The lithosphere contains the crust and the uppermost part of the mantle and is somewhere between 80 to 150 kilometres thick.
Lithosphere is strong and relatively rigid and this rigid surface is broken into regions called Lithospheric Plates.
The lithospheric plates move relative to each other, and it is these interactions which cause earthquakes, volcanoes and mountainbuilding.
Convergent plate boundaries are where the lithosphere is being consumed. This can occur where oceanic crust is subducted beneath oceanic crust or continental crust. Convergent plate boundaries usually create volcanoes on the overlying plate. Andesite is the most common form of igneous rock found in these convergent boundaries.
A good example of this is the boundary along the Java trench.
Divergent plate boundaries are where new crust is being created along the spreading axis of a mid ocean ridge. Basalt is the most common type of igneous rock formed at divergent boundaries.
A good example, close to Australia, is the mid- ocean ridge between Australia and Antarctica.
Conservative plate boundaries are where one plate is sliding past another. In this case the lithosphere is neither created nor destroyed. Conservative plate boundaries result in metamorphic rocks in the twisting and folding as the two plates slide past each other.
A good example of this is the Alpine Fault in New Zealand, where the Australian and Pacific plates slide past each other on a transform fault.
The most widely accepted hypothesis used to explain plate motion is the convection current hypothesis. This hypothesis uses the idea that heat exchange from the mantle causes convection currents in the liquid rock. The friction between the liquid rock of the asthenosphere and the rigid plates above causes those plates to move.
Another proposed hypothesis is that of ridge push and slab pull .This hypothesis proposes that the expansion at the mid ocean ridge causes the lithosphere to be higher. Being higher it would slide, under gravity, to a lower place (the trench).
The islands of Japan and Indonesia are examples of mountains formed at ocean/ocean boundaries. One piece of oceanic plate will readily sink beneath another in this process. Sediments from the sea floor saturated with water are subducted into the upper mantle. When this material melts it is less dense than surrounding material and begins to rise up through the overlying layer of the lithosphere to create volcanoes which are very explosive. The large amounts of water contained in the source material results in magma containing more quartz and feldspar making rocks like andesite.
The mountains formed at ocean/continent boundaries are usually much higher and contain not only volcanics but large amounts of folded and faulted sediment shaved off the subducting plate.
The Andes mountain range of South America is a good example of mountains formed an ocean/continent boundary.
The mountains formed at continent/continent boundaries usually contain faulted and folded sediments. These mountain ranges result from the collision between two plates both containing continental crust. Continental crust cannot be subducted so when the two plates to meet they crush and create a large mountain range.
A good example of this is the Himalayan Mountain range where the Indian plate collided with the Asian plate.
The oldest rocks in Australia are found in Western Australia in the Pilbara block and the Yilgarn block. The processes which formed these rocks are uncertain simply because they are so old(2300 million years old).
During the period from 2500 to 900 million years ago these cratons were separated by linear mobile belts which underwent more than one period of mobility. As these belts built into cratons they added the new material to the existing land mass. By 900 million years ago, two-thirds of the present-day Australia was built.
The Tasman fold built in the East, and developed until the end of the Paleozoic. By the end of the Triassic the Australian mainland was built. The final stage of this was the breakup of Gondwana, which began in the Jurassic.
The plate tectonic supercycle relates to to the formation of supercontinents and their subsequent break up.
If you take plate tectonics to its logical extreme, you will see that the continents must at some stage re-form once subduction is complete.
Because seafloor spreading will continue , ocean crust will be subducted beneath the supercontinent. This causes acidic volcanic mountain ranges to form at the edges of the continent, creating weaknesses. These seafloor subduction zones eventually choke with sediment and a new ocean-ocean convergent boundary begins further off the coast. This creates island arc volcanics with a back arc basin, with a similar structure to that of Japan and the China Sea.
The weaknesses created by the volcanic activity may be enough to start a new cycle of seafloor spreading across the supercontinent, creating many smaller continents once again.
It is believed by those who proposed the plate tectonic supercycle that this process has repeated itself many times over.
Earthquakes and volcanoes are more likely to occur on, or near, plate boundaries. It was in fact the plotting of earthquakes on a map of the earth that led to the true shape of the plates and their locations.
Most volcanoes are a result of plate convergence. Island arc volcanics, like those found in Japan , the Pacific Islands and Indonesia, are all located very close to convergent boundaries. The volcanoes found on the West Coast of the Americas all result from convergent boundaries between ocean and continental crust.
A few rare volcanoes occur in the middle of a plate and are the result of a hot spot which melts its way through the lithosphere.
Many earthquakes occur at Conservative boundaries because of the friction and release as one plate slides past another plate. The Alpine Fault in New Zealand and the San Andreas Fault in California are two places where this occurs.
Scientists use equipment which can be adjusted to measure any sort of change in distance, or a change in angle. A geodimeter is one type of instrument which measures changing distance from one place to another, Strain gauges do the same.
With earthquakes it is frequently small increases in distance which indicate an increase in tension from one side of a fault to another. With experience, scientists can predict the chance of an earthquake occurring.
Volcanologists use frequency of earthquakes and deformation of the volcano itself, as well as increase in temperature to predict the likelihood of an eruption.
Volcanic regions have rich soils and consequently lush vegetation.
Being tall mountains, volcanoes can also create their own microclimate with increased rainfall.
Very rich soils are also extremely productive for agriculture and so people will exploit this opportunity.
Most volcanoes remain dormant for many years, possibly even lifetimes for human beings. Many people will take the risk of living near a volcano because of the great rewards in the productivity of their soils.
Earthquakes are massive vibrations within the earth. When these vibrations hit the surface of the earth anything on the surface will also vibrate. Buildings not made to withstand the vibration of the earthquake wave will collapse.
If the earthquake occurs beneath the ocean then it will transfer its energy to a water wave which will not appear very dangerous to ships at sea. When the wave gets close to shore, the crest of the wave increases in proportion to its energy and crashes on the shore destroying anything in it’s path. This wave is called a tsunami.
Poisonous gas emissions - Lake Nyos, in Africa 1700people killed when the crater lake released a poisonous gas into the village valley
Ash flows - Pyroclastic flows which contain high temperature ash, fall quickly and can bury whole cities eg Pompeii.
Lava Flows – are usually more damaging when the lava flows quickly. Intra plate hot spot volcanoes(like Hawaii) have extensive lava flows which actually build the Islands. The rapid flows of lava bury any human settlements in their path.
Better prediction saves lives
The bigger the earthquake the greater the damage. The closer the earthquake epicentre is to human settlement, the greater the potential for damage.
Northridge, California and Kobe, Japan(1995 richter scale 7.2) are two towns built close to faults. When these faults gave way, the resultant damage was extensive and expensive. Buildings were toppled, freeways lay on their sides, bridges collapsed and people were killed by some of these collapses.
Earthquakes shake in 3 ways – push (P waves) , shear (S waves) and surface waves(L waves), surface waves being the most destructive as landform is lifted and dropped. The Richter scale measures the intensity of an earthquake on a scale from 1-10, each number being 10 times more damaging than the last. A 9.5 earthquake in Chile had total destruction of built environments and natural areas …. waves were seen in the land surface.
· The most disastrous Australian earthquake in the last 200 years was the Newcastle earthquake of 28 December 1989. It was a magnitude 5.6 earthquake that caused $1.2 billion damage. The most likely cause was by readjustments along the Hunter-Mooki Thrust, a curved fault running from Newcastle and through Maitland, Murrurundi, Quirindi. Narrabri and Mackay, The fault is sporadically active due to strong easterly compression from the expanding Pacific Ocean floor.
For more detail refer to Plimer Professor I.R., How, when, where and why an earthquake occurs, Newcastle Herald, 30 December 1989
· Some earthquakes in South Australia are said to be a result of the flexing of the centre of the plate as Australia twists to the north and west.
· Plate margin earthquakes are rare in Australia. Plate margin earthquakes occur more frequently than, and are of higher magnitudes than, intra plate quakes as well.
predict the possible effects of explosive volcanic activity on global and local climates
• describe and explain the potential and observed impacts of volcanic eruptions on global temperature and agriculture
Explosive Volcanics put large amounts of SO2 and dust into the atmosphere. At upper levels of the stratosphere, these droplets and dust reflect sunlight and cool the earth.
Cooler temperatures create slower growth of crops but much better ski seasons in Australia.
(b) Most candidates handled this part very well and provided good explanations of the likely impact on both local and global agriculture. Some responses were too general, using terms such as agriculture ‘suffered’ or ‘was hindered’.
• outline the relationship between the plate tectonic super-cycle and the occurrence of ice ages (the icehouse/greenhouse cycle)
One super continent will be hotter and drier than many smaller continents around the equator.
Once the supercontinent broke up the oceans were free to circulate heat better, causing more rain. Increased rainfall carried atmospheric CO2 to the oceans where it was precipitated. The drop in CO2 caused a drop in gobal temperature possibly causing Ice Ages.
Volcanic activity continues Cycling CO2 back into the atmosphere causing a cycle between Icehouse and Greenhouse conditions.
The ocean currents around Antarctica today are the main reason it is covered in Ice. Previously( when Australia was closer) it would have melted due to ocean currents rising to higher latitudes and warming as in the El Nino.
sample of student band5/6 answer
For Example ….