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Course Outline
GEOG 3
WORLD
CITIES
TECTONICS
Plate Theory
• Earth Structure
• Plate
Movement
• Theory and
Evidence (Alfred
Wegener)
Plate Margins
Earth Structure
Earth Structure
Crust
• Thin, outer-layer of the earth.
• It is as thin as the skin of an apple is to
its flesh.
• Oceanic Crust: a 6-10 km thick layer
composed of mainly basalt. At its
hottest point it is up to 1200C
• Continental Crust: up to 70 km thick.
Separated from the mantle by the moho
discontinuity.
Earth Structure – Mantle
• Composed mainly of silicate rocks, which
contain a lot of magnesium and iron.
• It extends to a depth of 2900km.
• Temperatures can reach 5000 degrees
Celsius, which generate convection currents.
• This is the ridge top layer of the mantle.
Together with the crust these 2 layers make
up the lithosphere.
• asthenosphere, this is the lower semi molten
state of the mantle.
Earth Structure - Core
• Contains iron and nickel.
• Situated approximately 6371km below
the surface.
• Temperature is around 5500 degrees
Celsius.
• outer core – semi molten
• inner core – Solid
The Theory: AW
Plate tectonics grew out of a theory that was first developed in the
early 20th century by the meteorologist Alfred Wegener. In 1912,
Wegener noticed that the coastlines of the east coast of South
America and the west coast of Africa seemed to fit together like a
jigsaw puzzle.
Plate Movement – Sea Floor Spreading
Paleomagnetism
What is Paleomagnetism?
A volcano erupts underwater, the lava which contains many different elements
including that of iron is heated and forced through the gap in the crust of the earth.
These Iron particles within the lava have become magnetic they now start to align
themselves with the earths polarity. (which changes every 400,000 years) Due to this
change in polarity we can see by the direction of the particles how long ago the
eruption was.
Theory: Jigsaw
• Further examination of the globe revealed that all of the
Earth's continents fit together somehow and Wegener
proposed an idea that all of the continents had at one time
been connected in a single supercontinent called Pangaea.
He believed that the continents gradually began to drift
apart around 300 million years ago - this was his theory
that became known as continental drift.
Theory: Geology
• Rocks that are similar in origin and age can be found in
both Brazil and South Africa, this suggests that originally
these two were touching and closer than they are today.
• The trends of the mountains in the Eastern USA and
Europe are similar
• Glacial deposits from Antarctica, South America and
India now 1000s of kilometres apart share some similar
trends.
• Striations which show the same orientation are present
when you compare Brazil to west Africa.
Theory: Fossil / Living
• Again similar fossil formations are found on both sides of
the Atlantic
• The Mesosaurus is only found in South America and
South Africa.
• Evidence for humid swaps which later formed coal fields
have been found in Antarctica but also in India
• Marsupials (kangaroos and koalas) only survived in
Australia because it had drifted away from the super
continent before predators could wipe them out.
Theory: Climate
• Places that are today, far apart contain the same coal
deposits suggesting that at one point they were close
together and in a different climate area. (Antarctica,
North America, Svalbard ( a Norwegian island) and the
UK) These places are no longer in tropical climates that
are needed for the “coal” swaps and so they must have
drifted apart
Plate Margins
• Constructive
• Destructive
- Oceanic to continental
- Oceanic to Oceanic
- Continental to Continental
• Conservative
Plate Margins
Plate Margins
Plate Margins
Plate Margins
Plate Margins
Constructive
Also known as a divergent margin. Plates move away from each other, for
example, N. American and Eurasian plates, at oceanic plates this creates
mid-ocean ridges such as the Mid Atlantic Ridge and at continental plates
they produce rift valleys. The space between the diverging plates is filled
with basaltic lava upwelling from below. Constructive margins are
therefore some of the youngest parts of the Earth's surface, where new
crust is being continuously created.
Constructive - Ocean Ridges
• Oceanic ridges are the longest continuous uplifted features on the surface of the
planet, and have a total length of 60,000km. In some parts they rise 3,000m above the
ocean floor. Their precise form appears to be influenced by the rate at which the
plates separate:
• A slow rate (10-15mm per year), as seen in parts of the mid-Atlantic ridge, produces a
wide ridge axis (30-50km) and a deep (3,000m) central rift valley with inward-facing
fault scarps
• An intermediate rate (50-90mm per year), such as that on the Galapogos ridge
(pacific), produces a less well-marked rift (50-200m deep) with a smoother outline
• A rapid rate (>90mm per year), such as on the east Pacific rise, produces a smooth
crest and no rift
• Volcanic activity occurs along the ridge, forming submarine volcanoes, which
sometimes rise above sea level, e.g. Sutsey to the south of Iceland. These are
volcanoes with fairly gentle sides because of the low viscosity of basaltic lava.
Eruptions are frequent but relatively gentle.
• As new crust forms and spreads, transform faults occur at right angles to the plate
boundary. The parts of the spreading plates on either side of these faults may move at
differing rates, leading to friction and ultimately to earthquakes. These tend to be
shallow-focus earthquakes, originating near the surface.
Constructive – Rift Valleys
At constructive margins in continental areas, such as
east Africa, the brittle crust fractures as sections of it
move apart. Areas of crust drop down between
parallel faults to form rift valleys. The largest of
these features is the African rift valley which extends
4,000km from Mozambique to the Red Sea. In some
areas the inward-facing scarps are 600m above the
valley floor and they are often marked by a series of
parallel step faults.
The area is also associated with volcanic activity (for example the
highest mountain in Africa, Mt Kilimanjaro). The crust here is much
thinner than in neighbouring areas, suggesting that tension in the
lithosphere is causing the plate to thin as it starts to split. The line if
the African rift is thought to be an emergent plate boundary, the
beginning of the formation of a new ocean as eastern Africa splits
away from the rest of the continent.
Destructive
Oceanic to Continental
Where the two plates meet, the denser oceanic lithosphere plate is forced down and
under the more buoyant continental lithosphere of the continental Plate.
The friction between the plates prevents the sub-ducting oceanic plate from sliding
smoothly. As it descends, it drags against the overlying plate, causing both to fracture
and deform. This results in frequent shallow focus earthquakes that get deeper as the
ocean plate descends further.
See Case Study
Source
Oceanic to Oceanic
The more dense plate will be subducted under the other during the collision. At the subduction
zone a very deep trench is formed in the ocean floor.
Oceanic and oceanic plate convergence result in the formation of volcano chains and island arcs.
The crust that is pulled under or subducted melts to form magma. This magma rises to the top of
the overriding oceanic plates and erupts on the ocean floor.
Over millions of years, the lava and
debris from the volcanic eruptions
pile up on the ocean floor until the
volcano rises above sea level to form
a volcanic island. These types of
islands are usually formed as chains
called island arcs, which run parallel
to the trench at the subduction zone.
Oceanic-oceanic plate convergence
experience similar powerful
earthquakes to oceanic-continental
convergences.
Source
Continental to Continental
Continental to Continental
Case Study
When continental plates collide neither can be subducted due to their low
density/buoyancy. This causes the continental crust to thicken due to folding and
faulting by compressional forces. The continental crust in the Himalayas is twice
the average thickness at around 75 km. The thickening of the continental crust
marks the end of volcanic activity in the region as any magma moving upwards
would solidify before it could reach the surface.
At continental plate boundaries fold mountains such as the Himalayas are formed.
Source
Conservative
• Conservative margins are also known as transform faults. Transform
faults are mainly found on the ocean floor, where they offset mid
ocean ridges and enable to ocean to spread at different rates. It
was through the work of John Tuzo Wilson that these faults were
recognised as the connection between the ocean ridges (divergent
margins) and ocean trenches (convergent margins).
• At conservative margins, plates slide past each other, so that the
relative movement is horizontal, and classified as either sinistral (to the
left) or dextral (to the right). Lithosphere is neither created nor
subducted, and whilst conservative plate margins do not result in
volcanic activity, they are the sites of extensive shallow focus
earthquakes, occasionally of considerable magnitude.
SOURCE
Plate Margin Case Studies
•
•
•
•
•
•
Conservative margin – San Andreas Fault
Oceanic/ Continental plate convergence – Nazca
Rift Valley – E.Africa Rift Valley
Ocean Ridge – Mid Atlantic Rift
Ocean Trench – Tonga Trench
Fold Mountains- Himalayas
Conservative
San Andreas Fault
Although both plates are moving
in THE SAME north westerly
direction, the Pacific Plate is
moving faster than the North
American Plate, so the relative
movement of the North American
Plate is to the south east. The Pacific
Plate is being moved north west due
to sea floor spreading from the East
Pacific Rise (divergent margin) in the
Gulf of California. The North
American Plate is being pushed west
and north west due to sea floor
spreading from the Mid Atlantic
Ridge (divergent margin).
Movement along the fault is not smooth and
continual, but sporadic and jerky. Frictional
forces lock the blocks of lithosphere together
for years at a time. When the frictional forces The average rate of movement along the San
are overcome, the plates slip suddenly and Andreas Fault is between 30mm and 50mm per
shallow focus earthquakes are generated.
year over the last 10 million years
Oceanic/Cont.
CONVERGENCE NAZCA PLATE
Oceanic/Cont.
CONVERGENCE NAZCA PLATE
• The Nazca Plate is moving eastwards, towards the South American Plate,
at about 79mm per year.
•
The effect of the collision of the two plates deforms the leading edge of the
South American Plate by folding the rocks. This crustal shortening
increases the vertical thickness whilst reducing the width of the lithosphere
in the collision zone (imagine a car hitting a solid wall) and so produces the
fold mountains of the Andes.
• Andesitic magma is less dense than the surrounding material, and can
have a temperature of 1000oC. It is viscous, trapping gases as it rises.
The water and gases in andesitic magma account for the explosive
activity of andesitic volcanoes, which typically lie dormant for many
hundreds or thousands of years. These volcanoes typically produce ash
and pyroclastic flows, as well as small amounts of andesitic lava.
• Andean volcanoes such as the stratovolcano Láscar, in northern Chile,
are a good example of this type of activity. Láscar erupted ash and
pyroclastic flows in 1993 and was still active in 2012.
RIFT VALLEY – East Africa
The East African Rift System (EARS) is one the geologic wonders of
the world, a place where the earth's tectonic forces are presently
trying to create new plates by splitting apart old ones. In simple
terms, a rift can be thought of as a fracture in the earth's surface that
widens over time, or more technically, as an elongate basin bounded
by opposed steeply dipping normal faults.
The exact mechanism of rift formation is an on-going debate among
geologists and geophysicists. One popular model for the EARS
assumes that elevated heat flow from the mantle (strictly the
asthenosphere) is causing a pair of thermal "bulges" in central Kenya
and the Afar region of north-central Ethiopia.
SOURCE
RIFT VALLEY – East Africa
A hot spot seems to be situated under central Kenya,
as evidenced by the elevated topographic dome
there (Figure 1). This is almost exactly analogous to
the rift Ethiopia, and in fact, some geologists have
suggested that the Kenya dome is the same hotspot
or plume that gave rise to the initial Ethiopian
rifting.
SOURCE
Ocean Ridge –
Mid Atlantic Rift
The North American and Eurasian Plates are moving away from each other
along the line of the Mid Atlantic Ridge. The Ridge extends into the South
Atlantic Ocean between the South American and African Plates. The ocean
ridge rises to between 2 to 3 km above the ocean floor, and has a rift valley
at its crest marking the location at which the two plates are moving apart.
The Mid Atlantic Ridge, like other ocean ridge systems, has developed as a
consequence of the divergent motion between the Eurasian and North
American, and African and South American Plates.
As the mantle rises towards the surface below the ridge the pressure is
lowered (decompression) and the hot rock starts to partially melt. This
produces basaltic volcanoes when an eruption occurs above the surface
(Eyjafjallajökull in Iceland) and characteristic basalt “pillow lava” in
underwater eruptions. In this way, as the plates move further apart new
ocean lithosphere is formed at the ridge and the ocean basin gets wider. This
process is known as “sea floor spreading” and results in a symmetrical
alignment of the rocks of the ocean floor which get older with distance from
the ridge crest.
Ocean Ridge –
Mid Atlantic Rift
Ocean Trench –
Tonga Trench
Underwater mountains are being dragged westward on the Pacific plate and
subducted into the Tonga Trench .
submarine trench in the floor of the South Pacific Ocean, about 850 miles
(1,375 km) in length, forming the eastern boundary of the Tonga Ridge; the
two together constitute the northern half of the Tonga-Kermadec Arc, a
structural feature of the Pacific floor completed to the south by the
Kermadec Trench and Ridge. The Tonga Trench has an average depth of
20,000 feet (6,000 m) and a width of about 50 miles (80 km); it reaches a
maximum depth of 35,702 feet (10,882 m).
FOLD MOUNTINS –
Himalayas
Continental Collision Boundaries fold mountains fold as the
material is crumpled against each other. The Example for
fold mountains is the Himalayas in India.
There is no volcanic activity but there are intense
earthquakes like Sichuan 2008 and NE India in 2001.
FOLD MOUNTINS –
Himalayas
The Himalayas, which stretch some 2,900 kilometres between India, Pakistan, China, and
Nepal, is the world’s tallest mountain range. In addition to Mount Everest, the world’s tallest
mountain by peak elevation standing at 8,848 meters tall, the range also features several
other mountain peaks over 8,000 meters. It is the only mountain range to boast mountains
over 8,000 meters—the runner-up is a mountain range in South America, whose tallest peak
is just 6,962 meters tall.
Millions of years ago, these mountain peaks didn’t exist. The Asian continent was mostly
intact, but India was an island floating off the coast of Australia. Around 220 million years
ago, around the time that Pangea was breaking apart, India started to move northwards. It
travelled some 6,000 kilometres before it finally collided with Asia around 40 to 50 million
years ago. Then, part of the Indian landmass began to go beneath the Asian one, moving the
Asian landmass up, which resulted in the rise of the Himalayas. It’s thought that India’s
coastline was denser and more firmly attached to the seabed, which is why Asia’s softer soil
was pushed up rather than the other way around.
FOLD MOUNTINS –
Himalayas
The mountain range grew very rapidly in comparison to most mountain ranges, and it’s
actually still growing today. Mount Everest and its fellows actually grow by approximately
a net of about a centimetre or so every year. That’s in comparison to the Appalachian
Mountains, which developed some 300 million years ago or more, which is actually
decreasing in peak elevations as it erodes.
The continued growth in the Himalayas is likely due to the Indian tectonic plate still
moving slowly but surely northward. We know the plate is still moving in part because of
the frequent earthquakes in the region.
Now, if you do the math, you’d find that if the Himalayas had been growing at the current
rate for 40 million years, they should be about 400 km tall! Once the infrastructure was in
place, this would have given us a much cheaper way to put things into low Earth orbit and
beyond. (For reference, the International Space Station typically orbits at between 300 km
to 400 km.)
So what happened? In part, the rate of vertical growth has varied over time, including in
favor of more horizontal growth. And, of course, gravity and erosion having limited the
mountains’ growth significantly.
FOLD MOUNTINS –
Himalayas
India merging into Asia became the accepted theory about how the Himalayas were
formed around 1912. That’s when Alfred Wegener, a German meteorologist, came
up with the “Theory of Continental Drift” which gave us our first ideas about
Pangea, tectonic plates, and the thought that continents were moving away from or
closer to each other.
What does the future for the Himalayas look like? Undoubtedly, the mountains will
continue to grow, though at the same time eroding too; but the net is expected to
continue to grow as the Indian tectonic plate doesn’t look like it’s going to slow
down any time soon. That means more earthquakes and, over time, slightly taller
mountains to climb.
Hot spots associated with plumes of magma and their relationship to
plate movement.
Some volcanoes do not occur at plate margins
For example the Hawaiian Islands which are volcanic in origin, were formed in the
middle of the pacific plate. 3200km from the nearest plate boundary.
Hot spots are the result of plumes of magma deep in the mantle which are super
heated by radio active decay which occurs within the core.
Volcanoes erupt over the hotspots and new islands are formed. The plate is moving
past the hot spot and so the islands formed will also over time drift past the hotspot.
This creates an Island Arc.
Hotspots
HOTSPOTS
Hawaii
• Before 2009, The theory of Hotspots hadn’t been proven, however
in December 2009 scientists and geologists were able to prove the
existence of the Hawaii Hotspot (HHS)
• After two years of research and analysis the PLUME group released
the first image of the HHS which stretches a depth of 1500km
• The Pacific Plate is moving at a rate of 5-10cm a year. And so the
newly created islands are slowly moved away from the hotspot.
• The next volcano is the Lo’ihi Seamount which is forming on the
sea floor, however it will not form an island on the surface for about
the next 200,000
ISLAND ARCS
• The Island of Hawaii is the youngest island in the chain. The most
southeastern part of the Island presently sits on top of the hot spot.
The active submarine volcano Lö'ihi, will form the next volcanic
island. With the possible exception of Maui, the other Hawaiian
islands have moved northwest beyond the hot spot and have
become cut off from the sustaining magma source and are no
longer volcanically active.
• The progressive northwesterly drift of the islands from their point
of origin over the hot spot is well shown by the ages of the
principal lava flows on the various Hawaiian Islands from
northwest (oldest) to southeast (youngest), given in millions of
years: Ni'ihau and Kaua'i, 5.6 to 3.8; O'ahu, 3.4 to 2.2; Moloka'i,
1.8 to 1.3; Maui, 1.3 to 0.8; and Hawai'i, less than 0.7 and still
growing.
SOURCE