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Earth and Environmental Science
Preliminary Course
Stage 6
Dynamic Earth
Part 4: Earthquakes and volcanoes
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Number: 43180
Title: Dynamic Earth
This publication is copyright New South Wales Department of Education and Training (DET), however it may contain
material from other sources which is not owned by DET. We would like to acknowledge the following people and
organisations whose material has been used:
Photographs courtesy of Upgrade Business Systems and Ric Morante
Part 2 p 14,
Part 4 p 8
Photograph of tillite courtesy of Barbara Gurney
Part 2 p 14
Photograph courtesy of Tim Reid
Part 4 p 7
Photographs of Japanese mountains courtesy of Richard Alliband
Part 4 pp 16, 17
Diagram from Veevers, JJ et al (1991) Australian Journal of Earth Sciences 38 p 384, courtesy
of Geological Society of Australia
Part 6 p 25
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pursuant to Part VB of the Copyright Act 1968 (the Act).
The material in this communication may be subject to copyright under the Act.
Any further reproduction or communication of this material by you may be the
subject of copyright protection under the Act.
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Published by
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© State of New South Wales, Department of Education and Training 2008.
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Contents
Introduction ............................................................................... 2
Volcanism at plate boundaries .................................................. 4
Igneous rocks - a review ......................................................................4
Constructive plate boundaries .............................................................5
Destructive plate boundaries ...............................................................8
Earthquakes at plate boundaries............................................. 19
Location of earthquakes.....................................................................19
Causes of earthquakes ......................................................................22
Earthquakes and seismic waves .......................................................27
Suggested answers................................................................. 31
Exercise – Part 35
Part 4: Earthquakes and volcanoes
1
Introduction
In this part you will build upon the knowledge about plate boundaries
you gained in Part 3. Upon completion of this part you will be able to
relate the tectonic processes operating at each of the plate boundaries
with the type of volcanism produced.
You will be required to analyse patterns in the location of earthquake foci
and relate these patterns to the three types of plate boundaries. Once you
become familiar with the tectonic processes leading to the formation of
different types of igneous rock, you will then be able to relate these
processes to particular types of volcanism and the subsequent
landforms produced.
In this part you will be given opportunities to learn to:
2
•
identify regions where sea floor spreading is now occurring and
describe the composition of igneous rocks formed at mid oceanic
ridges
•
describe the characteristics of volcanic activity associated with sea
floor spreading
•
describe the characteristics of igneous rocks and volcanic activity
associated with subduction zones
•
analyse the inferences about processes occurring at subduction zones
with data collected from earthquakes
•
explain how granites and andesites are formed.
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In this part you will be given opportunities to:
•
process information to plot the occurrence of explosive volcanic
activity around the world and relate the pattern produced to crustal
movements
•
gather information from secondary sources to identify and describe
the main features of igneous rocks associated with effusive volcanic
activity at mid-ocean ridges.
Extract from Earth and Environmental Science Stage 6 Syllabus © Board of
Studies NSW, amended October 2002. The most up-to-date version can be
found on the Board’s website at
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_liste.html
Part 4: Earthquakes and volcanoes
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Volcanism at plate boundaries
Different tectonic processes produce different types of volcanism.
In turn this variation in volcanism produces different rock types.
Igneous rocks - a review
By way of revision complete the following self-correcting questions
about igneous rocks.
1
How would you define what an igneous rock is?
_____________________________________________________
_____________________________________________________
_____________________________________________________
2
a) Recall the names of the two main categories of igneous rock.
__________________________________________________
__________________________________________________
b) What is the difference between these two categories?
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
Check your answers.
Igneous rock type depends on the composition of the molten material
from which it forms. This initial molten material is known as the melt.
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Constructive plate boundaries
The melt from which ocean floor material is being produced at
constructive plate boundaries is basaltic in nature. Therefore the rock
producing this new ocean floor at mid oceanic ridges is basalt.
Refer back to the map of plate boundaries in Part 2. Identify locations
where there are constructive plate margins.
_________________________________________________________
_________________________________________________________
The composition of rocks produced at mid oceanic ridges all over the
world varies little from one spreading centre to the next. This means that
oceanic crust is very similar in mineralogy from one ocean to another.
The lava being ejected out of these fissures produces basalt that is high in
iron and magnesium. This type of basalt is known as MORB (mid
oceanic ridge basalt). Basalt is a fine grained volcanic igneous rock.
Basalt is a fine grained igneous rock. (Photo: © LMP Tim Reid)
1
Why is basalt fine grained?
_____________________________________________________
2
Why is basalt classified as an igneous rock?
_____________________________________________________
3
Why is basalt classified as volcanic igneous rock?
_____________________________________________________
Check your answers.
Part 4: Earthquakes and volcanoes
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The table following shows the proportion of elements that basalt and
granite typically contain.
Elements
Basalt (%)
Granite (%)
silicon
48
72
aluminium
15
13
iron-magnesium
18
3
calcium
10
2
sodium and potassium
5
8
minor constituents
4
2
Mid oceanic ridge basalt (MORB) can form into structures known as
pillow basalts. This is a descriptive name for the structure of basalt that
forms underwater.
To see a site that shows a diver examining pillow basalt that has been
erupted from Hawaii's Mt Kilauea, see a site on the Earth and Environmental
Science website at: http://www.lmpc.edu.au/science
As soon as the lava is ejected from the fissure (crack in Earth’s surface)
it comes in contact with the cold ocean water. As a result, the lava is
cooled very quickly on its outermost edges and forms a tube-like
structure similar to a large pillow. Once the outside of this pillow
solidifies, the lava within the pillow also cools and solidifies into solid
rock forming a new section of ocean floor.
Pillow basalt is not only formed at mid oceanic ridges. Pillow basalt
forms anywhere where basalt is injected into water. Therefore, pillow
basalt has also formed in places such as Hawaii where vents have
injected basalt beneath the surface of the water.
Basaltic lava has low viscosity. This means that the lava is very fluid
and runny. As a result, gases and steam are able to easily escape from
the lava releasing any pressure.
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Low viscosity basalt erupting on the Island of Hawaii. (Photo: Tim Reid)
What is happening at mid oceanic ridges?
Because mid oceanic ridges are several kilometres below the surface of
the ocean, it has been difficult to observe the volcanic processes
operating at these depths. There is, however one place where a
mid oceanic ridge is operating on land.
The Reykjanes ridge is the name given to the mid oceanic ridge that
separates the Eurasian plate and the North American plate. This ridge
passes almost directly through the middle of Iceland and is the only place
on Earth where a mid oceanic ridge is operating on land.
Iceland
Reykjanes
Ridge
North American
Plate
Eurasian
Plate
Map showing the Reykjanes mid oceanic ridge passing directly through Iceland.
Part 4: Earthquakes and volcanoes
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Part of the Reykjanes Ridge. (Photo: Upgrade Business Systems Pty Ltd)
The type of volcanism at mid oceanic ridges is a non-explosive type of
eruption, ejecting lava from long elongated fissures or cracks in Earth’s
surface rather than from a single cone-shaped volcano.
To see photograph of fissure eruptions see the links on the Dynamic Earth
web page for this Part at: http://www.lmpc.edu.au/Science
Destructive plate boundaries
You will recall that there are three types of destructive plate boundaries:
•
continental – ocean collision
•
continental – continental collision
•
ocean – ocean collision.
These boundaries always involve subduction zones, where one plate is
moving under the other. If you’d like to revise these concepts before you
continue, look back at Part 3.
Igneous rocks formed at subduction zones
An enormous range of igneous rocks have been found to form at
destructive plate margins. The reason for this diversity in rock type is the
wide range of crustal types being remelted at subduction zones. The type
of igneous rock produced depends on the type of rock being melted.
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Think of the variety of minerals that are melted at an oceanic – continent
plate boundary. The subducting oceanic crust is composed of basaltic
minerals. These minerals then mix with the minerals from oceanic
sediments at subduction zones as well as with the minerals from the
continental crust.
The continental crust can itself vary tremendously in mineralogy from
one type of continental crust to another. As a result almost the full range
of igneous rocks have been found to form at destructive plate boundaries.
Classification of igneous rocks
The following table shows a simplified classification system for the
range of igneous rocks.
Orthoclase feldspar
(Pink - K rich)
Plagioclase feldspar
(White - Na rich)
65%
Granodiorite
(Trachyte)
Gabbro
Olivine
0%
Biotite
Diorite
(Andesite)
Amphibole
Syenite
Ferromagnesian minerals
Quartz
(Dacite)
Pyroxene
Granite
(Rhyolite)
(Basalt)
Igneous rock classification. Note the arrow directions show an increasing
tendency for a particular mineral to be present.
Plutonic igneous rock name
(Volcanic igneous rock name)
Part 4: Earthquakes and volcanoes
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Key:
•
Orthoclase is a pink coloured mineral. It is a potassium (K) rich
feldspar.
•
Plagioclase is a white coloured mineral. It is a sodium (Na) rich
feldspar.
•
Ferromagnesian minerals are rich in iron and magnesium.
These minerals give the rock a darker colour.
•
Quartz (SiO2) is made up of silica and oxygen. Quartz is a clear to
white mineral and helps give the rock a lighter colour.
•
Olivine is an iron-magnesium silicate. It is dark green or greenish
brown in colour.
•
Pyroxenes are very dark coloured minerals which cleave at 90°.
•
Amphiboles are similar to pyroxenes and are dark green to brown.
Amphiboles cleave at 120°.
•
Biotite is a type of mica and cleaves at 180° forming very thin
sheets. Biotite is brown to green in colour. When sheets of biotite
are stacked on top of each other biotite can appear as a black
reflective surface.
•
Rock names appearing within brackets are volcanic igneous rocks.
(They are rocks formed from lava flows over Earth’s surface.)
•
Rock names appearing in bold type above names in brackets are
plutonic igneous rocks. (They are rocks formed from magma
cooling beneath Earth’s surface.)
Other comments:
The rocks occupying the position between granite and granodiorite
are collectively known as the granitoids because of their similar
mineral composition.
Granite and rhyolite are identical in chemical and mineral composition.
The only difference is that granite is plutonic and is therefore coarse
grained because it has cooled slowly. Rhyolite on the other hand is
volcanic and is fine grained because it has cooled quickly on Earth’s
surface. This also applies to the other pairs of volcanic and plutonic
rocks listed in the table.
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Refer to the igneous rock classification table as you answer these questions.
1
What type of rock has very high amounts of ferromagnesian minerals
(giving the rock almost a black appearance)? This rock cooled very
quickly and therefore is very fine grained. It also has a high proportion
of plagioclase feldspar and contains no quartz.
_____________________________________________________
2
What plate boundary produces only this type of rock?
_____________________________________________________
3
What type of rock has very low amounts of ferromagnesian minerals
and has a large percentage of quartz (giving the rock a lighter colour)?
This rock cooled beneath Earth’s surface and so cooled very slowly.
This slow cooling gave the rock large crystals. It also has a high
proportion of orthoclase feldspar that gives the rock a pinkish colour.
_____________________________________________________
Check your answers.
Basalt and granite are often referred to when talking about igneous rocks
because they are at completely opposite ends of the spectrum. One is
volcanic, the other is plutonic. One is fine grained, the other is coarse
grained. One is dark coloured, the other is light coloured. All the other rock
types referred to in the table fall between these two ends of the spectrum.
Basalt. (Photo: © LMP Tim Reid)
Granite. (Photo: © LMP Jane West)
It is important to realise that when igneous rocks are being classified,
quite often they do not fall neatly into individual categories. This is
because mineral composition between rocks varies along a continuum
and gradually changes from one category into the next.
Part 4: Earthquakes and volcanoes
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Granite
When two continental crusts collide, the resulting magmatic plumes
contain material that has been melted from two plates high in silica,
quartz and water. The result is often the production of granitic plutons
that then force their way through the overlying continental crust.
Andesite
As stated above, continental crustal material can include a whole range of
igneous rocks. However, the average continental crustal composition is
much higher in silica and lower in ferromagnesian minerals than oceanic
crust. Continental crust commonly contains granite as well as other
rocks that have a high proportion of quartz.
When oceanic crust is subducted beneath continental crust and is melted,
magmatic plumes rise into the overlying continental crust melting some
of this surrounding rock type as it rises. The result is a mix of minerals
often leading to the formation of andesite, a rock which lies in between
rocks that typically go to make up oceanic and continental crust.
1
Where is andesite located on the classification table of igneous rocks?
_____________________________________________________
_____________________________________________________
2
The Andes Mountain Range in South America is the result of the
Nazca oceanic plate being subducted and melted underneath the
South American continental plate.
How do you think the name andesite was derived?
______________________________________________________
______________________________________________________
Check your answer.
Volcanism that produces andesite is often very explosive due to the
relatively high amounts of silica. This explosive volcanism ejects
material into the air from great depths beneath Earth’s surface.
As a result, cooling of material is very quick. Andesite is very fine
grained, and because of its high silica content, can sometimes contain
glass. Rhyolite can sometimes contain glass as well.
To see some samples of andesite and rhyolite in colour see links on the
Dynamic Earth webpage for this Part at : http://www.lmpc.edu.au/Science
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Andesite is also common at ocean – ocean plate boundaries.
Oceanic sediment is dragged down the trench and melted in with the
subducted oceanic crust. The magmatic plumes produced as a result
are often andesitic in composition leading to andesitic eruptions at
island arcs.
Variation in rock types at plate boundaries
It is often stated that andesite is formed at ocean – ocean and
ocean – continent destructive plate boundaries, with andesite and granite
being formed at continental – continental destructive plate boundaries.
However, it must be realised that these are very generalised statements
and in reality a very large range of igneous rocks have been found to
form at these plate boundaries with compositions from granitic magmas
to basaltic magmas.
Metamorphic rocks at subduction zones
The tremendous amount of heat and pressure generated from the contact
of colliding plates causes rocks to deform. During this deformation the
rocks are folded and the minerals recrystallise to form metamorphic
rocks. Many different types of metamorphic rocks are produced at
destructive plate margins.
One example is granitic gneiss, formed from the recrystallisation of
granite, a well known continental igneous rock.
Granite – an igneous rock
(Photo: © LMP Tim Reid)
Part 4: Earthquakes and volcanoes
Granitic gneiss – a metamorphic rock
(Photo: © LMP Tim Reid)
13
Name two other metamorphic rocks and identify the parent rock
from which they originated.
_________________________________________________________
_________________________________________________________
Check your answers.
Two metamorphic sequences of rocks produced at destructive plate
boundaries, which are characteristic of this type of boundary, include
ophiolites and blueschists.
Ophiolites
Ophiolites are a metamorphic sequence of rocks that represent ocean
floor material that has been metamorphosed together with oceanic
sediment. The sequence of rocks metamorphosed include gabbro, basalt,
peridotite (containing predominantly olivine) and oceanic sediments.
These rocks when metamorphosed produce metamorphic rocks such as
serpentinite. These rocks go to make up the accretionary wedge at
subduction zones.
Blueschists
Blueschists are characteristic of subduction zones due to the high
pressure low temperature environment of the trench region.
Blueschists obtain their distinctive blue colour from a particular type of
amphibole mineral known as glaucophane. With increasing pressure
these blueschists may crystallise out the mineral garnet and form the
metamorphic rock eclogite.
How volcanic island arcs form
When two oceanic plates collide and one oceanic plate is subducted
beneath another, a chain of volcanoes appears in the overlying oceanic
plate. These volcanoes are formed by rising magmatic plumes originating
from the melting of the subducted plate. The region between the island arc
and the continental mainland is known as the back arc basin. Island arcs
are said to be on the trench side of the subduction zone.
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oceanic crust
100 km
200 km
oceanic trench
subducting oce
ani
c lit
hos
asthenosphere
island arc
p he
continental crust
asthenosphere
re
melting
continental lithosphere
A ocean – ocean subduction zone with an island arc forming.
One distinct feature of this type of volcanic chain is that they tend to
form in the pattern of an arc rather than a straight line. The name
island arc is derived from this pattern.
Look at the map below showing the main lithospheric plate boundaries.
Shade over the destructive plate margins where island arcs have formed.
▲
▲
▲ ▲
▲
PACIFIC PLATE
▲
▲ ▲
▲
▲▲
▲
▲
▲
SOUTH
AMERICA
PLATE
▲
▲
NAZCA
PLATE
▲ ▲ ▲
▲ ▲
▲
INDO-AUSTRALIA PLATE
▲ ▲
CARIBBEAN
PLATE
▲ ▲
▲
▲ ▲ ▲
▲
▲ ▲ ▲
▲ ▲
COCOS
PLATE
▲
▲ ▲
▲ ▲
▲
PHILIPPINE PLATE
▲ ▲
▲
▲
▲
▲ ▲
▲ ▲
▲
▲
NORTH
AMERICA
PLATE
▲
▲
AFRICA PLATE
▲
▲
▲
▲
▲
▲ ▲ ▲ ▲ ▲ ▲
▲ ▲ ▲ ▲ ▲
▲ ▲ ▲ ▲ ▲
▲
▲
▲
▲
▲
▲ ▲
EURASIA PLATE
▲
▲
▲
▲ ▲
Constructive plate margins
Destructive plate margins
ANTARCTIC PLATE
Please check your answer.
Explosive volcanism and rock types
Large explosive volcanoes such as Mt Krakatoa (Indonesia) and
Mt Pinatubo (Philippines) are the result of volcanoes having a viscous
melt resulting from the collision of two plates. This high viscosity means
that the lava and magma are less fluid and behave more like honey.
Part 4: Earthquakes and volcanoes
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This high viscosity enables pressure within the volcano to build up because
gases and steam are unable to continually escape. The pressure is
eventually released in an explosion. These explosive volcanoes can cause
immense destruction and have been known to hurl rocks the size of a car
kilometres through the air.
To see photos of volcanic bombs see a link on the Dynamic Earth page for
this Part at: http://www.lmpc.edu.au/Science
Volcanic bombs ash and other particles ejected from volcanoes are
collectively termed pyroclastics. Ash can settle and form rocks called
pumice. Pumice can be so light that it can float on water.
Two different types of pumice. Both show holes known as vesicles
produced by escaping gas. (Photos: © LMP Tim Reid and Jane West)
Due to the high viscosity of this magma, volcanoes form with steeply
angled sides such as Mt Fuji in Japan. Here are some examples.
Volcano, Hokkaido, Japan (Photo: Richard Alliband)
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Sakurajima, Japan. (Photo: Richard Alliband)
Kaimondake, Satsuma Peninsula. (Photo: Richard Alliband)
These volcanoes are known as a stratovolcanoes because they are built
up of layers or strata from alternating lava and ash deposits from
different eruptive events. Sometimes these volcanoes are also referred to
as composite volcanoes.
The degree of silica in the melt largely determines the viscosity of the
magma and lava, and therefore determines how explosive the volcanism
will be.
Basalt has a lower amount of silica, therefore the viscosity will not be as
great and the volcanism will not be as violent. The lava will be allowed
to flow rather than be blasted around the countryside, and gases will be
allowed to escape with relative ease.
Andesite and rhyolite on the other hand, have a higher amount of silica
producing very viscous magma. This then gives rise to very
explosive volcanism.
Part 4: Earthquakes and volcanoes
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Nuees ardente
The term nuees ardente originates from French, meaning glowing cloud.
Nuees ardentes are a special type of pyroclastic eruption. They are
composed of a mixture of very fine dense ash and extremely hot gas.
These explosive eruptions are caused by the rapid degassing of viscous
lava within the volcano. A nuees ardente is often the first explosion in a
violent eruption and can travel down the limbs of a volcano at around
100 km per hour, devastating everything in its path.
To see photos of Mt St Helens nuees ardente eruptions go to the links for
this Part on the Dynamic Earth page at: http://www.lmpc.edu.au/Science
Such an extremely hot gas, ash and rock eruptions from Mt St Helens in
1980 was initially propelled at 300 km per hour. In 1902 Mount Pele
erupted on the Carribean island of Martinique. The nuees ardente that
resulted from this eruption demolished the town of St Pierre and killed
almost all of the 28 000 inhabitants within a few minutes.
Turn to the end of this part and complete Exercise 4.1.
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Earthquakes at plate boundaries
The focus of an earthquake is the point within Earth’s crust where rocks
break and shock waves are generated. It is specified by indicating its
latitude, longitude and depth. The epicentre is the point on Earth’s
surface directly above the focus.
weakest shock
epicentre
focus
fault line
strongest shock
Focus and epicentre.
Part 4: Earthquakes and volcanoes
19
Location of earthquakes
Look at the following map showing the distribution of Earth’s volcanoes.
Location of Earth's volcanoes.
Now look at the following map showing the distribution of the epicentres
of major earthquakes.
Epicentre locations on the surface of Earth.
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Now complete the following activity.
This is a bit like a complicated join-the-dots puzzle. On both of the maps
above try to join up the dots by drawing a single line through regions with
the most dots.
Make at least two comments about the location of the earthquakes and
volcanoes shown in the maps.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
Although earthquakes (and volcanoes) have been found to occur in areas
away from plate boundaries, overwhelmingly the large majority of
earthquakes occur along constructive and destructive plate boundaries.
Earthquakes and volcano distribution patterns provide additional
evidence for locating and determining the nature of plate boundaries.
When the earthquake epicentres are plotted on a map they form narrow
zones that delineate the edges of crustal plates.
There are a few volcano and earthquake areas found away from plate
boundaries, such as the Hawaiian – Emperor Seamount chain and the
line of extinct volcanoes on the eastern extent of mainland Australia
(eg. the Warrumbungles, Mt Kaputar, Mt Warning and the
Glasshouse Mountains).
These are thought to have formed as a result of the plate moving over a
stationary hotspot in the mantle. This hotspot then literally burns its way
through the crust. The fact that the mountains in these chains become
older in one direction suggests the plate on which they are formed is
moving over the hotspot. This also supports the concept of crustal
plate movement.
But why do earthquakes occur?
Part 4: Earthquakes and volcanoes
21
Causes of earthquakes
Earthquakes are a response to pressure being built up within Earth’s crust
and then being released. This release of pressure occurs along existing
faults or results in new faults being produced.
Tectonic forces cause pressure to build up within the crust. When this
pressure exceeds the frictional forces either side of the fault plane,
the built up stress is released as the rocks either side of the fault plane
suddenly move. This movement is known as seismic slip.
When rocks fracture in Earth’s crust an earthquake may result.
The features of an earthquake include shock waves, rock movements and
vibrations of the ground.
Strain builds up due to earth movement.
Energy is suddenly released when
rocks break.
The rocks either side of the fault line become more and more deformed
as stress is built up. When seismic slip occurs, the energy is released in
the form of earthquake waves.
The rocks are no longer under stress and return to their previous state
(but not to their original positions). This recoil after stress is known as
elastic rebound.
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Earthquakes at constructive plate margins
(or at mid oceanic ridges)
The foci of the earthquakes
associated with mid oceanic
ridges are generally shallow to
intermediate in depth.
Notice the positions of foci in
the diagram.
Mid Oceanic Ridge
Sea Level
➩
✳ ✳
✳✳
However, most earthquake
activity associated with these
margins actually occurs along
transform faults.
➩
✳ Earthquakes
Areas of earthquake activity at a mid oceanic ridge.
Earthquakes at conservative plate margins
(or at transform faults)
Large earthquakes are generated along transform faults near mid oceanic
ridges. Because these faults make up the edge of oceanic plates it is a
site where two different plates can move alongside each other travelling
in opposite directions.
ocean ridge-rift
Plate
A
transform
fault
Plate
B
lithosphere
asthenosphere
va
rising la
Mid oceanic ridge and transform faults.
On the diagram above, colour in the section along the transform fault where
the plates are moving in opposite directions on either side of the fault plane.
Part 4: Earthquakes and volcanoes
23
transform fault
➩
The friction produced
between these two plates
produces large earthquakes.
Because transform faults
move in a lateral or sideways
motion, the earthquake foci
are generally quite shallow.
➩
✳
✳
✳
✳
✳
✳
✳✳
✳
✳
✳ earthquakes
Earthquake activity at a transform fault
The very well known San Andreas Fault, that runs through San Francisco in
North America, is actually part of a sequence of transform faults separating the
Pacific plate and the North American plate. The damage caused by earthquakes
in San Francisco and Los Angeles are well documented and reported.
Earthquakes at destructive plate margins
(or at subduction zones)
In 1949 a seismologist by the name of Benioff noticed that when
earthquake foci are plotted in a cross-section across a subduction zone,
the earthquake foci are shallow at the trench itself and then gradually
become deeper as they pass under the island arc.
This earthquake region beneath the subduction zone has became
known as the Benioff zone and extends deep into the upper mantle.
Earthquakes associated with mid oceanic ridges and transform faults are
much shallower by comparison.
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oceanic plate
continental plate
sea level
✳
✳
➩
✳ ✳
✳
✳
✳✳
✳
✳ ✳
✳
➩
➩
✳ earthquakes
Earthquake activity at an ocean – continent boundary (destructive plate margin).
Plotting earthquake foci
What to do:
Plot the following points on the grid below.
Depth of earthquake foci (km)
50
Distance (km) and
direction from coast line
0
300
100 east
385
450 east
60
80 east
125
250 east
200
70 east
690
400 east
25
40 west
500
700 east
515
385 east
330
340 east
50
100 east
300
500 east
485
280 east
Part 4: Earthquakes and volcanoes
25
660
550 east
90
90 west
520
200 east
90
90 west
520
200 east
90
25 west
440
600 east
640
660 east
50
100 west
75
100 west
N
Tropic of Capricorn
0
200
300
400
500
Depth in kilometres
100
600
700
WEST
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100 0 100 200 300 400 500 600 700
Distance East or West of coast in kilometres
EAST
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Use your plotted grid to complete these tasks.
1
What type of plate boundary is suggested by the pattern of
earthquake foci?
_____________________________________________________
2
Give a possible location for this type of plate boundary.
_____________________________________________________
3
State another type of plate boundary and describe its pattern of
earthquake foci.
_____________________________________________________
_____________________________________________________
Check your answers.
Earthquakes and seismic waves
You should now understand that the release of stress in Earth’s crust can
lead to the production of earthquakes. The energy being released at these
faults is in the form of seismic waves. It is these seismic waves that
shake Earth’s surface in the form of an earthquake.
Seismologists have discovered that there are basically two kinds of
seismic waves produced at the focus during an earthquake. These travel
from the focus through Earth’s interior. They are called primary waves
known as P waves, and secondary waves known as S waves.
Both P waves and S waves are also called body waves because they are
able to travel through Earth and not along surfaces only.
P waves
P waves travel faster and are compression waves. That is, they push or
pull the rocks in the direction the wave is travelling. They can travel
through both liquids and solids.
P waves cause the particles in the material to move back and forth in the
same direction the wave moves. This is similar to creating a longitudinal
wave on a slinky spring.
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direction of particle vibration
direction of movement
Longitudinal waves can be demonstrated using a slinky spring.
S waves
S waves travel more slowly than P waves and are transverse waves.
They shake or shear the rock particles at right angles to the direction of
travel. They can travel through solids, but not through liquids.
S waves oscillate at right angles to the direction the wave moves. This is
similar to creating a wave by flicking a rope or garden hose up and down
and observing the wave travelling forward.
direction of particle vibration
direction of wave movement
S waves are transverse waves.
L waves
There is a third group of complex waves, collectively known as L waves.
They are made up of waves which move in elliptical or horizontal transverse
motions. They are generated at the surface when body waves reach there.
L waves travel only on the surface of Earth and are responsible for most of
the damage done. Because these surface waves are confined to a narrow
region near the surface, they are not spread throughout Earth as are body
waves. They therefore maintain their maximum amplitude longer and can
disperse their energy through greater distances than either P and S waves.
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As these surface waves also have longer periods (time between crests or
troughs) they are sometimes referred to as long waves.
Relative motion of P, S and L waves.
Complete these tasks about earthquake waves. You need to use the formula:
distance
time =
.
speed
1
The speed of P waves through granite in the crust is about 6 kms–1;
S waves travel at 3·5 kms–1.
a) How long does it take P waves to travel through 1000 km of
granite? __________________________________________
b) How long does it take S waves to travel through 1000 km of
granite? __________________________________________
c) What is the difference in time between P and S waves travelling
a distance of 1000 km? ______________________________
2
The speed of P waves through water is 1·5 kms–1.
How long does it take P waves to
travel through 1000 km of water? _________________________
3
After an earthquake, the difference between the arrival times for
P and S waves at a seismic station is 2 min 30 s. It is known that
P waves travel at 7·5 kms-1 through this portion of Earth, while
S waves travel at 4·4 kms–1.
a) How much further than an S wave can a P wave travel in 1 s?
_________________________________________________
b) How much further than an S wave can a P wave travel in:
i
1 min? ________________________________________
ii
2 min? ________________________________________
iii 5 min? ________________________________________
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c) Looking at your answers to b), what can you say about the
distance between a P wave and an S wave as time increases?
__________________________________________________
__________________________________________________
d) This diagram presents information about the P and S waves
detected at the seismic station.
x
focus
y
S wave
P wave
z
seismic
detection
station
With a time difference of 2 min 30 s between the arrival of the
two waves, it is known that a P wave can travel 660 km further
than an S wave.
Which of the values marked on the diagram –
x, y, or z – corresponds to this distance? _________________
e) It has been calculated that with a time difference of 2 min 30 s,
an S wave can travel 940 km.
How far is the focus of this earthquake from the seismic
detection station?
__________________________________________________
__________________________________________________
Check your answers.
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Suggested answers
A review of igneous rocks
1
An igneous rock is one that has crystallised from a molten mass of
rock known as a melt.
2
a) The two main categories of igneous rock are volcanic and
plutonic.
b) Volcanic igneous rocks are formed by the cooling of lava on
Earth’s surface. This relatively quick cooling gives volcanic
igneous rocks a very fine grain structure. Plutonic igneous
rocks are formed from magma within the ground. They
crystallise out after relatively slow cooling, giving them a coarse
grain structure.
Constructive plate boundaries
1
Basalt is fine grained because it cools quickly in the open air or
in oceans.
2
Basalt is igneous because it is formed from the cooling of molten rock.
3
Basalt is volcanic (and not plutonic) because it is formed from the
cooling of lava after having been extruded from volcanoes (and not
formed from the cooling of magma beneath Earth’s surface).
Classification of igneous rocks
1
basalt
This rock is the volcanic equivalent of gabbro. It can contain
olivine, more pyroxene and ferromagnesian minerals, less amphibole
and no biotite than the other igneous rocks. Basalt generally
contains no quartz and is high in plagioclase feldspar.
2
mid oceanic ridges, or constructive plate margins
3
granite, a plutonic rock
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Andesite
1
Andesite is located in between basalt and dacite, and to the right of
trachyte.
2
Andesite was given its name after it was found to occur in large
quantities in the Andes mountain range.
Metamorphic rocks at subduction zones
Other metamorphic rocks include quartzite (from sandstone), slate (from
shale) and marble (from limestone).
How volcanic island arcs form
Island arcs are found along:
•
the subduction zone separating the Philippine plate and the Eurasian
plate
•
the northern most subduction zone on the Pacific plate
•
the subduction zone on the northern border of the Australian plate,
known as the Java trench.
Location of earthquakes
Two comments about the location of earthquakes on Earth are:
•
earthquakes appear to occur in the same regions as volcanoes
•
the positions of earthquake epicentres (and volcanoes) outline the
positions of the plate boundaries.
Plotting earthquake foci
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1
Destructive plate margin.
2
From about 100 km west and 50 km depth to about 600 km east and
650 km depth.
3
Transform fault has foci along the fault near the surface.
Constructive plate boundary has foci along the mid-oceanic ridge.
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Earthquakes and seismic waves
1
a) time = 1000/6 = 170 s
b) time = 1000/3.5 =280 s
c) 280 – 170 = 110 s
2
1000/1.5 = 670 s
3
a) 7.5 – 4.4 = 3.1 km
b) i)
3.1 x 60 = 186 km
ii) 3.1 x 120 = 372 km
iii) 3.1 x 300 = 930 km
c) The distance between a P wave and an S wave increases with
time.
d) y
e) 940 + 660 = 1600 km.
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Exercises – Part 4
Exercise 4.1 to 4.2
Name: _________________________________
Exercise 4.1: Plate boundaries
1
Use the words below to complete the following passage on divergent
plate boundaries. Use each word only once.
Iceland
spreading
rate
continually
diverging
ridges
ocean
away
Mid Atlantic Ridge
MORB
sea floor
mountain
asthenosphere
Most divergent boundaries, where plate
occurs,
are situated at the peaks of mid oceanic
.
At these ridges the plates move
from the ridge
axis and the fissures that result are filled with molten rock that oozes up
from the
.
The rock type produced at these ridges is called a
which stands for mid oceanic ridge basalt. This material cools to form
floor.
new sections of
New oceanic material is
between the
mechanism for
being injected
plates. This provides the
spreading.
The typical
between 2 and 8 cm per year.
of spreading at these ridges is
is one of the most investigated mid
The
oceanic ridges in the world. It is a gigantic
range that rises 2500 to 3000 m above the adjacent ocean floor.
It extends southward from the Arctic Ocean to beyond the southern
tip of Africa.
is an example of where the Mid Atlantic
Ridge has actually grown above sea level.
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35
2
Give an explanation as to why the rock type produced at
constructive plate boundaries varies very little from one mid oceanic
ridge to the next.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
3
Explain why andesite is often found at destructive plate margins.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
4
Explain the type of volcanism occurring at destructive plate margins
and give an explanation as to its cause.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
5
Describe the link between the type of volcano produced at
destructive plate margins and the magma and lava present.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
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6
Plate
boundary
Summarise some of the main features of plate boundaries by
completing the following table.
Movement of
plates
Part 4: Earthquakes and volcanoes
Depth of
earthquakes
Rock types
produced
Explosive or
non-explosive
volcanism
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Exercise 4.2: Examining igneous rocks
In this exercise, you will need to identify data, plan, choose resources
and make and record observations.
Locate a sample of a plutonic igneous rock such as granite, and a sample
of an igneous rock associated with explosive volcanisms such as pumice.
If you cannot find samples or your teacher does not supply you with
samples go to the www.lmpc.edu.au/science website photographs in
8.3 The Local Environment.
Using dot points and in table format, compare the features of these two
rock types. In addition, you may decide to include sketches of the rocks
you study.
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