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Environmental Science
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Nick Eyles

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C HAPTER 2: P LATE T ECTONICS 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 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 theseafloor  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  Seafloor 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 curiepoint  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 o 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 thebenioff 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 <200 million yrs old → they formed during the Mesozoic and Cenozoic eras  The earth = 4.55 billion yrs old →deep sea floor covering more than half of the earth’s th surface preserves less than 1/20 of its rock and sediment  Young age of the sea floor rocks → explained by Hess’ sea-floor spreading  New, young sea-floor is continuously being formed by basalt eruptions at the ridge crest → basalt is carried sideways by convection and is subducted into the mantle at an oceanic trench o Old sea floor is being destroyed at trenches & new sea floor is being formed at the ridge crest  Young sea floor has little sediment b/c basalt is newly formed  Old sea floor farther from the ridge crest has been moving under a constant rain of pelagic sediment – building up a thicker layer as it moves along o Youngest sea floor = at the ridge crest o Older sea floor = at the trenches Measuring Plate Movement In Real Time  GPS data can be used to measure plate movement direction & on a global scale  1970s → space geodesy – a space based technique measuring points on the earth’s surface  Most common space-geodetic techniques: o Very long baseline interferometry (VLBI)→ uses pairs of radio telescopes pointed toward a common quasar o Satellitelaser ranging (SLR) o Global positioning system (GPS) → most useful of the 3 techniques for studying movement of the earth’s crust and measure relative motion b/w plates  Plate motions are recorded on a yearly basis around the world → rates and directions of plate movement measured by a GPS are close to the those calculated on the basis of geological data  Measurement of relative plate movement provides valuable data regarding potential earthquake activity alongactive plate margins  GPS data is currently being used to monitor interactions b/w the pacific plate and surrounding continental plates to learn about the earthquake and volcanic activity in the pacific ring of fire What Are Plates And How Do They Move?  Plate → large, mobile slab of rock that is part of earth’s surface o Surface may be made up entirely of sea floor (Nazca plate) or of both continental and oceanic rock (NA plate) o Most small plates are entirely continental while large plates contain some sea floor o Plates = part of lithosphere which consists of rocks of the crust and the uppermost mantle  Age of Lithosphere → ↑es with both age and thickness w/ distance from the crest of the mid- oceanic ridge  Oldest → far from crest of the mid-oceanic ridge (100km thick)  Youngest → close to mid-oceanic ridge (10km thick)  Continental lithosphere = thicker o 125km thick to as much as 200-250km thick beneath the oldest, coldest and most inactive parts of the continent  Interior of a plate = inactive tectonically  Plates move away from the mid-oceanic ridge crest  If plate is made up of mostly sea floor (Nazca and pacific plates) → plate can be subducted down into the mantle forming an oceanic trench and its associated features  If the leading edge of the plate is made up of continental rock (SA plate) it will not subduct because it is less dense than oceanic rock and too light  Athenosphere→ zone of low-seismic wave velocity hat behaves plastically b/c of ↑ed temp and pressure below the rigid lithosphere o Plastic Athenosphere acts as a lubricating layer under the lithosphere allowing plates to move o Made up of upper-mantle rocks o Below the Athenosphere = more rigid mantle rock  Plate boundaries o Divergent plate boundary → boundary b/w plates that are moving apart o Convergent plateboundary → lies b/w plates that are moving toward each other o Transformplateboundary → one at which 2 plates move horizontally past each other How Do We Know That Plates Move?  Magnetic anomalies and seismicity of fracture zones convinced most geologists that plates do indeed move  Paleomagnetic Evidence o Earth’s magnetic field has periodically reversed its polarity in the past → north magnetic pole and south magnetic pole exchange positions → Magnetic Reversals  Normal polarity → magnetic lines of force flow from the south pole to the north pole and compass needles point to the north  Cooling dikes are normally magnetized  Reversedpolarity → lines of magnetic force run flow from the north pole to the south pole and compass needles point south  Dikes that cool when the field is reversed are reversely magnetized o Paleomagnetism → study of ancient rocks can tell us about changes in the earth’s magnetic fields in the past o Lava flows contain abundant magnetic minerals and can be isotopically dated → stacked continental lava flows have been used to construct a magneticpolarity timescale which measures the pattern of magnetic reversals over time o Reverses ever 500 000yrs and takes 10 000 yrs for a magnetic reversal to develop o Strength of the earth’s magnetic field also changes over time  Anomaly → deviation of magnetic strength from the average  Magnetometer → instrument that measures the strength of the earth’s magnetic field  MarineMagnetic Anomalies o Most magnetic anomalies on these floor are arranged in bands that lie parallel to the rift valley of the mid-oceanic ridge o Alternating (+) and (-) anomalies form a stripe like pattern parallel to the ridge crest o Morley-Vine-Matthews Hypothesis  Pattern of magnetic anomalies was symmetrical about the ridge crest → pattern on 1 side of the ridge was a mirror image of the pattern on the other side  Same pattern exists over different parts of the mid-oceanic ridge  Pattern of magnetic anomalies at sea matches the pattern of magnetic reversals already known from studies of lava flows on the continents  Proposed explanation of magnetic anomalies → Dikes  There is a continual opening of tensional cracks w/in the rift valley on the mid- oceanic ridge crest which are filled by basaltic magma → cools to form dikes o Cooling magma in the dikes records the earth’s magnetism at the time the magnetic minerals crystallize o dikes preserve a record of the polarity that prevailed during the time the magma cools → extension produced by the moving se floor then cracks a dike in 2 and the 2 ½’s move in opposite directions down the flanks of the ridge o new magma intrudes the newly opened fracture which cools, is magnetized and forms a new dike (repeat cycle) How Fast Do Plates Move?  Morley-Vine-Matthews Hypothesis o Allows us to measure:  Rate of sea-floor motion (plate motion)  Since magnetic reversals have already been dated from lava flows on land. The anomalies caused by these reversals are also dated and can be used to discover how fast the se-floor has moved  Sea floor age  By matching the measured anomaly pattern with the known pattern of anomalies, the age of the sea floor (& therefore rocks) in the region can be predicted  Fracture zones & Transform Faults o Mid-ocean ridges are offset along fracture zones  They were once cont
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