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Chapter 2

Chapter 2 - Plate Tectonics

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Department
Environmental Science
Course Code
EESA06H3
Professor
Nick Eyles

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CHAPTER 2 - PLATE TECTONICS
What is Plate Tectonics?
Tectonics is the study of the origin and arrangement of the broad structural features
of the Earths surface. (ie folds, faults, mountain ranges, etc..)
Plate tectonics describes the Earths surface as being divided into a few large,
thick plates that move slowly and change in size. At the boundaries of plates there is
intense geologic activity.
oExplains several diverse features on the Earth: Earthquakes, origin of
mountains, sea-floor topography.
oIdea developed in the 1960s; a combination of two theories, continental drift
ad sea-floor spreading.
oContinental Drift Idea that the Earths CONTINENTS moved freely over
the Earths surface, changing their shape and position relative to other
continents over time.
oSea-floor spreading - Idea that the Earths sea floor forms at the crests of
the mid-oceanic ridges, moves horizontally away from the ridge and
eventually sinks at the oceanic trenches; like a conveyer belt.
How did the Plate Tectonics Theory Evolve?
Alfred Wegner was the first to make a strong case for the continental drift. He
noticed:
oThat the continents all seemed to fit together like a puzzle.
oFossils from several continents had the same Palaeozoic rocks and fossils.
Wegner called the super continent Pangaea, which he believed split into Laurasia
(Todays Northern Hemisphere continents) and Gondwanaland (had all of today’s
southern hemisphere continents + India).
More Evidence Based on the distribution of the continents today, a huge ice age
would have to cover the entire planet to account for the Palaeozoic glacial deposits
we have found around the world. However, those deposits would make more sense if
we assume the continents were all joined together at one point. THEN, we would
only observe glaciations deposits in the southern hemisphere and India.
www.notesolution.com
When examining sedimentary rocks from the paleoclimatic reconstructions, Wegner
found that the distribution of the polar positions were very different in the past
compared to now. Hence, he hypothesized that in the past continents had moved.
Skepticism about Continental Drift
oWegner believed that the continents ploughed through he oceanic crust,
compiling mountain ranges in the front. Knowing the strength of rocks at the
time, this was a bad explanation.
oWegner also proposed that based on a combination of two forces, the
centrifugal force and gravitational forces that continental drift was possible.
HOWEVER, through carful calculations it was shown that these forces are
way too small to account for the movements of continents.
The study of the sea-floor and geophysical research (rock magnetism) renewed
interest in continental drift in the 40s and 50s.
Geophysical Research- When rocks are formed they have within them magnetite
(cooled basaltic lava flow or other metallic minerals) that point towards the poles
and once cooled get frozen in time. This frozen compass needles tell us where the
magnetic poles were at the time of rock formation. It was originally believed based
on the results, that the poles were moving ver time. Now, however its believed that
the continents (the rocks) were the ones moving.
oAssuming there was only one pole, and lets take out the atlntic ocean in
between North America and Europe....then the polar wandering paths for
Europe and North America would be the EXACT same. Hence, this suggests
that at one time Europe and NA were both joined together (supercontinent)
and that over time the Atlantic ocean formed in between.
More Evidence Rock types matches: boulders in South America have been traced to
sources in Africa. Rock Matches show continents were at one point together.
Pangea split up to 200 million years ago, but evidence shows that the continents
have been moving for up to 2 billion (some say 4 billion) years now.
What is Sea Floor Spreading?
Harry Hess suggested that the sea floor was spreading; totally opposite idea to
Wegners (that the sea floor was still while the continents moved through them)
www.notesolution.com
oProposed that the sea floor moves away from the mid-oceanic ridges towards
trenches and sinks as a result of MANTLE CONVECTION (uneven heat
distributions).
oAt the ridge crest, convection drives the hot mantle material upwards which
then diverges as it encounters the surface. The movement also creates
tension at the ridge crest which causes the cracking open of the oceanic
crust....hence allows for the formation of rift valleys and shallow-focus
earthquakes..
oAs the mantle rock moves horizontally away from the ridge it carries with it
the basaltic oceanic crust. Eventually the sea floor slides beneath a continent
or an island arc (process called SUBDUCTION)
The sea floor is younger than 200 million years. HOW is that possible???... (The
Earth is supposed to be 4.5 billion years old). Ans: Cause of the sea-floor spreading,
the sea floor is continuously being formed by basalt eruptions at ridge crests. Ocean
floor moves 1-10cm/year depending on the location.
Older Sea floor is further away from the ridge crest. Younger sea floor is close to the
ridge crest.
What are Plates and How do They Move?
A plate is a large, MOBILE slab of earth that is a part of the Earth`s surface (called
the lithosphere). Can be made up of completely sea floor (Nazea Plate), completely
continental surface (these are usually very small), or both continental and sea floor
(most common; usually found in the larger plates)
Lithosphere is outer most surface of the Earth
oMakes up all the plates.
oIncludes rocks of the crust and the uppermost mantle.
oThe lithosphere gets older and thicker as you move further away from the
mid-oceanic ridge.
oContinental lithosphere is thicker (125-250km) than oceanic lithosphere
(ranges from 10 to 100km based on distance from the mid-oceanic ridge)
Asthenosphere Plastic like zone (cause of high temp and pressure) below the
lithosphere. Acts like a lubricating layer that allows for the plates to move.
www.notesolution.com

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Description
CHAPTER 2 - PLATE TECTONICS What is Plate Tectonics? Tectonics is the study of the origin and arrangement of the broad structural features of the Earths surface. (ie folds, faults, mountain ranges, etc..) Plate tectonics describes the Earths surface as being divided into a few large, thick plates that move slowly and change in size. At the boundaries of plates there is intense geologic activity. o Explains several diverse features on the Earth: Earthquakes, origin of mountains, sea-floor topography. o Idea developed in the 1960s; a combination of two theories, continental drift ad sea-floor spreading. o Continental Drift Idea that the Earths CONTINENTS moved freely over the Earths surface, changing their shape and position relative to other continents over time. o Sea-floor spreading - Idea that the Earths sea floor forms at the crests of the mid-oceanic ridges, moves horizontally away from the ridge and eventually sinks at the oceanic trenches; like a conveyer belt. How did the Plate Tectonics Theory Evolve? Alfred Wegner was the first to make a strong case for the continental drift. He noticed: o That the continents all seemed to fit together like a puzzle. o Fossils from several continents had the same Palaeozoic rocks and fossils. Wegner called the super continent Pangaea, which he believed split into Laurasia (Todays Northern Hemisphere continents) and Gondwanaland (had all of todays southern hemisphere continents + India). More Evidence Based on the distribution of the continents today, a huge ice age would have to cover the entire planet to account for the Palaeozoic glacial deposits we have found around the world. However, those deposits would make more sense if we assume the continents were all joined together at one point. THEN, we would only observe glaciations deposits in the southern hemisphere and India. www.notesolution.com When examining sedimentary rocks from the paleoclimatic reconstructions, Wegner found that the distribution of the polar positions were very different in the past compared to now. Hence, he hypothesized that in the past continents had moved. Skepticism about Continental Drift o Wegner believed that the continents ploughed through he oceanic crust, compiling mountain ranges in the front. Knowing the strength of rocks at the time, this was a bad explanation. o Wegner also proposed that based on a combination of two forces, the centrifugal force and gravitational forces that continental drift was possible. HOWEVER, through carful calculations it was shown that these forces are way too small to account for the movements of continents. The study of the sea-floor and geophysical research (rock magnetism) renewed interest in continental drift in the 40s and 50s. Geophysical Research- When rocks are formed they have within them magnetite (cooled basaltic lava flow or other metallic minerals) that point towards the poles and once cooled get frozen in time. This frozen compass needles tell us where the magnetic poles were at the time of rock formation. It was originally believed based on the results, that the poles were moving ver time. Now, however its believed that the continents (the rocks) were the ones moving. o Assuming there was only one pole, and lets take out the atlntic ocean in between North America and Europe....then the polar wandering paths for Europe and North America would be the EXACT same. Hence, this suggests that at one time Europe and NA were both joined together (supercontinent) and that over time the Atlantic ocean formed in between. More Evidence Rock types matches: boulders in South America have been traced to sources in Africa. Rock Matches show continents were at one point together. Pangea split up to 200 million years ago, but evidence shows that the continents have been moving for up to 2 billion (some say 4 billion) years now. What is Sea Floor Spreading? Harry Hess suggested that the sea floor was spreading; totally opposite idea to Wegners (that the sea floor was still while the continents moved through them) www.notesolution.com
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