Tectonic Impacts

Introduction

nDavid Tweed B.Sc. Dip. Ed.

nScience Coordinator and Stage 6 Coordinator P.A.C.

Lithospheric plates and their motion

ncharacteristics of lithospheric plates

The rigid surface layer of the earth is called the lithosphere

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.

noutline the motion of plates and distinguish between the three types of plate boundaries (convergent, divergent and conservative)

ncomposition of igneous rocks and plate boundary type

 

Convergent Plate Boundaries

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

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 Boundaries

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.

assess current hypotheses used to explain plate motion

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).

 

Plate Motion

 

The movement of plates results in mountain building

nmountains formed at

u  ocean/ocean boundaries

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.

 

u  ocean/continent boundaries

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.


u  continent/continent boundaries

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.

 

 

Continents evolve as plate boundaries move and change

noutline how the Australian continent has grown over geological time as a result of plate tectonic processes

u  tectonic map of Australia in terms of age Perspectives p446

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.

 

nsummarise the plate tectonic super-cycle

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.


Natural disasters are often associated with tectonic activity

npredict where earthquakes and volcanoes are currently likely to occur based on the plate tectonic model

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.

 

n       describe methods used for the prediction of volcanic eruptions and earthquakes

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.

n       describe the general physical, chemical and biotic characteristics of a volcanic region and explain why people would inhabit such regions despite the risk

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.

n       describe hazards associated with earthquakes, including ground motion, tsunamis and collapse of structures

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.

 

n       describe hazards associated with volcanoes, including poisonous gas emissions, ash flows, lahars and lava flows and examine the impact of these hazards on the environment, on people and other living things

 

Poisonous gas emissions - Lake Nyos, in Africa 1700people killed when the crater lake released a poisonous gas into the village valley

 

Pyroclastic Flow

nMount Saint Helens 1980 before

nMount Saint Helens 1980 After

Ash flows  - Pyroclastic flows which contain high temperature ash, fall quickly and can bury whole cities eg Pompeii.

 

Lahar

nLahars – Mt Pinatubo. Often once the volcano has erupted it releases trapped water with the ash, creating a mud flow called a LAHAR.

nhttp://volcanoes.usgs.gov/Products/Pglossary/lahar.html

nPhotograph by J.N. Marso on 14 August 1989

 

 

Lava Flows

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.

n       justify continued research into reliable prediction of volcanic activity and earthquakes

 

Better prediction saves lives

 

n       describe and explain the impacts of shock waves (earthquakes) on natural and built environments with reference to specific examples

 

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.

 

n       distinguish between plate margin and intra-plate earthquakes with reference to the origins of specific earthquakes recorded on the Australian continent

·         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.

 

Plate tectonics and climate

 

     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.

n2001 Exam Q17 Tambora Volcano

 

3 

(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)

The movement of the continents has caused changes in climate.

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.

 


Preparation For the exam

nStudy what was assessed last year

u  Use the standards package

sample of student band5/6 answer

nPredict the possible questions this year

nPrepare for these questions

 

What This Means

nDo some hard work in learning….  There is no easy road!!

nDo it now!

 

Websites to use

 

1 UNISERVE

http://science.uniserve.edu.au/school

 

2 HSC Online

http://hsc.csu.edu.au

 

3 USGS  This Dynamic Earth

http://pubs.usgs.gov/publications/text/dynamic.html

 

4 Board of Studies

http://www.boardofstudies.nsw.edu.au

 

5 My EES Website

http://www.angelfire.com/sc2/ees

 

6 Email me

 david_tweed@hotmail.com


2001 exam

Candidates who used general information to answer questions addressing these skills were usually unable to reach a high standard.

         i.e. You must detail specifics, give examples from your studies.

 If you don’t have examples then get some!!

nRefer to any source material given in the exam.

nGeneral statements such as “agriculture suffered” or “agriculture was hindered ”  are to be avoided, make some detailed comments about the degree of the effect…..examples are even better.

nPoorer answers lacked clarity-so be clear!  (know what you are talking about)

 

 

 

 

 

 

 

 

 

2002 exam - What can they ask?

Same as last year

 

 

Things left out from last year

 

 

 

Mixture of both

 

 

 

 

For Example ….

 

Name the most common igneous rock type found at Conservative plate boundaries

Name of the most common igneous rock type found at the divergent plate boundaries

Assess one hypothesis used to explain plate motion.

Compare the formation and  rock type of mountain belts formed as a result of thermal uplift and rifting with those resulting from different types of plate collision.

Describe the plate tectonic supercycle concept.

Identify the technology used to measure crustal movement at collision boundaries and describe how this is used.

Identify three hazards associated with earthquakes

Describe three hazards associated with volcanoes and examine the impact of these hazards on the environment, on people and on other living things.

Distinguish between plate margin and intraplate earthquakes with reference to earthquakes recorded on the Australian continent.

Outline the relationship between the plate tectonic super-cycle and the occurrence of ice ages (the icehouse/greenhouse cycle)

 

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