Tectonic Impacts

8/4/2019 Tectonic Impacts

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Tectonic Impacts HSC E & E Science

Lithospheric Plates and their Motion

Describe the characteristics of lithospheric plates

Lithospheric plates are the plates that comprise the fractured surface of the earth. There

are two types, continental plates comprised mainly of granitic type rock rich in silica andaluminium and oceanic crust is comprised mostly of basaltic-type rocks rich inmagnesium and aluminium. Oceanic crust is denser than the continental crust.Earthquake and volcanic activity is common along lithospheric plate boundaries.

Oceanic Crust Continental Crust

Mean Thickness 7 km 30-50 km Thicker under mountains

Identify the relationship between the general composition of igneous

rocks and plate boundary typeIgneous rocks are classified by two main properties:

- mineral crystal size- mineral composition

• Mid-ocean ridges produces basalt which covers the seafloor, these basalts produced are hotter and less dense than the cooler basalts before they aresubducted

• Gabbro forms at divergent plate boundaries

• Convergent plate boundaries – andesite is produced in conjunction with other igneous rocks such as granite

Type of Boundary Type of Igneous Rock

Divergent Basaltic (mafic)

Convergent Oceanic – oceanic Basaltic early and as it matures andesite

Oceanic – Continental

Andesite

Continental – continental

Granite (placed as plutons)

Transform (or conservative) None

Outline the motion of plates and distinguish between the three typesof plate boundary

Convergent

• This is where two plates collide as a result of tectonic motion

• There are different types of convergent plate boundaries these are:o Oceanic – oceanic (Tonga – Philippine and Australian Plates)

o Oceanic – continental (The Andes – Nazca and South American Plates)

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o Continental – continental (Himalayas – Indo-Australian and Eurasian

Plates)

Divergent

•Basaltic magma is injected between two plates causing the plates to be pushedapart

• Originate along fault lines and these fault lines produce rift valleys

• Mid ocean ridges are also formed by this type of plate boundary – MORs arehundreds of kms wide and thousands of kms long

• Divergent plate boundaries cause tensional forces that pull the plates apart andaway from each other, creating normal faults

• The different types of divergent plate boundaries are:o Oceanic – oceanic (North Atlantic Rock)

o Continental – continental (East African Rift Valley – African plate

boundary)

Conservative or Transform

• When these boundaries occur on land they are subject to greater weathering andare therefore not as steep

• San Andreas Fault (Juan de Fuca and North American Plates) and Alpine Fault(Australian and Pacific Plates)

• Plate material is neither produced nor destroyed at this plate boundary but it may be metamorphosed

MUST PUT IN DIAGRAMS OF EACH PLATE BOUNDARY

Describe current hypotheses used to explain how convectioncurrents and subduction drive plate motion

The three hypotheses are:- slab push from the MOR - Slab pull at subduction zone- Gravity slide

Slab Push

- Rising magma at a spreading centre creates new lithosphere and in the process

pushes the plates away from each other - PROBLEM – pushing involves compression, but the existence of rifts alongMORs indicates a state of tension

Slab Pull

- lithosphere breaks and starts to sink through the asthenosphere because coldlithosphere is more dense than the hot asthenosphere

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- PROBLEM – lithospheric plates are brittle and they are much too weak totransmit large-scale pushing and pulling forces without major deformationoccurring in the middle

Gravity Slide

- The lithosphere grows cooler and thicker away from a spreading centre,

therefore the boundary between the lithosphere and the asthenosphere slopesaway from the spreading centre. If the slope is as little as 1 part in 3000, thelithosphere’s own weight could cause the lithosphere to slide at a rate of several centimetres per year.

Of the three ideas it is believed that subduction starts when old, cold lithosphere breaks

and begins to sink, pulls on the plate and starts the movement. Once movement starts,downhill slide and ridge push combine to keep the movement going.

The movement of plates results in mountain building

Distinguish between mountain belts formed at divergent andconvergent plate boundaries in terms of general rock types

Oceanic – oceanic divergent boundary

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E.g. North Atlantic Ridge☻ ascending hot currents in the asthenosphere push the whole area up☻ dominant rock type is a volcanic lava of basaltic composition☻ extruded as pillow basalts☻ thin veneer of sediments

☻ normal faults are produced as tensional forces pull the lithosphere apartGET DIAGRAM FROM CLEZY’S REVISION

Oceanic – oceanic convergent boundary

E.g. Tonga (immature)☻ as subduction occurs the molten basaltic material rises to form an arc of

volcanic islands☻ restricted largely to igneous activity (initially basaltic) and does not include

widespread metamorphism or granitic intrusions☻ when two oceanic plates converge one is subducted under the other, and in the

process a trench is formed

☻ subduction processes in oceanic – oceanic plate convergence also result in theformation of volcanoes☻ such volcanoes are typically strung out in chains called island arcs☻ magmas that form island arc are produced by the partial melting of the

descending plate☻ over millions of years, the erupted lava and volcanic debris pile up on the

ocean floor until a submarine volcano rises above sea level to form an islandvolcano

☻ the descending plate also provides a source of stress as the two plates interact,leading to frequent moderate to strong earthquakes

☻ these earthquakes are shallow to deep focus and the pattern produced istermed a benioff zone

E.g. Japan (mature)☻ a more complex situation in which heat and or pressure combine to produce

distinctive rock associations and deformational patterns☻ rock sequences of oceanic sediments, pillow basalts from the oceanic crust, in

addition to sediment from erosion of the volcanic arc☻ partial melting produces andesitic/granitic magmas☻ A zone of high pressure, low temp. metamorphism (blue schist)☻ an inner zone of high temp metamorphism (green schist)☻ crustal deformation results in older rock to be compressed, folded and broken

by thrust faults

Continental – oceanic convergent boundary

E.g. Andes☻ before subduction sandstone, shale and limestone can accumulate along the

continental margin☻ deep marine sediments on the oceanic plate can be crumpled and deformed☻ slabs of oceanic crust, called ophiolite complexes, shear off

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Continental – continental convergent boundary

E.g. Himalayas☻ the convergence of two continental plates generates a mountain belt with

several different characteristics

☻ shallow and deep water sediments occur along the margin of the continents☻ a trench forms with all the characterisitics of a oceanic/continental subductionzone

☻ as continents approach segments of the remaining oceanic crust (ophiolite) aredeformed by overthrusting and are eventually squeezed between the plates

☻ as the continent moves into the subduction zone, its buoyancy prevents it fromdescending into the mantle. It can be thrust under the overriding plate,however, so that a double layer of low-density continental crust is produced.This layer rise buoyantly to create a wide belt of deformed rock.

☻ These regions are characterised by intense folding, thrust faulting andmetamorphism

☻ The oceanic slab of lithosphere, descending intot he mantle, ultimately becomes detached and sinks. When the slab has been consumed, the volcanicactivity and earthquakes it generates cease.

Folding and Faulting

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Gather, process and present information from secondary sourceswhich compares formation, general rock type and structure of mountain belts formed as a result of thermal uplift and riftingwith those from different types of plate convergence

SummaryDivergence

(Tensional force)

Convergence

(Compressional force)

Oceanic/oceanic Oceanic/oceanic Oceanic/

continental

Continental/

continental

Igneous rock Basalt Basalt to

andesite

Andesite

volcanics and

granite plutons

Granitic plutons

Metamorphic little

little

widespread

e.g. Slate, schist,

gneiss

widespread

e.g. Slate, schist,

gneiss

Sedimentary Small amounts

of ooze

oozes andigneous material

eroded from arc

shallow marinesediments e.g.

Limestones,shales,

sandstones

shallow marinesediments e.g.

Limestones,shales,

sandstones

Faulting type Normalassociated with

tensional force

Reverse Reverse & thrust Reverse, thrust& nappes

Folding Little or none

minor

Intense folding

Intense folding

Continents evolve as plate boundaries move and change

Outline the main stages in the growth of the Australian continent over geological time as a result of plate tectonic processes

Stage 1- still with Gondwana- 3500 mya = Archean Rock - Pilbara and Yilgarn were formed 2500 mya- Granite greenstone were cratorised, these have formed andesite and basalt

through metamorphismStage 2

- still with Gondwana

- Proterozoic – 2500-570 mya- Mobile belts

o Active linear mountain

o Chains – metamorphic rock

o Highly deformed and folded

- Formation of cornerstone blocks and fusion of cratonso Growler Block

o Kimberley Block

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o Mt Isa Block

Stage 3- still with Gondwana- Palaeozoic 570 – 230 mya- Subduction – accretion

- Tasmin belt formed – active island arcs and trench- Eastern rocks exhibit evidence of the presence of former island arcs and oceantrenches

- Major mountain building – Lachlan Fold Belt and New England Fold Belt- Active/mobile belt move east – producing Lord Howe Island- By 200 mya eastern third of Australia was cratorised

Stage 4- Mesozoic – 190 – 65 mya- Beginning of the breakup of Gondwana- Great Artesian Basin formed, then flood to form an inland sea and then it

dried and became non-marine

- Tasmin sea spreading- Sydney Basin – Foreland basin subsided as the magmatic arc eastward – Foreland Basin filled with sediment

Stage 5- Intra-continental earthquakes, hot spots and volcanoes- Tensional stresses – acting within plate boundaries to north pushing against

asian and pacific plates- Very old faults joined

Stage 6- still moving northward- interaction between the converging Australian and Pacific plates produce the

current New Guinea mobile belt

DRAW MAP OF AUST. WITH ALL NECESSARY MOVEMENTS

Summarise the plate tectonic super-cycle

- plates move due to thermal and gravitational affects, that is, by seafloor spreading and subduction

- continents were once apart of the supercontinent Pangea- approx. 200 mya Pangea split into Laurasia and Gondwana. Gondwana

includes the present continents of:o Australia

o Antarticao Africa

o South America

o Indian sub continent

Laurasia split into:o North America

o Eurasia

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Natural Disasters are often associated with tectonic activity and environmental conditions caused by this activity may contribute to the problems experienced by people

Identify where earthquakes and volcanoes are currently likely to

occur based on the plate tectonic modelEarthquakes and volcanoes occur along plate boundaries in particular around the “Ring of Fire”

Describe methods used for the prediction of volcanic eruptions andearthquakes

Scientist estimate earthquake probabilities in 2 ways:

- By studying the history of large earthquakes in a specific area- The rate at which strain accumulates in the rock

Volcanoes are monitored through the recording and analysis of measurable phenomenasuch as:- seismicity- ground movements- geophysical properties- gas geochemistry- hydrologic regime

Other instruments used include- strainometers- seismometers- lasers (surface mvt)

- tiltmetersDescribe the general physical, chemical and biotic characteristics of a volcanic regionand explain why people would inhabit such regions at risk

- traditional land ownership- fertile soils- biodiversity- geothermal energy- economic mineral deposits such as copper, gold, silver, lead- tourism- entertainment

Describe hazards associated with earthquakes including groundmotion, tsunamis and collapse of structures

Name of Hazard Type of effect Description

Ground motion Primary Ground motion results from the movement of seismic waves, especially surface waves,

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through surface-rock layers and soil. The

motion can damage and sometimes completely

destroy buildings. Proper design (including

such features as a steel framework and a foundation tied to bedrock) can do much to

prevent such damage, but in a very strong earthquake even the best buildings may suffer some damage.

Faults Primary Where a fault breaks the ground surface,buildings can be split, roads disrupted, and

any feature that crosses or sits on the fault

broken apart.

Slip and

collapse

Secondary In regions of steep slopes, earthquake

vibrations may cause soil to slip and cliffs to

collapse. This is particularly true in Alaska, parts of southern California, China, and hilly

places such as Iran and Turkey. Houses,roads, and other structures are destroyed byrapidly moving soil flows.

Tsunami Secondary Tsunamis are started by the sudden movement of the seafloor due to an earthquake and have

been particularly destructive in the Pacific

Ocean. Seismic sea waves travel at speeds up

to 950 km/h and have wavelengths up to 200km. Amplitudes of the waves are so low they

can rarely be seen in the ocean, but as a wave

approaches the shore the water is piled up

rapidly to heights of 30 m or more. About 5 hafter a severe submarine earthquake near

Unimak Island, Alaska, in 1946, for instance, atsunami struck Hawaii. The wave had traveled

at a speed of 800 km/h. Although the amplitude

of the wave in the open ocean was less than 1

m, the amplitude increased dramatically as thewave approached land, so that when it hit

Hawaii, the wave had a crest 18 m higher than

normal high tide. This destructive wavedemolished nearly 500 houses, damaged a

thousand more, and killed 159 people.

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Describe hazards associated with volcanoes, including poisonousgas emissions, ash flows, lahars and lava flows and examinethe impact of these hazards on the environment, on people andother living things

Volcanic hazards Descriptions

1. Gas The volcanic gases that pose the greatest potential hazard are

• Sulfur dioxide

• Carbon dioxide

• Hydrogen fluoride

2. Lahars A mixture of water and rock fragmentsflowing down the slopes of a volcano

3. Landslides Large masses of rock and soil that fall,slide, or flow very rapidly under the force

of gravity4. Lava flows Streams of molten rock

5. Pyroclastic flows Are high-density mixtures of hot, dry rock fragments and hot gases that move awayfrom the vent at high speeds

6. Tephra General term for fragments of volcanicrock and lava regardless of size that are blasted into the air

7. Ash Flows Ash plumes carried downwind are animmediate threat to aircraft, crushinghouse, blocking roads, destroying crops and

killing livestock

Justify continued research into reliable prediction of volcanic activityand earthquakes

Seismic activity provides us with info on the interior of the earth, the structure of thecrust, density of rocks and possible resources and plate motion and geomorphology.Research helps to give warnings of potentially damaging earthquakes early enough toallow appropriate response to the disaster, minimising loss of life.

Describe and explain the impacts of shock waves (earthquakes) onnatural and built environments

Seismic events vary in destructiveness. Through the use of the modified Mercalli scale ananalysis can be prepared of the extent of the damage from an earthquake.

Kobe Earthquake, Japan 1995 Magnitude 7 - 5000 people died- 25 000 injured- 300 000 homeless- $80 billion damage

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Distinguish between plate margin and intra-plate earthquakes withreference to the origins of specific earthquakes recorded on theAustralian continent

Intraplate earthquakes are those occurring in the interior of plates, away from plate boundaries. They take place in the crust (up to 50km deep) and are caused by

compressive stress. An example of an intraplate earthquake in Australia was the one inMeckering, Western Australia in 1968 – it was a result of compressive force causingdamage to pipelines, roads and railway lines.

Gather, process and present information from secondary sources tochart the location of natural disasters worldwide associatedwith tectonic activity and use available evidence to assess thepatterns in terms of plate tectonics

Identify data, plan an investigation and gather information fromsecondary

CreepmetresAlignment arraysGeodometers

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Case Study of a Natural Disaster

Fact File

Name: Pakistan EarthquakeDate: 8th October 2005Magnitude: 7.6

Location: 34°N 73°EDeaths: over 80,000 affecting mainly Pakistan but also India and AfghanistanLocation of Epicentre: not far from Muzaffarabad in Nth PakistanExtent of Earthquake: felt as far west as Central Afghanistan and as far east as the westcoast of Bangladesh

Tectonic Movement

- Lies on the boundary of the Indo-Australian plate and the Eurasian plate- Convergent plate boundary – one plate was subducted underneath the other - Indo-Australian plate was subducted under the Eurasian plate- Indo-Australian plate moves northward at a rate of 4cm/yr

- Common in the regionDistance from the area of disaster

- Area impacted was vast – as far west as the middle of Pakistan and as far eastas the west coast of Bangladesh, as far north as the border of north-west chinaand as south as Jaipu in India

- Area of most devastation is central and north Pakistan and north west

India

- Worst damage was in Muzaffarabad- Cities of Pakistan– Abbottabad, Gujrat, Gujranwala, Lahore, Islamabad,

Rawalpindi- Cities of Kashmir – Jammu, Baramula, Srinagar

Recurrence- Area was overdue for a major earthquake with the last big one in Pakistan

being 1935- “It [an earthquake] might not appear for 10 to 20 years but if it occurred

tomorrow it wouldn’t be a surprise” Steven Wesnousky, Neotectonic Studiesat the University of Nevada

- This area is capable of much bigger earthquakes however the unpredictabilityof it is very high as there is a part of the fault which has not shifted or brokensince 1100

Technology

- Seismographs – record and measure earthquakes through the vibrations

created in the earth during an earthquake (seismograms)- Natural method – behavioural patterns in animals such as birds and insects for

an early warning sign- Strainmeter – currently under development in California measures the motion

of a geographical point in comparison with another geographical point- Other forms used include creep meter, leveling surveys, radon-gas counter, tilt

meter and the gravity meter Possible solutions used to minimise destruction

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- improvement of construction materials and designs of buildings- ductile materials such as steel and aluminium perform better than brittle

material such as brick, stone and unstrengthened concrete – Pakistan’sinfrastructure is mainly brittle material

- early warning systems and evacuation drills like they do in Japan

- improve country’s economy because in concern to Pakistan there is no moneyto have improved infrastructure

Plate Tectonics and Climate

Predict the possible effects of explosive volcanic activity on globaland local temperatures

Volcanoes affect the climate through- gases- dust particle emissions

The effect of volcanic dust may warm or cool the earth’s surface depending on howsunlight interacts with the volcanic material.Volcanic Gas – Sulfur

- volcanoes that release large amounts of sulfur compounds affect the climatemore strongly than those that just eject dust

- the sulfur compounds are gases that rise easily into the stratosphere- they combine with water to form a haze of tiny droplets of sulfuric acid- the effect is these tiny droplets are very light in colour and reflect a great deal

of sunlight – these reflective hazes can cause significant cooling of the earthfor as long as 2 years after a major-sulfur bearing explosion

Volcanic Dust

Volcanic dust blasted into the atmosphere causes temporary cooling- the amount of cooling is dependant on the amount of dust in the air - duration of cooling is dependant on the size of the dust particles

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Tectonic Impacts