Download 6-Plate Tectonics

Plate Tectonics
– plate margins; dynamics; effects Jarðsaga 1
-Saga Lífs og Lands –
Ólafur Ingólfsson
Excellent web pages: http://earth.leeds.ac.uk/dynamicearth/index.htm
http://pubs.usgs.gov/publications/text/dynamic.html
Earth´s inner structure
What are the continental plates?
The Earth's rocky outer crust is not a solid shell; it is broken up into
huge, thick plates that drift atop the soft, underlying mantle.
Over long periods of time, the plates drift across the surface of the
globe, and they also change in size as their margins are added to,
crushed together, or pushed back into the Earth's mantle. The plates
are generally between 80-400 km thick.
Plate margins and motions
Divergent= constructive; convergent= destructive; transform= conservative
Constructive plate boundaries
...where new lithosphere is created - by sea floor spreading.
Divergent boundaries occur along spreading centres where plates are
moving apart and new crust is created by magma pushing up from the
mantle.
Sea Floor Spreading
Types of transform fault
Atlantic Mid-Ocean
Ridge – the best
known constructive
plate margins
Submerged MOR
The mid ocean ridges extend for
some 70,000 km along the floor
of the ocean. Note the sediment
blankets away from the ridge
Pillow basalt along the midAtlantic ridge.
Submerged open fissures
Black smokers – cradles of life...?
Black smokers are vents of mineral laden waters circulating
through the hot rocks in the rift valley. This warm water
provides a unique habitat for exotic life that subsists without
sunlight.
Mid-Oceanic rift
reflected in the
geological history of
Iceland
The Icelandic Hotspot
Idealized cross section of a mid
ocean ridge
Crustal thickness in Iceland
Hotspots in the world
A hotspot is an area where there has been a continous
volcanism for a very long time. They occur both on diverging plate boundaries and inside plates. Were thought to
be the major force driving plate tectonics, but that view is
not prevailing today.
The Hawaiian hotspot
East African Rift Valley
East Africa may be the site of the Earth's next major ocean.
• Continental rifting occurs
when divergent plate margins
develop in continents.
• They typically have normal
faults, shallow earthquakes, and
basalt and rhyolitic magmatism
What drives
the plates?
• Until the 1990s, prevailing explanations about
what drives plate tectonics have emphasized
mantle convection.
• Some geologists argue that the intrusion of magma into the spreading
ridge provides an additional force (called "ridge push") to propel and
maintain plate movement.
• Most scientists now favor the notion that forces associated with
subduction are more important than seafloor spreading.
Convergent Plate Boundaries
• The size of the Earth has not
changed significantly since shortly
after its formation 4.6 billion
years ago.
•The Earth's unchanging size
implies that the crust must be
destroyed at about the same rate
as it is being created.
• Such destruction of crust takes
place along convergent boundaries
where plates are moving toward
each other, and sometimes one
plate sinks (is subducted) under
another.
• The location where sinking of a
plate occurs is called a subduction
zone.
Ocean-Continent convergence
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Oceanic-Continental Plate Margins
Off the coast of South America the oceanic Nazca Plate is pushing
into and being subducted under the continental part of the South
American Plate. The overriding South American Plate is being lifted
up, creating the towering Andes mountains, the backbone of the
continent. Strong, destructive earthquakes are common in this region.
The convergence of the Nazca and South
American Plates has deformed and pushed
up limestone strata to form towering
peaks of the Andes
Andean margin
Andean volcanism
Ruiz, in Columbia (5,321 m), capped
by snow and ice. The 1985 eruption
of Ruiz produced mudflows which
reached the town of Armero and
killed more than 23,000 people
Mt Villaricha in Chile. 55
eruptions since 1558
Some Andean
volcanoes
Colima (4100 m) is the most active volcano in
Mexico. Eruptions often explosive, producing
tephra as well as avalances. Most recent eruption
was in 1994
Arenal, in Costa Rica, was regarded as a dormant stratovolcano. That changed
in July of 1968. An explosive eruption produced hot avalanches and ejected
blocks that devastated the west flank of the volcano and killed 78 people.
Arenal has been continuously active since then. Strombolian eruptions occur
at intervals of several minutes to hours.
The Ring of Fire...
Oceanic-oceanic margins:
Island Arches
Japanese
Volcanoes
Sakurajima is a stratovolcano
on the the island of Kyushu,
and one of the most active
volcanoes on Earth. Since it's
first recorded eruption in
A.D. 708, Sakurajima has
been in nearly constant
eruption.
Aso (1,592 m)
Aso has produced more explosive
eruptions than any other volcano
in the world: It is a caldera
about 20 km in diameter. The
first documented eruption in
Japan was at Aso 553. Since
then it has erupted 167 times,
most recently in 1993.
Sakurajima (1117 m)
Subduction-zone earthquakes
- Often very violent and destructive -
- The destruction of Tokyo On 1 September 1923, one of the worst earthquakes in
world history hit the Kanto plain and destroyed Tokyo,
Yoko-hama and the surroundings. About 140,000 people
fell victim to this earthquake and the fires caused by it.
The destruction...
The destruction was of the same order as the effects of the
American nuclear bombs at Hiroshima and Nagasaki in 1945...
The Kobe-earthquake in 1995:
Indonesia – a subduction zone
Indonesia:
a line-up
of
volcanoes
Some of Earth´s most
violent volcanoes are in
Indonesia
The 1815 eruption of Tambora
was the largest eruption in
historic time. About 150 cubic
kilometers of ash were erupted.
Ash fell as far as 1,300 km
from the volcano. An estimated
92,000 people were killed by
the eruption.
The eruption of Krakatau in 1883 produced one of the largest explosions
recorded on Earth. It destroyed much of Krakatau, leaving only a remnant,
killing about 30,000 people. Since 1927, small eruptions have been frequent
and have constructed a new island, Anak Krakatau
Other subduction zones also have
violent volcanoes
Mt. Pelee in the West Indies (1337 m) is
famous for the May 8, 1902 eruption which
killed 29,000 people and destroyed the city of
St. Pierre. Mt Pelee is at the plate boundary
where the N American plate subducts the
Caribbean Plate.
The destruction was
caused by a nuee
ardente, a type of
pyroclastic flow that
consists of hot solid
particles and gases
Continental-continental
convergence
The Himalayan mountain range demonstrates
one of the most spectacular consequences of
plate tectonics. When two continents meet
head-on, neither is subducted because the
continental rocks are relatively light and, like
two colliding icebergs, resist downward
motion. Instead, the crust tends to buckle
and be pushed upward or sideways. The
collision of India into Asia 50 million years
ago caused the Eurasian Plate to crumple up
and override the Indian Plate. After the
collision, the slow continuous convergence of
the two plates over millions of years pushed
up the Himalayas and the Tibetan Plateau to
their present heights. Most of this growth
occurred during the past 10 million years.
Continent-Continent Convergent plates
Where Continental plate meets continental
plate, granite is produced due to enormous
heat and pressure.
Increasing metamorphic change occurs with increasing
temperatures and pressures. If the melting point is reached,
the change is no longer metamorphic, and igneous rocks are
formed.
Granite
• Granite is a coarse-grained igneous rock composed
mostly of light-colored, light-density, nonferromagnesian minerals. The earth's continental areas
are dominated by granite and by rocks with the
same mineral composition of granite.
Formation of the
Himalayas
The initial mountain building process
started 70-60 MY when the two land
plates of India and Eurasia began to
collide with each other. As a result, the
shallow seabeds of the Tethys Sea
rapidly folded and raised into
longitudinal ridges and valleys. A
second major phase occurred about 25
MY, and the last phase started about
800 TY. The former sea-bed was
elevated to form a great mountain
range.
Visual Evidences of Himalayan
Formation
Very precise measurements by GPS have shown that the Indian plate
is continuously moving north at the rate of about 2 cm/yr. Because of
this reason the Himalayas are rising at the rate of about 5 mm/yr.
The thrusting is demonstrated by the strongly folded longitudinal
ridges and consistent directions of thrust, folding and overthrust
Mt Everest
The fossil-bearing marine strata in
the folded and upthrusted mountains
and ridges show the uplift. The top of
Mt Everest is of stratifed,
fossiliferous marine sediments.
Mt Everest is still rising, at the
rate of 8.2 mm/year.
Active
tectonics
Because the Himalayas are
still rising, they are
tectonically unstable (large
earthquakes frequent) and
geomorphologically unmature
and very active
The Mediterranean area
Another region where a continental plate meets another
continental plate in collision is the Mediterrean area.
There, the African plate and the Eurasian plate
collide. This causes the frequent earthquakes in e.g.
Greece and Turkey. Volcanism in Italy and Greece is
related to complex plate boundaries...
Santorini and Hellenic volcanism:
The eruption of Santorini in Greece in 1,650 B.C. was one of the
largest in the last 10 ka. About 30 cubic km of ash was erupted in
a huge plinian column, causing the volcano to collapse, producing a c
aldera. Ash fell over a large area in the eastern Mediterranean and
Turkey. The eruption probably caused the end of the Minoan
civilization on the island of Crete
Santorini has had eleven eruptions since
197 B.C. The most recent eruption at
Santorini was in 1950
50 m of Minoan tephra
Stromboli, Italy
Stromboli is one of the Aeolian Islands
of Italy. The island is about 2 km in
diameter and reaches 900 m above sea
level. It rises 3,000 m above the floor
of the Tyrrhenian Sea. It has been in
nearly continuous eruption for about
2,000-5,000 years.
Vesuvius, Italy
(1,281 m)
Vesuvius is famous for the
catastrophic eruption in 79 A.D.
that buried the towns of
Herculaneum and Pompeii. It has
erupted 30 times since, last time in
1943-1944
Transform boundaries
The zone between two plates sliding horizontally past one another
Most transform faults are found on
the ocean floor. A few occur on land,
for example the San Andreas fault
zone in California.
The San Andreas fault zone is about
1,300 km long. It slices through two
thirds of the length of California.
Along it, the Pacific Plate has been
grinding horizontally past the North
American Plate for 10 million years, at
an average rate of about 5 cm/yr.
Land on the west side of the fault
zone (the Pacific Plate) is moving in a
north-westerly direction relative to
the land on the east side of the fault
zone (the North American Plate).
The 1906 San Fransisco earthquake
The 1906 earthquake in
San Francisco is one of
the most famous
earthquakes in history although not the largest
recorded nor the most
killed, it was one of the
first major earthquakes
to be comprehensively
studied, reported and
photographed.
7 m of displacements
The main epicentre of the earthquake was near San Francisco, but a
length of some 430 km of the San Andreas fault had given way with
about 7 m large horizontal displacements 50km NW of San Francisco.
What is the rate of continental drift?
http://cddisa.gsfc.nasa.gov/926/slrtecto.html
Rate of continental
drift measured by
GPS and satellites
Rates usually between 2050 mm/yr. Highest rate
measured on Easter
Island, 150 mm/yr
The effects of continental drift and plate
tectonics on climate and environment
• Changes in the global
configuration of the
Continental landmasses
alter Ocean currents.
Ocean currents largely
control heat transfer
between high and low
latitudes.
• Continental drift
controls periods of ice
age climate, like the
present.
The effects of continental drift and plate tectonics on
climate and environment...
• Mountain-building episodes alter
planetary waves and atmospheric
circulation patterns.
• Build-up of high mountains and
plateaux change runoff and
watersheds, albedo, vegetation,
animal habitats etc
• Erosion and wethering of high
mountains radically changes
lowland and nearshore
environments
The effects of continental drift
and plate tectonics on climate and
environment...
• Volcanism associated with
plate tectonics affects the
chemical composition of
Earth´s atmosphere and
thus directly and indirectly
affects life, environments
and climate...
The effects of continental drift and plate tectonics on climate and
environment...
Plate tectonics and continental drift create and destroy
sea-way passages and land connections, hindering or
enhancing migrations of plants and animals
Examples: Break-up of Gondwana, closing of the Panama sea-way,
isolation of Antarctica and Australia, opening of the Bering Strait...
Bering Strait
Plate tectonics are largely
responsible for Earth as we know it
Witout Plate Tectonics, Earth would be an area of
peneplain continents, much impoverished in environments
that sustain biological diversity
Plate tectonics have affected Earth´s
environments for most part of its 4,6 BY history
4.0-3.2 BY: Development of the Lithosphere begins.
3.2-2.6 BY: Development of continental plates
and cratons
2.4-2.0 BY: First break-up of continents
2.0-1.6 BY: Major crustal growth
1.6-1.4 BY: First Supercontinent forms
1.0-0.5 BY: Two Supercontinents
0.5 BY to Today
1.
Precambrian Supercontinents break up
2. Pangaea Supercontinent forms
3. Pangea breaks up, first into Gondwana and
Laurasia, then into the various tectonic plates
that are recognized today.
The future...
Continental drift and
plate tectonics will
continue to shape and
reshape the Earth´s
surface...
In the long run, plate
tectonics continue to
control Earth´s
environments...
References, web-pages etc
• Stanley, Earth System History, chapters 8 and 9.
• This Dynamic Earth - The Story of Plate Tectonics:
http://pubs.usgs.gov/publications/text/dynamic.html#anchor10790904
• Stórskemmtileg “on-line” kennslubók, “The Physical Environment” með góðum
texta, myndum og kvikmyndum:
http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/title_page.html
Kvikmynd um landreks- og plötukenninguna og þau gögn sem liggja að baki:
http://www.scctv.net/annenberg/Earth_Revealed_05.asx
• http://www.scotese.com/earth.htm
• http://www.ucmp.berkeley.edu/geology/tectonics.html
• http://volcano.und.nodak.edu/vw.html
• http://en.wikipedia.org/wiki/Eduard_Suess
• http://www.geology.utoronto.ca/glg105h/lecture16/CDsummarylast.htm
• http://www.seismo.unr.edu/ftp/pub/louie/class/100/plate-tectonics.html
• http://www.uh.edu/~jbutler/anon/quakes.htm