CHAPTER 2: PLATE TECTONICS
CHAPTER 2: PLATE TECTONICS What Is Plate Tectonics? * * *
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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 earth’s 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
Contrasted with Wegener who thought that the ocean floor remained stationary as the continents ploughed through it Initial concept → sea floor is moving like a conveyor belt away from the crest of the mid-oceanic ridge, down the flanks of the ridge, and across the deep-ocean basin, finally disappearing by plunging beneath a continent or island arc Spreading axis/center → the ridge crest, with sea floor moving away from it on either side Subduction → sliding of the sea floor beneath a continent or island arc Convection → a circulation pattern driven by the rising of hot material and/or the sinking of cold material o Hot material – lower density → rises o Cold material – higher density → sinks o Slow convection circulation of a few cm/year is set up by temperature differences w/in the mantle & convection can explain many sea-floor features as well as the young age of the sea-floor rocks If convection drives sea-floor spreading → hot mantle rock must be rising under mid-oceanic ridges Hess → showed how the existence of ridges and their high heat flow are caused by the rise of this hot mantle rock o Basalt eruptions on ridge crests are also related to this rising rock b/s the mantle rock is hotter than normal & begins to undergo partial melting o As hot rock continues to rise beneath ridge crests, the circulation pattern splits and diverges near the surface → mantle rock moves horizontally (carrying the sea floor w/ it) away from ridge crests on each side of the ridge (becoming cooler and denser, sinking deeper beneath the ocean surface), causing tension at the ridge crest and cracking open the oceanic crust to form rift valleys and associated shallow-focus earthquakes Downward plunge of cold rock accounts for the existence of the oceanic trenches & their low heat flow values Also explains the large negative gravity anomalies associated with trenches →sinking of the cold rock provides a force that holds trenches out of isostatic equilibrium o As the sea-floor moves downward into the mantle along a subduction zone, it interacts with the stationary rock above it → causing the benioff zones of earthquakes associates with trenches How old is the sea floor? o Determined through isotopic dating and sediments (fossils) Discovered that all the rocks and sediments from the deep sea floor proved to be
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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 earth’s 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
Contrasted with Wegener who thought that the ocean floor remained stationary as the continents ploughed through it Initial concept → sea floor is moving like a conveyor belt away from the crest of the mid-oceanic ridge, down the flanks of the ridge, and across the deep-ocean basin, finally disappearing by plunging beneath a continent or island arc Spreading axis/center → the ridge crest, with sea floor moving away from it on either side Subduction → sliding of the sea floor beneath a continent or island arc Convection → a circulation pattern driven by the rising of hot material and/or the sinking of cold material o Hot material – lower density → rises o Cold material – higher density → sinks o Slow convection circulation of a few cm/year is set up by temperature differences w/in the mantle & convection can explain many sea-floor features as well as the young age of the sea-floor rocks If convection drives sea-floor spreading → hot mantle rock must be rising under mid-oceanic ridges Hess → showed how the existence of ridges and their high heat flow are caused by the rise of this hot mantle rock o Basalt eruptions on ridge crests are also related to this rising rock b/s the mantle rock is hotter than normal & begins to undergo partial melting o As hot rock continues to rise beneath ridge crests, the circulation pattern splits and diverges near the surface → mantle rock moves horizontally (carrying the sea floor w/ it) away from ridge crests on each side of the ridge (becoming cooler and denser, sinking deeper beneath the ocean surface), causing tension at the ridge crest and cracking open the oceanic crust to form rift valleys and associated shallow-focus earthquakes Downward plunge of cold rock accounts for the existence of the oceanic trenches & their low heat flow values Also explains the large negative gravity anomalies associated with trenches →sinking of the cold rock provides a force that holds trenches out of isostatic equilibrium o As the sea-floor moves downward into the mantle along a subduction zone, it interacts with the stationary rock above it → causing the benioff zones of earthquakes associates with trenches How old is the sea floor? o Determined through isotopic dating and sediments (fossils) Discovered that all the rocks and sediments from the deep sea floor proved to be
