Geog 3 Tectonics
Full recap – everything in one handy
PowerPoint....enjoy
Mr V
What do we need to know?
Plate movement
Earth structure, plate tectonics theory: convection
currents and sea-floor spreading.
Continental drift and palaeomagnetism.
Plate boundaries
Destructive, constructive and conservative plate
margins.
Associated landforms: young fold mountains, rift valleys,
ocean ridges, deep sea trenches and island arcs.
Hotspots
Hot spots associated with plumes of magma and
their relationship to plate movement.
Volcanoes
(vulcanicity)
types of volcanoes
Earthquakes
(seimisity)
The causes and main characteristics of earthquakes:
focus and epicentre; seismic waves and earthquake
measurement.
Tsunamis
Characteristics and causes.
Forms of intrusive activity – dykes, sills, batholiths.
Minor forms of extrusive activity – geysers, hot
springs and boiling mud.
What case studies you we need?
Volcanoes
Two case studies of recent (ideally within the last 30
years) seismic events should be undertaken from
contrasting areas of the world. In each case, the
following should be examined:
• the nature of the seismic hazard;
• the impact of the event;
• management of the hazard and responses to the
event.
Earthquakes
Two case studies of recent (ideally within the
last 30
years) seismic events should be undertaken
from
contrasting areas of the world. In each case,
the
following should be examined:
• the nature of the seismic hazard;
• the impact of the event;
• management of the hazard and responses to
the
event.
Structure of the Earth
• Earth’s interior
divided into its four
concentric spheres:
the crust, the
mantle, the outer
core and inner core.
Lithosphere and Asthenosphere
Key terms Defined
•
•
•
•
Crust
Earth’s crust made up of 3 kind of rock:
Igneous
Sedimentary
Metamorphic
• 2 types of crust:
• Oceanic is dense and basaltic in name. approx
10km thick.
• Continental crust less dense up to 70 km thick.
• Lithosphere = Rigid ‘plate’ comprising of the
crust and upper mantle.
• Asthenoshere = Plastic like in upper layer.
Allows the plate to move
• Mantle = Dense solid layer. Rich in iron and
magnesium. Beneath this is the semi-molten
outer core and a solid inner core.
How do we know this structure is
true?’
• Answer = we use measurements from
earthquakes
• Both P and S waves travel through the
interior of the earth and are recorded
on seismographs. P and S waves travel
at different velocities according to the
density of the material which they
travel.
• E.g S waves are not able to travel
through liquids. Using this data
scientists can build a picture of the
thickness and composition of the
layers.
Continental Drift Theory
• Earthquakes, volcanoes and fold mountains all occur in similar
areas. They occur in long narrow bands thousands of miles long but
only hundreds of kms wide. These narrow bands are the Plate
Boundaries.
• Over millions of years the continents have drifted apart from an
original supercontinent called Pangea.
• The idea of Continental Drift was first put forward by Alfred
Wegener in 1912. His evidence came from things like:
• The similarity in the shape of the coastlines of S. America and Africa
• Similar glacial deposits from 290 million years ago are found in S.
America, Antarctica and India
• Similar geological sequences are seen in Scotland and eastern
Canada.
• Fossil remains are found on different continents
• Coal reserves are found in Antarctica - the climate there must have
been much warmer.
Plate Tectonics
•
•
•
•
•
•
•
•
•
•
•
The more modern theory of Plate Tectonics comes from much later – 1960s.
Supporting evidence came from:
discovery of the mid-Atlantic Ridge in 1948
studies of palaeomagenetism in the `1950s
sea floor spreading – in 1962 Harry Hess confirmed that the newest rocks were in the middle
of the Atlantic (near Iceland) and got older to the edges e.g. USA. He said that the Atlantic
could be widening at 5cm/year.
If the Atlantic was getting wider, and if the earth was not getting bigger, then somewhere
else old crustal rocks must be being ‘eaten up’. Evidence for this came from the fringes of the
Pacific Ocean.
Many of the oceans are under 200 _____________ Years old.
Some of the plates are only oceanic crust e.g. ____________
Others carry oceans and continents on their back e.g. _____________
Continental plates (40kms) are much thicker than ocean plates (10kms) and are thickest
under _____________________An example of thick continental crust is ______________
The_______________ crust is much older than the ______________Crust.
The crust and the top part of the mantle makes up the lithospheric plates which float on the
underlying ___________________
Evidence for sea floor spreading
• Palaeomagnetism.
What drives the motion of the plates?
• Answer = The movement of the
tectonic plates is driven by
thermal convection currents in
the upper mantle using heat
derived from the radioactive
decay of minerals deep within
the Earth and residual heat
from the Earth’s formation
(some 4,600 million years ago).
This heat causes plumes of
magma to rise, then cool and
fall.
Plate boundaries
•
The plates form the solid lithosphere (crust and top part of the mantle) – which is
thought to ‘float’ on the underlying asthenosphere.
•
Rates of movement of the plates varies between about 60mm / year and 100mm
/year. On the map low rates are seen _________________________
Constructive
• In some places – Constructive plate boundaries – new molten lava pours out
onto the surface along mid-ocean ridges.
• An example is the mid-Atlantic ridge. New basalt lava pours out onto the
ocean floor and by sea-floor spreading gradually widens the sea floor pushing
two continents apart.
• The Atlantic was made like this over a period of about 200 million years. These
plate boundaries have both volcanoes and earthquakes.
Destructive
• At other places old oceanic crust slides down into the
mantle and is melted. This happens in N. America where
the little Juan de Fuca ocean plate descends (subducts)
underneath North America.
• This created the Cascade Range of volcanic peaks like
Mount St. Helens and Mt. Rainier. These volcanoes can be
destructive. There are also earthquakes.
Collision Boundary
• At a collision boundary, two continental plates
move together. The collision forces them upwards
forming mountains. This kind of plate movement
has caused the Himalayan mountain range to form.
We get earthquakes at collision boundaries, but not
volcanoes. This is because the two plates are rising
upwards rather than being subducted and melting
to form magma.
Conservative or Transform
• At Conservative Plate Boundaries two plates slide past each other.
There is no volcanic activity but shallow focus earthquakes are
common along transform faults. An example is the San Andreas
fault in California. Here the Pacific Plate moves north at about
60mm/year whilst the North American Plate only moves at
10mm/year. The different rates of movement leads to a build up of
pressure which is released as an earthquake when the rocks
fracture.
Hotspots
•
In some places – like in Hawaii – there is a rising plume of magma within the mantle.
This has found a weak spot in the Earth’s crust and so basaltic lava pours out onto
the ocean floor.
This builds huge underwater shield volcanoes which gradually grow even larger to
make islands. As the ocean plate slides over the Hot Spot new islands to the south of
the earlier ones are made. Currently Kilauea – the world’s most active volcano – is
above the Hot Spot.
Hotspots
• Hot Spots The places known as hot spots in geology are
volcanic regions thought to be fed by underlying mantle
that is anomalously hot compared with the mantle
elsewhere.
• They may be on, near to, or far from tectonic plate
boundaries. There are two hypotheses to explain them.
One suggests that they are due to hot mantle plumes that
rise from the core-mantle boundary.
• The other hypothesis suggests that it is not high
temperature that causes the volcanism, but lithospheric
extension that permits the passive rising of melt from
shallow depths.
• Source: Wikipedia
Associated landforms: young fold mountains, rift valleys,
ocean ridges, deep sea trenches and island arcs.
Constructive Plate Boundary –
initial stage
Rift Valley
e.g. East African rift valley
Initial doming from
upwelling magma
Leads to Y-shaped rifting.
Rift Valley with stepped faults, and
tilted blocks (horsts)
A shallow sea basin may form – lots
of evaporite salt deposits.
Middle Stage
Later stage
Mid ocean ridge created by sea floor spreading.
Magnetic stripes as evidence for the spreading.
Fissure volcanoes e.g. Iceland
Underwater mountain range
Central rift valley
Offset by transform faults
Destructive Plate Boundary – type 1
Ocean plate v. Ocean plate
Ocean trench e.g. Aleutian trench
Island arc volcanoes e.g. Aleutians (off Alaska)
Pacific Ring of Fire
Destructive Plate Boundary – type 2
Ocean plate v. Continental plate (Sometimes called a SUBDUCTION ZONE)
Ocean plate subducts as it is denser
Ocean trench e.g. Peru-Chile trench
Cordillera range of Young Fold Mountains e.g. Andes
Andesitic volcanic peaks e.g. Nevada del Ruiz
Destructive Plate Boundary – type 3
Continental plate v. continental Plate collision
A collision zone
Major fold mountains e.g. Himalayas and High Plateau e.g. Tibet
e.g. Ocean plate carrying India drifted northwards until India collided with
Eurasian plate.
Conservative Plate Boundary
No volcanic features
Plenty of earthquakes e.g. San Andreas fault.
Fault line scarp slopes
Horizontal movement faults
Fault displacement features
Conservative Plate Boundaries are
transform faults
which displace mid ocean ridges in
Constructive plate Boundaries
Transform fault
Mid ocean ridge
The Main Plates
•
Convergent boundaries = Destructive Plate Boundaries: e.g ____________________
•
Divergent Boundaries = Constructive Plate Boundaries e.g_____________________
•
Transform Faults = Conservative Plate Boundaries e.g_________________________
Vulcanicity (Volcanoes)
• Volcanoes can be classified by their size and
shape or by the nature of their eruptions.
However, these features are connected as the
type of eruption affects the consequent
landform. Eruptions may be categorised as
either effusive or explosive. Effusive eruptions
involve the outpouring of magma that is
relatively low in viscosity and in gas content,
whereas explosive eruptions generally involve
magma that is more viscous and acidic.
Shield Volcano
• Basalt lava is low in silica which is called a 'basic' chemical
composition. They are hotter at 1000*C to 1200*C and have low viscosity.
Gases are released more easily and the eruptions tend to be less explosive
like the fissure eruptions of Constructive Plate Boundaries like Heimaey
1973 in Iceland or like lava flows from Kilauea in Hawaii at an Intra-Plate
Hot Spot which builds shield volcanoes.
• Shield Volcanoes have gentle sides because the lava is runny and moves
away from the vent quite freely before solidifying.
• They are very tall wide structures compared to conical (composite)
volcanoes - although in places like Hawaii much of the volcano may be
below sea level.
composite volcanoes.
• These volcanoes have steeper sides and are
smaller in scale than shield volcanoes - they have
the typical volcano conical shape and are often
alternating layers of ash and pyroclastic lavas
called a composite volcano or stratovolcano.
Composite volcanoes
• Lavas with high silica content have an acidic chemical
composition and form rocks called rhyolites. They are
viscous and have relatively low temperatures of 600*C
to 1000*C. The lavas flow slowly, the gases do not
escape easily and build up to produce more explosive
eruptions. They are typical of Destructive Plate
Boundary volcanoes either ocean v. ocean plate
collisons or ocean v. continent plate collisions.
• Andesite lava is intermediate but rather silica rich and
therefore also likely to be quite explosive in eruption.
Andesite volcanoes are also typical of Destructive Plate
Boundary subduction zones.
Composite Cones
• Examples include Mount Fuji in Japan,
Cotopaxi in Ecuador and Merapi in Indonesia,
all located around the Pacific Ring of Fire. The
sequence of slow-flowing lava and pyroclastic
material erupted produces steep layers of ash
and lava domes. This increases the risks of
landslides.
•
•
•
Hawaiian – These occur from a central vent or along
fissures. The lava may feed lava streams that flow
downslope or be erupted to a height of several hundred
metres. They are rarely explosive but may be accompanied
by jets of gas. Kilauea in Hawaii (a hotspot) erupts in this
way.
Icelandic – These occur at constructive plate margins, emit
large volumes of fluid lava from fissures, are often several
kilometres in length and spread in sheets over the land.
Strombolian – named after the Italian volcano, these are
moderate explosions accompanied by scoria (porous
sponge-like lava) and a white vapour cloud. The gases rise
faster than the magma and escape with small explosions,
throwing out lumps of molten lava which then flow
downslope in streams.
•
Plinian and Vulcanian – These eruptions have more viscous
lava that forms a solid crust over the crater between the
infrequent eruptions. Ash fallout can affect large areas
hundreds of kms downwind. Lava flows from the crater and
then from fissures on the sides of the cone. Fast moving
pyroclastic flows often occur during these eruptions, which
include Mount St. Helens in 1980 and Pinatubo in 1991.
•
Pelean – named after Mount Pelee on the island of
Martinique, these eruptions produce the most viscous lava
and the greatest explosions as the magma is blocked by a
plug in the vent forcing a horizontal blast. A nuee ardente
(glowing cloud) is formed as hot gas, dust, ash and lava
fragments are blown out and which rushes down the
mountain slopes at speeds up to 160kph. These volcanoes
often collapse to form large calderas.
•
Krakatauan – these explosions are cataclysmic – a huge
caldera is formed in these ‘ultraplinian’ events.
Types of
Eruption
Intrusive and Extrusive
Landforms:
GEYSERS:
• This is where volcanic activity heats up water which
eventually explodes on to the surface. These features are
only temporary as the only last up to a few thousand years.
The magma needs to be relatively near the earth’s surface
in order to heat the water up, this is why these features are
only found in volcanic areas. These are fairly rare as they
need very specific hydrological conditions to occur. When
an eruption occurs there are four main stages. First of all
the steam rises from the geothermally heated water, this
leads to pulses of water swelling up wards. Eventually the
surface is broken and the ejected water explodes.
• Examples include the Haukadalur spring in Iceland. Almost
50% of all geysers are situated in Yellowstone National Park,
USA
Hot Spring:
• Hot springs are caused by the emergence of
geothermally heated water on the surface. Boiling Mud
is water which is heated up by volcanic activity that
doesn’t always explode onto the surface. Occasionally
it mixes with surface deposits which cause boiling mud.
This kind of feature usually occurs in Iceland. These
springs have a very high mineral content which makes
them a very popular tourist attraction. However, some
of them contain Biota which makes them dangerous to
humans.
• Examples include Bath and Iceland.
Batholiths, Dykes and Sills:
• Batholiths are formed deep below the surface
when large masses of magma cool and solidify.
• Dykes are vertical intrusions with horizontal
cooling cracks. They cut across the bedding
planes of the rocks into which they have been
intruded.
• Sills are horizontal intrusions along the lines of
bedding planes. They have vertical cooling cracks.
Why do earthquakes occur?
• Fractures, faults
• Energy released
and propagates in
all directions as
seismic waves
causing
earthquakes
epicenter
focus
Where do earthquakes occur:
1) Most earthquakes occur along the edge of the
oceanic and continental plate
2) Along faults: normal, reverse, transform
definitions
• Earthquake = Vibration of the Earth produced by the
rapid release of energy
• Seismic waves = Energy moving outward from the
focus of an earthquake
• Focus= location of initial slip on the fault; where the
earthquake origins
• Epicenter= spot on Earth’s surface directly above the
focus
Seismic waves: forms
• P-waves:
– called compressional, or push-pull waves
– Propagate parralel to the direction in which the wave is moving
– Move through solids, liquids
• S-waves:
– Called shear waves
– Propagate the movement perpendicular
to the direction in which the wave is
moving
• Surface waves (L-waves or long waves).
–
–
–
–
Complex motion
Up-and-down and side-to-side
Slowest
Most damage to structures, buildings
Seismic waves: properties
• Velocity: function of the physical properties of the
rock the wave is traveling through
– Velocity increases with rock density
– Velocity changes when passing from one material
to another (increases/decreases)
– Liquids: S-waves do not get transmitted through
liquid; P-waves slow down
• Why is this important?
–If we know the velocity of the wave, we can infer
the type of rock it traveled through- that’s how we map
the interior of the Earth!!!
Measuring earthquakes
• Seismometers:
instruments that detect
seismic waves
• Seismographs
Record intensity, height
and amplitude of seismic
waves
Earthquake size: two ways to measure
1) Magnitude: Richter Scale
•
•
•
•
Measures the energy released by fault
movement
related to the maximum amplitude of the S
wave measured from the seismogram
Logarithmic-scale; quantitative measure
For each whole number there is a 31.5 times
increase in energy
•
eg. an increase from 5 to 7 on the Richter scale = an
increase in energy of 992 times!!
2) Intensity: Mercalli Scale:
– What did you feel?
– Assigns an intensity or rating to measure an earthquake at
a particular location (qualitative)
– I (not felt) to XII (buildings nearly destroyed)
– Measures the destructive effect
• Intensity is a function of:
• Energy released by fault
• Geology of the location
• Surface substrate: can magnify shock waves e.g. Mexico
City (1985) and San Francisco (1989)
Frequency of Occurrence of Earthquakes
Descriptor
Magnitude
Average Annually
Great
8 and higher
1¹
Major
7 - 7.9
17 ²
Strong
6 - 6.9
134 ²
Moderate
5 - 5.9
1319 ²
Light
4 - 4.9
13,000
(estimated)
Minor
3 - 3.9
130,000
(estimated)
Very Minor
2 - 2.9
1,300,000
(estimated)
¹ Based on observations since 1900.
² Based on observations since 1990.
Largest earthquake in the world
Chile : 1960 May 22
19:11:14 UTC
Magnitude 9.5
More than 2,000 killed, 3,000 injured, 2,000,000
homeless, and $550 million damage in southern
Chile
tsunami caused 61 deaths
$75 million damage in Hawaii;
138 deaths and $50 million damage in Japan;
32 dead and missing in the Philippines;
and $500,000 damage to the west coast of
the United States.
Most Destructive Known Earthquakes on Record in the World
Date
Location
Deaths
Magnitude
Comments
May 31, 1970
Peru
66,000
7.9
$530,000,000
damage, great rock
slide, floods.
July 27, 1976
China,
Tangshan
255,000
(official)
7.5
Estimated death toll as
high as 655,000.
Sept 19, 1985
Mexico
Michoacan
9500
(official)
8.0
Estimated death toll as
high as 30,000
Old lake bed magnified
shock waves by 500%
2001 Jan 26
India
20,023
7.7
166,836 injured,
600,000 homeless
.
2004 Dec 26
Sumatra
283,106
9.0
Deaths from earthquake
and tsunami
Earthquake damage
• Ground Failure - constructions collapse
• Fires - from broken gas and electrical lines
• Landslides - EQ's triggered; occur in
hilly/mountainous areas.
• Liquefaction - water-saturated,
unconsolidated materials flow
• Tsunami (seismic sea waves; "tidal" waves) can grow up to 65 m
Case Study – Japan 2011
• Location
• Timing: 2.46pm Friday
• Sendai (Japan) Earthquake – March 11th 2011
Japan 2011 - Causes
Japan 2011 – nature of
the event
• Japan's most powerful earthquake since
records began - measured at 9.0 by the US
Geological Survey - has struck the northeast coast, triggering a massive tsunami.
Many modern buildings are earthquake
‘proof’ but cities like Tokyo also contain
many vulnerable older residential
communities with narrow wooden houses
in narrow streets, where roofs are
touching almost each other. Much more
damage could have occurred in Tokyo.
• After the earthquake came a 7metre high
tsunami wave that devastated 1300 kms of
the north east coast around Sendai. The
wave moved up to 10km inland in flat
coastal areas. The tsunami arrived just
over 1 hour after the earthquake struck.
14.46 and 15.55 JST.
Japan 2011 - Impacts
• The death toll one
week after the
disaster was at least
7,500 fatalities and
over 12,000 officially
listed as missing
(Sunday Telegraph).
Minamisanriku - feared more than half the
17,000 population were lost as the 8m wall of
water came crashing inland at 500mph an
hour on Friday 11th March after a huge
earthquake beneath the ocean.
• Fukushima – nuclear
reactors badly damaged.
Evacuation zone of 20kms
– at least 200,000 people
were evacuated.
• Other nations advised
their citizens to leave
Japan. Britain has advised
its nationals currently in
Tokyo and to the north of
the capital to consider
leaving the area, and to
keep outside an 80km (50
miles) radius of the
Fukushima plant, in line
with US state department
instructions.
Social Impacts
• Social Impacts Hundreds of thousands
of victims have been
made homeless, with
more than 400,000
struggling to survive in
emergency shelters with
no power and little food
or water.
One 80 year old woman
rescued alive from the
rubble of her home after
9 days.
Social Impacts
• 25% of Japan’s population is
elderly and many of them
struggled to survive in
shelters without heating or
electricity as supplies of food
and medicine ran low. Snow
fell and temperatures were 5*C in the week after the
earthquake.
• Many of the rural, seaside
towns hit by the tsunami
were in economic decline
and had seen an exodus of
young people, who moved to
major cities for work.
Economic impacts
• Economic Impacts: Some countries that
export energy and raw materials could see a
surge in demand from Japan.
But others that rely on Japan for
manufacturing components will be bracing
themselves for shortages in supply.
Cargo containers were strewn about in Sendai
Japan March 12.
Economic impacts
• Japanese shares have tumbled on the first
trading day after the massive earthquake and
tsunami. Amid record share trading, the Nikkei
index ended down 6.18% at 9,620 points.
The benchmark crude oil price fell to less than
$99 a barrel on weaker demand from the
world's third-largest economy. Meanwhile, the
Bank of Japan tried to support the economy,
saying it would inject 15 trillion yen ($183bn;
£114bn) into the banking system.
Economic Impacts
• Production was stopped at some of Japan's
best-known companies, heavily denting their
share prices. Carmaker Nissan was down by
9.5% after it shut all its plants, while Toshiba,
whose products include semiconductors and
nuclear reactors, fell 16%. European stocks
also began the day lower, with German shares
hit hardest. The main Dax index fell 1.3%,
dragged lower by power companies and
insurers.