northern NSW.
How are fold mountains formed ?
Give three examples of fold mountains;
Himalayas
Andes
Jura Mountains
Block or Fault Mountains
Block mountains form along cracks or faults where some parts rise to form mountains and other parts do not, forming valleys.
The layers of rock on one side of a fault line may be pushed up forming a block mountain. On the other side of the fault line a depression is formed, called a rift valley.
Examples of fault lines are the San Andreas Fault in California USA. The Sierra Nevada mountains in California are examples of block mountains.
Dome Mountains
Dome mountains form under different geomorphic (“earth changing” or “earth moving”) processes. Pressure from rising magma forces the surface rock layers to rise but the surface rocks do not break to form a hole (as a volcano does). So, instead of a volcano, a “dome” of uplifted “bent” rock layers forms a dome mountain.
What forces shape mountains ?
As soon as mountains are formed from the geomorphic forces of uplift below, they begin to be worn down by atmospheric and hydrologic processes over millions of years.
These atmospheric and hydrologic forces are referred to as:
The mountain ecosystem
Mountain areas have a unique ecosystem. This is mainly because of the effect of
altitude on temperature. Temperatures decrease as altitude increases, because the atmosphere becomes “ thinner “ and is unable to retain as much heat. Temperatures fall by 60 C for every 1000 m. Temperatures on the 5th April 2006 dropped significantly because of strong southerly and south westerly winds. Parramatta’s morning temperature was 90. Mount Kosciuszko (2228 m) received snow on this day. What might have been the approximate morning temperature at the summit of Mt Kosciuszko on this day? -7 degrees
At altitudes higher than 2000 m water may freeze and precipitation may fall as snow in winter months. At altitudes greater than 4000m, sub zero temperatures are “normal”, and water will stay frozen most of the time. Ice is frozen water.
Mt Everest is 8848 m high!
Also, some high mountains benefit from the (this is where moist air is
So, a high mountain or mountain range may contain more than one form of water as altitude increases.
Precipitation changes as altitude increases, mainly because of falling temperatures (and often greater amounts of moisture). Freezing conditions and ice are ‘stunting” on plant growth. This is why high mountains have a “tree line”, where trees cannot grow above a certain altitude because soil and air temperatures are too low to allow trees to grow. In the snowy mountains, the tree line is about 1850 m. Nevertheless, small shrubs and tussock grasses can grow at these latitudes. Beyond 5000 m virtually no plants will grow.
So, a flow chart could be used to explain these and other features in the following way…