Why live near volcanic zones?

Close to an erupting volcano the short-term destruction by pyroclastic flows, heavy falls of ash, and lava flows can be complete, the extent of the damage depending upon the eruption magnitude. Crops, forests, orchards, and animals grazing or browsing on the volcano's slopes or surrounding lowland can be leveled or buried. But that is the short-term effect. In the long run, volcanic deposits can develop into some of the richest agricultural lands on earth.

One example of the effect of volcanoes on agricultural lands is in Italy. Except for the volcanic region around Naples, farming in southern Italy is exceedingly difficult because limestone forms the basement rock and the soil is generally quite poor. But the region around Naples, which includes Mount Vesuvius, is very rich mainly because of two large eruptions 35,000 and 12000 years ago that left the region blanketed with very thick deposits of ash and rock which has since weathered to rich soils. Part of this area includes Mount Vesuvius. The region has been intensively cultivated since before the birth of Christ. 




The fertility of many farmlands of the North Island of New Zealand are due to volcanic soils of different ages. These have developed on older (4,000 and 40,000 years old) volcanic ash deposits of the Waikato and Bay of Plenty regions. Combined with ample rainfall, warm summers, and mild winters, these regions produce abundant crops.



Another thing that volcanic mountains and mountain ranges do is enhance local rainfall if the conditions are in its favour.  Moist air flows are pushed into cooler areas which the condense and fall back as rain.  Combined with the rich soils, food production can be very high in these areas.

The Tectonic Supercycle

The tectonic supercycle (also known as the Wilson cycle) describes the movement of continents where they congregate to fora a supercontinent, break up, and eventually rejoin again to form a new supercontinent.  The last supercontinent was Pangaea   The breakup of this continent was thought to have occurred about 250 million years ago.  The supercontinent Rodinia was thought to have begun forming 1100 million years ago and appears to have broken up about 750 million years ago.

Parts of Western and Central Australia were in the northern hemisphere when Rodinia was formed, but much of Australia as we know it already existed in the southern regions of Pangaea.  Continental drift over long periods of time has help explain this crossing of the hemispheres.




The cycle is as follows:

1. A stable cration (geologically stable region) begins rifting and pulling apart (African Rift valley)
2. Over time the extent of rifting allows for the ocean to flood in. Two new continents are made (South America and Africa, separated by the Atlantic Ocean)  
3. Eventually the denser oceanic lithosphere begins to get pushed under the continental lithosphere, causing a subduction zone to form.
4. As the subduction continues, an accretionary wedge and volcanic activity add land to this area.  The gap between the continents decreases.
5. Eventually the 2 continents collide, forming fold mountains.  Eventally motion stops.  
6. Erosion flattens the mountain and the craton becomes stabilised.

Plate Boundaries

Divergent plates

Divergent plates are where the upwelling of magma at tectonic plate boundaries causes lava to be extruded and pulls the 2 plates apart.  This part of the lithosphere contains mafic rock (high in iron and magnesium) from vulcanism and include basalt and gabbro.  Plate divergence is generally occurring under the sea, with the notable exception if Africa's rift valley and Iceland.





Oceanic oceanic convergent plates

This takes place were 2 oceanic lithospheric plates collide to form island arcs. Examples of these include Japan, Indonesia and The Phillipines.


The older cooler plate gets subducted due to its slightly higher density.  As the plate sinks sediments and water are also subducted.  This lowers the melting temp of the rock.  When it melts, the sediment and water may be incorporated into the rock altering its chemistry.  The pressure of the molten rock mixed with water forces it upwards where it erupts forming the island arcs.  Rock such as granite and rhyolite can be formed at these volcanoes.  These are termed felsic rocks.  These are lower in iron and magnesium, but higher in silica.



Continental oceanic convergent plates
When continental plate encounters a oceanic plate the oceanic plate is subducted.  Rather than forming a island arc, these plates form a line of volcanic mountain ranges near the coastline.  Examples of these are the Andes in South America and the Cascade mountains in the USA.  Rock types produced from this activity is the same as those in oceanic-oceanic boundaries.






Continental Continental convergent plates
When 2 continents collide fold and thrust mountains will occur.  This is caused by the compressional forces generated when the 2 continents are pushed together.  There is no volcanic activity in these plates so no new igneous rock is formed.  However frequent earthquakes occur due to the buildup and sudden releases of pressure on these rocks.








Transverse faulting

This occurs where 2 plates are moving in opposite directions relative to each other.  There is no volcanic activity, but earthquakes can be frequent and sometimes strong.  The figures below shows movement of trees in an orchid and fencposts relative to each other from slippages of the San Andreas Fault.

Lithospheric plates

Lithospheric plates are regions of Earth's crust and upper mantle.  This is the rigid solid part of the Earth.  This solid surface is fractured into plates that move across a the plasticine like mantle.  The term plasticine refers to something a bit like plasticine that you may be familiar with in your younger years.  While it's generally regarded as reasonably solid, it moves very slowly like extremely thick and viscous honey.  

There are 2 types of crust on the lithosphere, oceanic and continental.  Oceanic lithosphere is slightly denser and is up to 100 km thick.  Continental lithosphere is thicker (up to 150 km) and is made up of less dense rock.

Mine site rehabilitation. Mount Owen complex

The Mount Owen complex is made up of three sites and back onto Ravensworth state forest.  It is an open cut coal mine.  To ensure the environmental integrity of the area is restored by the end of mining operations, the following steps need to be undertaken.

1. A flora and survey needs to be done.

2. Topsoil is removed and and stored.

3. Spoil (rocks and deeper soil removed to access the coal) is stored in dumping areas.  

3. Following the exhaustion of this open cut mine, the spoil is placed back in the excavated areas.  Any remaining coal must be completely buried so that a bush fire does not ignite a seam of remaining underground coal and travel underground.

4.  The topsoil is then placed on top.  The existing seeds in the soil can be used to regenerate the area or deliberate replanting of the area with vegetation identified with the initial flora and fauna survey can be used.

5.  Revegetation may be a multi-staged work.  In some instances grass can be quickly established to stabilise the soil, followed by tree planting to restore the old vegetation. 

6.  Once completed, the area will need to be monitored over an extended period to ensure that regrowth is following projections and that native fauna is returning.







A summary of the whole process is below.

Stormwater Treatment.

The example given for you was Lake Macquarie on the Central coast of NSW.  Recent and rapid urban development caused urban erosion due to land clearing, increased impermeable surfaces and outdated stormwater drains were causing erosion. 


The immediate concern of the local council was to reduce the sediments being transported into the lake from the creeks and streams and the erosion occurring on the sides of the lake.

Revegetation was part of the solution to try and stop erosion from happening in the first place, but the most effective step appears to have been the construction of artificial wetlands.  These work for a number of reasons.

1. They slow the flow of water down reducing its potential to cause erosion.
2. Slowing the water down also allows sediments to fall out before they reach the lake
3. Plants in the wetlands also absorb nutrients and thus improve the quality if the water entering the lake.

Artificial wetlands also protect the lake oven when overflowing in heavy rain by reducing sediment flow into the main waterway.  Even though their capacity to remove nutrients and pollutants is greatly reduced, they continue to slow water down so that it is not able to carry as much sediment into the lake.

Environmental Flows and River Health

There have been 2 well known rivers in NSW that have had significant alterations to their health as a result of damming and water diversion for agriculture.  One is the  Snowy river and the other is the Murray-Darling.  We will concentrate on the Snowy River.

As a result of the Snowy hydroelectric scheme, water flows of the Snowy River fell by 95-99%.  Because of this severe degradation, the river had silted up, weeds and feral plants were established, and the fish life was dying out. Scientists had established that it would take 28% of the original flows to restore the river back to good health.

In the late 1990's the NSW and Victorian governments agreed to release this water into the river, gradually increasing it to the 28% level about a decade later.  The drought from 2000-8 caused the governments to hesitate because water was being diverted to agriculture.

Eventually court action by local environment groups force the release of water in 2011.  Water is released seasonally as to mimic the spring snowmelt and copy the behavior of the river in a natural state.  This cylce of large and rapid releases has cleared the river of a lot of the silt and debris, and it is hoped that indigenous water fauna will now return.




A YouTube link is below:

www.youtube.com/watch?v=55WUBhBibkw