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Lecture

EESA06H3 Lecture Notes - Very-Long-Baseline Interferometry, Mid-Ocean Ridge, Oceanic Trench


Department
Environmental Science
Course Code
EESA06H3
Professor
Nick Eyles

Page:
of 14
CHAPTER 2: PLATE TECTONICS
What Is Plate Tectonics?
* Tectonics study of the origin and arrangement of the broad structural features of the earth’s surface
(folds, faults, mountain belts, continent & earthquake belts)
* Basic idea of plate tectonics the earth’s surface is divided into a few large, thick plates that move slowly
and change in size
* Intense geological activity occurs @ the plate boundaries where plates move away, past or towards one
another 8 large plates and a few dozen smaller ones make up the outer shell of the earth ( crust & upper
part of mantle)
* Concept of plate tectonics was developed in the late 1960’s by combining 2 pre-existing ideas:
o Continental drift continents move freely over the earths surface, changing their positions
relative to one another
o Sea-floor spreading hypothesis that the sea floor forms at the crest of mid-ocean ridges, then
moves horizontally away from the ridge crest toward an oceanic trench
The 2 sides of the ridge are moving in opposite directions like slow conveyor belts
How Did The Plate Tectonics Theory Evolve?
Early case for continental drift
o Wegener noted that South America, Africa, India, Antarctica & Australia has almost identical late
Palaeozoic rocks and fossils
Reassembled the continents to form a giant supercontinent = Pangaea
If continents are arranged according to Wegener’s Pangaea reconstruction: glaciations in
the southern hemisphere is confined to small area and the absence of widespread
glaciations in the northern hemisphere becomes easier to explain
Pangaea Laurasia (Northern supercontinent) & Gondwanaland (Southern
supercontinent)
Laurasia (Northern supercontinent)
o North America & Eurasia
Gondwanaland (Southern supercontinent)
o Southern Hemisphere continents & India
Reconstructed old climate zones (Paleoclimatology) and the ancient sedimentary rocks
Discovered that paleoclimatic reconstructions suggested polar positions very
different to those at present evident for changes in position of the poles overtime
Scepticism about continental drift
o Wegener presented the best possible case in the early 1900’s for continental drift but evidence
provided was not clear cut and he didn’t have a good mechanism to account for continental
movement
Proposed that continents ploughed through the oceanic crust, perhaps crumpling up
mountain ranges on the leading edges of the continents where they pushed against the sea
floor
Geologists thought this violated what was known about the strength of rocks at the
time
Driving mechanism proposed by Wegener combination of :Centrifugal force from the
earth’s rotation & Gravitational forces that cause tides
Too small to move continents
Geologists in the southern hemisphere where Wegener’s matches of fossils and rocks b/w
continents were more evident were impressed while geologists in the northern
hemisphere were not
Renewed interest in continental drift
o Work in the 1940s and 1950s set the stage from the revival of the idea of continental drift w/ new
investigations in the areas: study of the sea floor & geophysical research (especially in relation to
rock magnetism)
o
Study of the sea floor
Oceans cover more than 70% of the earth’s surface difficult to study
Samples of rock and sediments can be taken from the sea floor in several ways:
Rocks can be broken from the sea floor by a Rock dredge open steel container
dragged over the ocean bottom @ the end of a cable
Sediments can be sampled with a Corer weighted steel pipe dropped vertically
into the mud and sand of the ocean floor
Sea floor drilling (both rocks and sediments) revolutionized field of marine
biology
o offshore oil platforms drill holes in the relatively shallow sea floors near
shore
o a ship with a drilling derrick on its deck can drill a hole in the deep sea floor
far from land the drill cuts long, rod-like rock cores from the ocean floor
Submersibles small research submarines which take geologists to many parts of
the sea floor to observe, photograph & sample rock and sediment
Single-beam echo sounder basic tool for indirectly studying the sea floor which measures
water depth and draws profiles of submarine topography
A sound signal send downward from a ship bounced off the sea floor and returns to
the ship determining water depth from the time it takes the sound to make the
round trip
Multibeam sonar uses a variety of sound sources to produce detailed shaded
relief images of the sea-floor topography
Sidescan sonar measures the intensity of sound reflected back to the tow vehicle
from the ocean floor and provides detailed images of the sea floor and information
about sediments and bedforms
Seismic reflection profiler works on essentially the same principles as the echo
sounders but uses a louder noise @ lower frequency
o Sound penetrates the sea floor and reflect from layers w/in the underlying
sediment and rock records water depth and reveals the internal structure
of the rocks and sediments of the sea floor (i.e. bedding planes, folds and
faults, unconformities)
Magnetic, gravity & seismic refraction surveys can also be made at sea & Deep sea cameras
can be lowered to the bottom to photograph the rock and sediment
Geophysical research
o Convincing new evidence about polar wandering came from the study of rock magnetism
Wegener’s world dealt with the wandering of earth’s geographic poles of rotation
Magnetic poles = close to geographic poles
The position of magnetic poles moves from year to year but the magnetic poles stay
close to the geographic poles as they move
Many rocks record the strength and direction of the earth’s magnetic field at the time the
rocks formed
Magnetite in a cooling basaltic lava flow acts like a tiny compass needle, preserving
a record of earth’s magnetic field when the lave cools below the curie point
Sedimentary rocks contain iron oxides and can also record the earth’s magnetism
Magnetism of old rocks can be measured to determine the direction and strength of
the magnetic filed in the past paleomagnetism
b/c magnetic lines of force are inclined more steeply as the north magnetic pole is
approached the inclination of the magnetic alignment preserved in the magnetite
minerals in the lava flows can be used to determine the paleolatitude as which the
flow formed
old rocks reveal very different magnetic pole positions to those at present it was once
thought this was due to movement of the poles (polar wandering)
Now known that it is due to the movement of the tectonic plates
Polar wandering paths now used to reconstruct continental movement over time
Every continent shows a different position for the Permian pole a single stood
still while continents split part and rotated as they moved
Wandering paths for north America and Europe = similar shapes but Europe path is to the
east of the north American path continents were once joined
Recent evidence for continental drift
o Paleomagnetic evident revived interest in continental drift
o By defining the edge of a continent as the middle of the continental slope rather than the constantly
changing shoreline, a better fit has been found b/w the continents
o Most convincing evidence greatly refined rock matches between now-separated continents
o Many of the boulders in south American glacial deposits have been traced to a source that is now in
Africa
o Now: there are an abundance of satellite geodetic data from the GPS that allow us to watch the
continents in real time
History of continental positions
o Rock matches show when continents were together after the split of continents, the new rocks
formed are no longer similar
o Paleomagnetic evidence indicates the direction and rate of drift allowing maps of old continental
positions to be drawn
What Is Sea-Floor Spreading?
Hess in 1962 - proposed that the sea floor moves away from the mid-oceanic ridge as a result of mantle
convection