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Lesson 3. Plate Tectonics Overview •  Prior to the 1970s, there was no unifying geologic concept that explained the distribuBon and mechanisms of geologic processes such as earthquakes and volcanoes. •  Local condiBons were believed responsible for the processes that produced these features. •  The examinaBon of the ocean floors in the decades aGer World War II changed all that with the development of the theory of plate tectonics. •  Knowledge of the ocean floors was crucial because they cover near 70 % of the Earth's surface and they contain features not evident on the conBnents. •  In addiBon, the seafloors are, in fact, much younger than the conBnents with simpler geology. Maps of the ocean floor reveal topography as complex as that characterisBc of the conBnents. Overview •  AGer World War II, there was a veritable explosion in our knowledge of the ocean floor. •  Included among the seafloor characterisBcs that must be explain by any general model of seafloor formaBon are: –  existence of the topographic features: mid-­‐ocean ridges, oceanic trenches and transform/fracture zones –  magneBc striping of the seafloor –  distribuBon of sediment thickness and oceanic crust age –  high heat flow of ridges and low heat flow of oceanic trenches –  difference between acBve and passive conBnental margins Age of ocean floor. Overview •  Geophysical observaBons at the Earth's surface provide another set of data that must be explained by any general geologic model for the Earth. These include: –  concentraBon of earthquakes in linear belts –  associaBon of these earthquakes with mid-­‐ocean ridges, transform/fracture zones and oceanic trenches –  variaBon in earthquake depth with different features –  associaBon of volcanism with ridges and trenches in linear belts Overview •  Geological invesBgaBons on the conBnents have been made for hundreds of years. These provide a different perspecBve from seafloor and geophysical observaBons. The main observaBons from conBnental geologic studies include: –  conBnental rocks are very old –  they contain complex and more varied rocks than the ocean basins –  conBnental crust is thicker than oceanic crust –  conBnental rocks are less dense –  conBnents are composed of many exoBc parts that have traveled long distances across the surface of the Earth Overview •  Plate tectonics explains the formaBon of the Earth's two major landforms: conBnents and ocean basins. •  It is accepted by most geologists because it explains so many observaBons about the Earth's surface and its processes. •  Basic tenets of plate tectonics include: –  Earth's outer surface is comprised of lithospheric plates that are approximately 100 km thick –  these plates move relaBve to each other –  there are three types of boundaries that separate plates: convergent, divergent and transform –  the plates are formed at divergent boundaries and destroyed at convergent margins –  along transform boundaries, plates are neither destroyed nor created –  most tectonic acBvity (volcanism, earthquakes, faulBng, mountain building, etc.) is concentrated at plate boundaries –  the centers of plates are geologically stable with liZle tectonic acBvity Overview •  Just like with Wegner's model of conBnental driG, the mechanism that drives plate tectonics is unknown. •  However, unlike before the evidence that plates move is so strong that most scienBsts accept the model even though there is no universally accepted driving mechanism. •  Proposed mechanisms for driving plate moBon include: –  mantle convecBon: layered or whole –  slab pull –  ridge push Overview •  The major features of plate tectonic theory can be summarized as follows: –  Earth surface covered by thick (100 km) rigid plates two mechanical layers: lithosphere (the plates) and asthenosphere –  three types of boundaries: •  convergent: plates move together, destrucBve, volcanoes, earthquakes •  divergent: plates move apart, construcBve, volcanoes, earthquakes •  transform: plates slide past each other, conservaBve, earthquakes –  driving mechanisms: mantle convecBon, slab pull, ridge push •  none work completely well –  explains Earth's two major landforms: ocean basins & conBnents Types of Plate Margins •  There are three types of plate margins each characterized by different types of geologic processes and landforms: –  divergent margins -­‐ earthquakes, volcanoes convergent margins -­‐ earthquakes, volcanoes transform boundaries -­‐ earthquakes •  Because they are subject to different types of geologic processes, each margin type is characterized by different types of geologic hazards as well as Earth resources. Types of Plate Margins •  Divergent boundaries are where new oceanic crust is created. –  also known as ridges, rises and spreading centers –  known as divergent margins because two plates move away from each other or diverge –  new oceanic crust is created below the axial riG valley when new material intruded/erupted from below –  rate of spreading varies from 2 to 15 cm/yr –  this process of creaBon of oceanic crust is seafloor spreading Divergent margins form a disBncBve bathymetric feature that encircles the globe. Types of Plate Margins •  Convergent boundaries are where two lithospheric plates move toward each other. •  Convergent boundaries (also known subducBon zones) are characterized by: –  one plate being subducted or thrust under another –  three different types of subducBon zones: ocean-­‐
ocean, ocean-­‐conBnent, conBnent-­‐conBnent The conBnent-­‐conBnent convergent zone between Asia and India has produced the Himalayans. Types of Plate Margins •  Transform margins are regions where two lithospheric plates slide past each other as along a strike-­‐slip fault. •  At these margins, lithospheric crust is neither created nor destroyed. •  Because two lithospheric plates move past each other in opposite direcBons along transform faults, they are regions of earthquakes. •  However, they are confined to shallow depths because the lithospheric plates are only 10 to 50 km thick. The plates aren't 100 km thick yet because they are sBll cooling and have not reached their full thickness. Block diagram illustraBng a transform margin connecBng two ridges. Types of Plate Margins •  Plate moBon on a sphere defines three types of plate boundaries in terms of how the plates move relaBve to each other. •  Each boundary is characterized by different types of processes, geologic hazards and Earth resources. The types of boundaries are: –  divergent margins: two plates move apart • 
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oceanic crust created spreading rates are 2-­‐15 cm/yr earthquakes, volcanoes not big source of hazards because mostly submarine –  convergent margins: lithospheric plates move toward each other •  crust is consumed or destroyed •  three types: ocean-­‐ocean, ocean-­‐conBnent, conBnent-­‐conBnent •  earthquakes, volcanoes –  transform boundaries: plates slide past each other •  crust is conserved, i.e. neither created nor destroyed •  earthquakes, but not volcanoes Divergent Margins •  Divergent boundaries are where new oceanic crust is created. They are called divergent because the two plates move away or diverge from each other. •  The characterisBcs of diverge margins include: –  also known as ridges, rises and spreading centers –  known as divergent margins because two plates move away from each other or diverge –  new oceanic crust is created below the axial riG valley when new material intruded/erupted from below –  rate of spreading varies from 2 to 15 cm/yr –  this process of creaBon of oceanic crust is seafloor spreading Divergent Margins •  The mid-­‐ocean ridge system is a large, linear, submarine mountain chain. •  It is the largest topographic and geologic feature on Earth with the following characterisBcs: –  extends for 80,000 km around the world –  found in all major ocean basins –  it rises 2-­‐3 km above the surrounding seafloor and is 1,000-­‐2,500 km wide –  the ridge is comprised almost exclusively of volcanic rock –  many secBons of the ridge are marked by a riG valley -­‐ 1-­‐2 km deep and several kilometers wide (same size as Grand Canyon) –  riG zone is present along the ridges in Indian and AtlanBc Oceans but missing from the ridge in the Pacific –  basalt is erupted along crest, which also registers high heat flow –  the ridge is the site of shallow (0-­‐20 km) earthquake acBvity –  site of hot springs and the mineral deposits derived from them Divergent Margins •  The mid-­‐ocean ridge system is the locaBon of the vast majority of the Earth's volcanic acBvity. •  Obviously, it poses liZle hazard to humans because it is mostly submarine. •  The highly volcanic nature of the mid-­‐ocean ridge system is demonstrated by the presence of hydrothermal vents or black smokers. •  The mid-­‐ocean ridge system comes to the surface in only one place in the world, i.e. Iceland. Much of Iceland is volcanic. However, the nature of the lavas means that the risk of loss of life to volcanic acBvity is remote. Black Smokers Divergent Margins •  The mid-­‐ocean ridge system is closely associated with a very narrow band of earthquakes (Earthquake DistribuBon Movie). •  In fact, the locaBon of earthquake epicenters is useful for outlining the edges of plate margins. •  CharacterisBcally, these earthquakes are very shallow. •  The vast majority of these earthquakes are not very strong and oGen detectable only by instruments. •  These weak quakes are very abundant, i.e. millions per year (East Pacific Rise Movie). •  Along the mid-­‐ocean ridge system, magma is intruded beneath the ridge axis. •  In this area, the athenosphere, a region of very hot mantle rock, comes close to the Earth's surface. •  Because of the high temperature at shallow depths, rocks beneath the ridge can fracture only at very shallow depth where the rocks are very cold. Divergent Margins •  An important step in the formaBon of the theory of plate tectonics was the recogniBon that there is a systemaBc relaBonship between the age of the ocean floor and its distance from mid-­‐ocean ridges. –  isochrons are lines that connect points on the oceanic crust that have the same age –  they get older with distance from the mid-­‐ocean ridge crest –  isochrons are parallel to the ridge crest Divergent Margins •  Humans have used the Earth's magneBc field to navigate from hundreds of years. AGer technological developments made in World War II, the ability to study and understand this magneBc field increased greatly. •  Earth's magneBc field has changed direcBons throughout geologic Bme. –  normal polarity is when magneBc north points toward geographic north pole –  reverse polarity is the condiBon where magneBc north points toward the geographic south pole •  Minerals that contain iron acquire a permanent magneBc field similar to the Earth's magneBc field when they cool through their Curie Point. Divergent Margins •  The discovery of systemaBcally distributed magneBc anomalies on the ocean floor lead to convincing arguments for sea-­‐floor spreading and ulBmately the concept of plate tectonics. •  MagneBc anomalies are created when new oceanic crust forms at the mid-­‐ocean ridge. New rocks acquire the magneBc field operaBng at the top they formed. –  posiBve anomalies forms over oceanic crust with normal polarity –  negaBve anomalies are characterisBc of crust that is reversely polarized –  anomalies are arranged parallel mid-­‐ocean ridge –  they are symmetrically arranged, i.e. the two sides are mirror images of each other, the ridge •  The width of an individual magneBc anomaly is a funcBon of the duraBon of the magneBc period and the rate of seafloor spreading. –  if magneBc period is constant, faster spreading creates wider stripes –  for a fixed spreading rate, shorter magneBc periods produce thinner anomalies than longer periods The posiBve and negaBve anomalies of the oceanic crust produce a magne:c striping of the ocean floor. These stripes run parallel to the ridge crest and form crude mirror images on opposite sides of the ridge. The discovery of these stripes was an important clue to the understanding seafloor spreading and plate tectonics. Divergent Margins •  Obviously, the volcanic and plutonic rocks formed by magma erupBon and emplacement along the axis of the mid-­‐ocean ridge is hot. •  The high temperature means the rocks are less dense and more buoyant. •  As the crust moves away from the ridge crest it is in contact with cold ocean water. •  Heat lost to the ocean leads to a cooler of the oceanic crust and an increase in its density. Thus, the ocean floor subsides as it moves away from the ridge. •  Because of this cooling and sinking, water depths of about 3 km along the ridge crest increase to an average of 6 km in the center of the ocean basins. •  The depth of the seafloor is accurately predicted by a cooling model that assumes heat is lost from the lithospheric plate by conducBon, i.e. movement of heat on an atomic scale through a solid. SchemaBc cross-­‐secBon through a mid-­‐ocean ridge showing the classic seafloor bathymetric profile produced by conducBve cooling of the aging lithosphere. Divergent Margins •  Divergent margins can also form under conBnents. When this happens, the divergent moBon of the two plates splits (ri;s) the conBnent apart thereby creaBng a large valley. •  As the valley lengths along the riG axis and deepens, it eventually intersects the conBnental coast line. •  At this point, the valley fills with ocean water creaBng a new ocean. •  ConBnued riGing creates an ever widening and deepening ocean basin. •  This process is what separated the Americas from Europe and Africa and created the AtlanBc Ocean. •  In regions of conBnental riGing, earthquakes and volcanism are potenBal geologic hazards. Divergent Margins •  Oceanic crust is created at divergent margins where two plates move away from each other. •  Divergent margins are characterized by mid-­‐ocean ridges that are the largest topographic features on the planet. •  Some of the important features of divergent margins include: –  a ridge system 80,000 km long, 1-­‐2 km high and 1,000-­‐2,000 km wide made almost exclusively of volcanic rock; –  shallow-­‐level (0-­‐20 km) earthquakes occur beneath the ridge crest –  the ridge is the site of hydrothermal hot springs that deposit a variety of metallic elements -­‐ these are future mineral deposits –  oceanic crust gets progressively older with distance from the ridge crest •  lines of equal age crust, i.e. isochrons, parallel the ridge crest on both sides –  alternaBng posiBve and negaBve magneBc anomalies running parallel the ridge crest produce magneBc striping of the ocean floor –  as the crust moves away from the ridge it cools thereby becoming denser and sinking. This conducBve cooling produces a characterisBc increase in ocean depth from the ridge crest (3 km) to the deep ocean basins (6 km) •  Divergent margins under conBnents cause the conBnent to riG and ulBmately create a new ocean basin. Convergent Margins •  Convergent boundaries are where two lithospheric plates move toward each other. •  CharacterisBcs of convergent boundaries (also known subducBon zones) include: –  one plate is subducted or thrust under another –  destrucBon of oceanic crust –  occurrence of earthquakes and possibly volcanoes depending upon convergent margin type –  three different types: ocean-­‐ocean, ocean-­‐conBnent, conBnent-­‐conBnent Convergent boundaries (shown on the map above) are where crust is destroyed. Convergent Margins •  An ocean-­‐ocean convergent boundary occurs when two oceanic lithospheric plates collide. •  Since the two plates are of equal density, one is subducted or thrust under the other. •  An ocean-­‐ocean convergent zone is characterized by: –  oceanic trench –  forearc region with an accreBonary wedge –  an arc-­‐trench gap that separates the oceanic trench from the volcanic arc –  volcanic arc (the island arc) –  back-­‐arc basin –  earthquake focii vary from shallow to deep A schemaBc cross-­‐secBon through an oceanic subducBon zone. Convergent Margins •  The island arc is typically 100+10 km above the subducBng plate. •  the arc-­‐trench gap varies with the angle of the subducBng plate. •  The subducBng plate is defined by a plane of steeply dipping earthquake focii. This is the Benioff-­‐Wadi zone. Differences in Bennioff-­‐WadaB zone in the western Pacific. Convergent Margins •  An ocean-­‐con:nent convergent boundary occurs when oceanic lithospheric and conBnental plates collide. •  Since the oceanic plates is always more dense than the conBnental plate, it is subducted beneath the conBnental plate. •  An ocean-­‐conBnent convergent zone is characterized by: –  an oceanic trench along the conBnent margin –  a forearc region with an accreBonary wedge –  an arc-­‐trench gap that separates the oceanic trench from the volcanic arc –  a volcanic arc on the conBnent (forms mountain chain on conBnental margin) –  thin, linear belts of earthquake acBvity Convergent Margins •  A con:nent-­‐con:nent convergent boundary occurs when two conBnental lithospheric collide. •  Since the two plates are of equal density and both less dense than the underlying mantle neither can be subducted. •  Thus, they pile up in a mountain range. •  A conBnent-­‐conBnent convergent zone is characterized by: –  very thick crust (~70 km) -­‐ thicker than normal crust –  earthquakes spread out over a very wide area –  neither plate is subducted -­‐ no crust destroyed –  absence of volcanoes and volcanic acBvity Convergent Margins •  Oceanic trenches are long, narrow features that are much deeper (2,000 -­‐ 5,000 m) than the normal seafloor. •  Trenches generally parallel a line of volcanic islands or a conBnental margin. –  on landward side of trench, the seafloor slopes into the trench at angles as steep as 25o –  marked by low heat flow and negaBve gravity anomalies, i.e. the gravity is lower than the Earth's average gravity –  characterized by intermediate and deep earthquakes on the same side of the trenches as the volcanoes –  this acBvity defines a plane dipping steeply beneath the conBnents or volcanic islands Convergent Margins •  Plate boundaries at which the two lithospheric plates move apart are convergent margins or boundaries. Important convergent margin characterisBcs include: –  one plate is subducted or thrust under another –  destrucBon of oceanic crust –  occurrence of earthquakes and possibly volcanoes depending upon convergent margin type –  three different types: ocean-­‐ocean, ocean-­‐conBnent, conBnent-­‐conBnent Convergent Margins •  Because of physical differences in the plates interacBng at different convergent margin types, the geologic structure of each type of margin is different. The important characterisBcs of each convergent margin type include: –  ocean-­‐ocean; moving from the subducBng to overriding plate the margin consists of an oceanic trench, forearc with accreBonary wedge, arc-­‐trench gap, volcanic island arc and possibly a back arc basin; marked by linear belts of earthquake epicenters of variable depth –  ocean-­‐conBnent: oceanic trench paralleling the conBnental margin, forearc with accreBonary wedge, arc-­‐trench gap, volcanic mountain range along the conBnental margin –  conBnent-­‐conBnent: since both plates are of light crustal material, neither is subducted, crust squeezed into thick pile 9~70 km), earthquakes of variable depth scaZered across a broad region, no volcanoes or volcanic acBvity, because crust not destroyed •  For the ocean-­‐ocean and ocean-­‐conBnent margins, the volcanoes sit 100+10 km above the Benioff-­‐Wadi zone. Thus, shallower dipping subducBon zones have wider arc-­‐trench gaps than steeply dipping ones. Transform Margins •  Transform boundaries are margins where crust is neither destroyed nor created. Plates simply slide past each other. –  produces shallow-­‐level earthquakes –  since crust is neither destroyed or created there is no volcanic acBvity •  Of the three types of plate margins, transform boundaries are the least common. They make up only a small percentage of known margins. •  They were first described by Canadian geologist J. Tuzo Wilson and mark an important point in the development of the concept of plate tectonics. Transform Margins •  A curious observaBon of the mid-­‐ocean ridge system and the transform faults along it is the distribuBon of earthquake epicenters. •  The earthquakes occur along the ridge crest and along the transform fault segment connecBon ridge crests. •  There are no earthquakes on the segments outside of the ridge crests •  This distribuBon is a result of the plate moBons along the ridge system. –  between crest two plates moving in opposite direcBons -­‐ fricBon produces quakes –  outside crests plates moving in same direcBon and at same rate -­‐ no fricBon to produce earthquakes –  the offset of the magneBc stripes was originally produced at the ridges and don't get bigger with Bme Transform Margins •  There are three types of arrangements for transform boundaries. These are: –  ridge -­‐ ridge (most common) –  ridge -­‐ trench –  trench -­‐ trench Transform Margins •  Ridge-­‐ridge connecBon: –  A close examinaBon of the mid-­‐ocean ridge system indicates it is not a simple conBnuous system but is offset periodically by long linear structures –  These features: • 
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are oriented at right angles to the ridge divide the ridge into segments 200-­‐800 km long extend for thousands of kilometers across the ocean floor displace magneBc anomalies and topographic features up to 1000 km form imposing cliffs that may be 1-­‐2 km high difference in age of the oceanic crust across the boundary –  Although buried by sediment near the conBnents, there is evidence these features extend right up to and possibly onto the conBnents. Transform Faults •  Ridge-­‐trench connec:on –  In this arrangement, the transform connects a ridge with a subducBon zone. Examples of this type of boundary include: •  San Andreas Fault •  Queen CharloZe Fault •  fault through Mediterranean region •  South AtlanBc Transform Margins •  Trench-­‐trench connec:on –  This type of boundary corresponds to the: •  in Asia •  Caribbean •  South AtlanBc Transform Margins •  Plate margins where crust is conserved are transform margins. •  Along these boundaries plates simply slide past each other. •  Transform margins are marked by shallow earthquakes, but typically lack volcanism. •  They connect different types of plate margins. The possible connecBons are: –  ridge -­‐ ridge (most common) –  ridge -­‐ trench –  trench -­‐ trench Studying the Seafloor •  Over 70 % of the Earth's surface is covered by water. •  Four major oceans: Pacific, AtlanBc, Indian, ArcBc •  Pacific is largest with 53 % of water on Earth •  Extensive ocean has existed on the planet for at least the last 3.5 billion years Studying the Seafloor •  Oceanographic Research –  scienBfic invesBgaBons of oceans did not begin unBl the 1700s –  first carried out by Great Britain –  Captain James Cook's voyages in 1768, 1772 and 1777 were mostly exploring and charBng –  first true oceanographic cruise was that of the HMS Challenger, 1872-­‐1876 –  most oceanographic work has been since World War II Studying the Seafloor •  Problems –  large area -­‐ the deep parts (> 2,000 m) of the oceans cover 3,120,000 km2 (8 Bmes surface of Mars) –  Great depth (averages 3,500 m) –  High pressure at the boZom –  no light -­‐ can't use direct observaBon –  because of these problems mapping and charBng by the method used on land -­‐ direct observaBon -­‐ is impossible Studying the Seafloor •  Techniques for studying the ocean floor –  Imaging Sounding by lead line -­‐ bathymetry (topography) Echo Sounder -­‐ bathymetry (topography) Seismic profiler -­‐ bathymetry and sediment thickness and structure Sea Beam -­‐ images the bathymetry of a swath of the ocean floor Sidescan Radar -­‐ determines the bathymetry of large porBons of the seafloor Satellite radar alBmeter -­‐ measures sea surface elevaBon, which in turn reflects bathymetry •  Heat flow -­‐ measures amount of heat flowing from the Earth's surface •  Magnetometer -­‐ measures the strength and polarity of the local magneBc field •  Submersibles and ROVs -­‐ take photographs and videos of very small porBons of the sea boZom • 
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–  Sampling • 
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Dredging -­‐ samples of sediment and ocean crust Gravity coring -­‐ sediment samples Drilling -­‐ samples of sediment and ocean floor Submersibles and ROVs -­‐ sediment, volcanic rocks, oceanic crust, biologic material Studying the Seafloor • 
Sounding by lead line –  for centuries the only method of determining ocean depth was "heaving the lead” –  1521: Ferdinand Magellan aZempt sounding -­‐ lowered line to 750 m ( G) but didn't touch boZom –  1840: James Clark Ross used this procedure to determine the depth of South AtlanBc as 4,365 m. OperaBon took four hours. –  1854: John Mercer Brooke devised a sounding device that provided posiBve evidence the boZom had been reached -­‐ cannonball with tube through it and twice diameter of cannonball. When hit boZom, tube driven into boZom and forced out of cannonball. Tube was pulled up. Became known as Brooke sounding device. –  mid-­‐1800s: MaZhew Fontaine Maury, superintendent U.S. Navy Depot of Charts and Instruments, started first systemaBc sounding program, produced first contour map of AtlanBc in 1854 (less than 200 measurements and none deeper than 1,000 fathoms [1 fathom = 6 feet]), it is first map to suggest existence of mid-­‐ocean ridge. –  laying of transatlanBc cable in 1860s made knowledge of ocean boZom topography of pracBcal importance –  during the first important oceanographic cruise, the BriBsh ship HMS Challenger (1872-­‐1876) took soundings -­‐ one in Pacific was as deep as 10,000 m –  1904: General Bathymetric Chart of the Oceans was first chart published in 1904 and based on 18,400 soundings Studying the Seafloor •  Echo Sounder –  The major advance in studying the ocean floor topography was the introducBon of remote methods of surveying. This allowed much faster and more accurate mapping of the ocean floor. –  this was the first "remote” system used to image the seafloor -­‐ major advance in studying the ocean floor –  sends out a sound wave –  some of energy reflected off seafloor and reflected back to the ship –  depth is calculated based on Bme to make trip and velocity of sound in water (varies with water temperature) –  provides a conBnuous recording of the depth under the ship –  disadvantage is that it reveals nothing to either side of the ship –  sBll, the first detailed bathymetric charts of the seafloor were made using this technique Studying the Seafloor •  Seismic Profiler –  Big advance over echo sounder was the seismic profiler. Not only did it provide a picture of the seafloor topography but revealed some informaBon about the material making up the sea floor. –  similar to echo sounder but uses a much more powerful sound wave –  some of energy reflected off seafloor but some also penetrates the ocean floor and is reflected by sediment layers and top of the oceanic crust –  yields informaBon about ocean floor topography as well as its structure Studying the Seafloor •  Sea Beam –  Both the echo sounder and the seismic profiler look at a thin line of the ocean floor under the ship. This is a very slow way of mapping the ocean floor. A major advance in studying the ocean floor topography was the introducBon of remote methods of surveying. This allowed much faster and more accurate mapping of the ocean floor. First technique was Sea Beam. –  Sea Beam is an acousBc mapping system developed for the Australian navy –  it emits a sound wave from transducers mounted on the ship's hull –  receivers, also mounted on the hull, pick up the sound wave aGer it has been reflected from the seafloor –  the system images a swath of the ocean floor as wide as 0.8 of the water depth –  because of the relaBon between water depth and image swath width, Sea Beam is best used in deep water –  the system can survey 2,667 km2/day –  a bathymetry map of the area surveyed is created by overlapping adjacent swaths Studying the Seafloor •  Sidescan radar –  While a major advance in mapping the ocean floor, Sea Beam sBll imaged only a small swath of the sea boZom. Gloria or sidescan sonar images a swath up to 26 miles wide. Allows much faster mapping of the sea floor. –  sidescan radar is a technique that images the sea floor on either side of a ship not directly beneath it –  radar signal is emiZed from towed vehicle called a fish –  the reflected radar signal is received by the fish, which transmits the informaBon to the towing ship –  by varying the wavelength of the signal used these systems can image objects as large as seamounts many miles across or as small as sand ripples a few inches apart –  sidescan radar systems can image a wider swath of the seafloor than Sea Beam systems, but do not cover the region directly below the ship –  Sea Beam and sidescan radar systems are oGen used in conjuncBon with each other One of the most commonly used sidescan radar systems is GLORIA (Geological LOng Range Inclined Asdic). It can image a swath of the ocean floor up to 45 km wide, but can only idenBfy objects greater than 30-­‐152 m in maximum dimension. The GLORIA system has been used extensively to map the U.S. Exclusive Economic Zone (EEZ). This GLORIA fish is being serviced prior to launching and will be towed behind a ship at 18 km/hr. GLORIA 'fish' being launched for a mapping run. The fish is 8 m long and 0.6 m in diameter. Out of water, it weighs 2.25 tons. In three months, a GLORIA system can survey 750,775 km2 of the ocean floor. This system can be used any where the water is deeper than 198 m. Studying the Seafloor •  Satellite AlBmetry –  Another new method of studying the ocean floor is to map the sea surface using satellites. This system uses a satellite to map the topography of the ocean floor over large areas very quickly. –  satellite alBmeter sends a radar signal that is bounced off sea surface and returned to satellite –  allows measuring of distance between surface and satellite to within a few cenBmeters –  sea surface height is determined by changes in gravity caused by variaBons in boZom topography and oceanic crust density –  a submarine mountain has greater gravity so it pulls water toward it. Since water is incompressible, the increased water mass above the mountain produces a mound of higher water above the mountain. A submerged mountain may have as much as a 5 m bulge in sea level above it. –  in contrast, a deep oceanic trench has lower gravitaBonal aZracBon above and the water above it is pulled away from the trench –  the decreased mass of water produces a 'hole' in the sea surface -­‐ sea surface can be as much as 60 m lower than average above a trench –  differences in sea surface height is measured by the satellite and these surface differences are translated into boZom topography These maps show the results of a satellite alBmeter invesBgaBon. The diagram on the leG shows the elevaBon of the sea surface. On the right, this informaBon has been converted to seafloor topography. Studying the Seafloor •  MagneBc Surveys –  During World War II, the U.S. and BriBsh navies developed ways to find submarines by detecBng their magneBc signature. AGer the war, ships leG the instruments for measuring the magneBc field on and measured the Earth's field as they sailed. Later, this informaBon lead to a major scienBfic breakthrough. –  the strength of the magneBc field above the ocean floor is measured by towing a magnetometer behind a ship –  records the local field strength –  in some areas, the measured field strength is less than that of the Earth's average field -­‐ nega:ve magne:c anomalies –  field strength in other areas is greater than the current field -­‐ posi:ve magne:c anomalies –  along a profile of the ocean floor, this variaBon produces a series of troughs and crests (figure next page) –  when rocks from regions of negaBve magneBc anomalies are measured, they have a magneBc polarity opposite the Earth's current field –  the opposite is true of rocks from areas with posiBve magneBc anomalies Studying the Seafloor •  Sampling by dredging –  Samples of sediment and volcanic rocks can be recovered by dredging. –  This sampling method involves dragging a steel basket across the seafloor. –  Sediment, loose rocks as well as fragments broken from outcrops are recovered. –  Samples recovered in this manner can weight up to tens of kilograms. –  Because of the jumbled manner in which the samples are recovered, the spaBal relaBonships of the samples recovered are unknown. Dredging returns samples from the ocean floor but it does not sample the ocean floor verBcally and the relaBonship of the samples recovered is uncertain. The dredge bucket is dragged over several tens of kilometers thereby mixing all the samples from this area together. Studying the Seafloor •  Sampling by drilling –  in 1968, the Deep Sea Drilling Project (DSDP) was begun as an internaBonal program to drill the ocean floor and recover sediment and oceanic crust in their original verBcal posiBons –  first ship was the Glomar Challenger which was capable of drilling in 6,000 m of water –  this program drilled more than 1,000 sites all around the world –  DSDP ended in 1983 when the Glomar Challenger was reBred from service –  the Ocean Drilling Program (ODP) replaced DSDP and is internaBonally supported and sBll funcBoning its drilling ship the JOIDES (Joint Oceanographic InsBtuBons for Deep Earth Sampling) Resolu=on was put in operaBon in 1985 Drilling ship JOIDES Resolu=on Nearly 1,000 sites have been drilled during the 15 year history of the Deep Sea Drilling Project. Because the seafloor is far more homogeneous than the conBnents, this limited set of sampling sites sBll provides an accurate picture of the nature of the seafloor. Studying the Seafloor •  Because 70 % of the Earth's surface is covered water, the study of the oceans and the methods of study are important. Oceanographic research did not begin unBl the 1700s, but most of the important discoveries have been post World War II. This late start in ocean research reflects the difficulBes in studying the ocean floor: –  large area, great depth, high pressure with no light, cold temperatures –  make direct observaBon difficult. •  Two basic approaches to studying ocean floor: –  imaging -­‐ indirect observaBon of sea floor sounding, echo sounder, seismic profiler, Sea Beam, Sidescan Radar, satellite radar alBmeter, heat flow, magnetometer, submersibles, ROVs –  sampling -­‐ acquisiBon of samples of the sea floor. dredging, gravity coring, drilling, submersibles, ROVs Hot Spots •  Most of the volcanoes on Earth are associated with plate boundaries. Their magmas result from the moBon of plates at these boundaries. •  There are, however, isolated volcanoes that are not associated with plate margins and generally occur in the interior of plates. •  Approximately, 100 of these volcanoes have been idenBfied. They are called hot spot volcanoes or more simply hot spots. Hot Spots •  Magma Origin –  Since hot spots are generally not associated with plate margins, their magmas cannot be the product of plate moBon as for plate boundary volcanoes. –  Instead, their magmas are generated deep within the mantle below the lithosphere. –  In these areas, mantle at depth is heated and rises as solid rock, i.e. mantle plumes. –  At the base of the lithosphere, the rising mantle plume melts due to decompression, i.e. lowered pressure but high temperature. –  The resultant magma ascends through the lithosphere and crust to produce the hot spot volcanoes. –  Mantle plumes and their associated hot spot volcanoes remained fixed in posiBon for very long periods of Bme, e.g. tens of millions of years. Hot Spots •  Hot spots occur both in the ocean basins as well as on the conBnents. •  There are al large number of hot spots in both the Pacific and AtlanBc Oceans. •  Hawaii and the Galapagos Islands in the Pacific are two very large hot spots. •  In the AtlanBc, the Azores and Canary Islands as well as St. Helena and TrisBan de Cunha are hot spots in the South AtlanBc. •  Because of the influence of conBnent crust, the trails for conBnental hot spots is much more diffuse than for oceanic ones. •  Although North America has only a few hot spots, Africa has a very large number. •  Yellowstone on the North American conBnent is a famous conBnental hot spot. Hot Spots •  The majority of hot spots are located in the interior of plates. However, a few occur near divergent margins. •  Because of the addiBonal magma output, these regions build up to higher elevaBons and may emerge above the surface. •  Iceland is just such a place. The Azores and Tristan de Cunha in the South AtlanBc are other ridge hot spots. The Galapagos are another famous ridge hot spot in the Pacific. •  Because the moBon of the plates away from these hot spots is in two direcBons, they produce two hot spot chains. The two chains are on opposite plates and form mirror images of each other. Hot Spots •  Although mantle plumes are fixed in space, the volcanoes they feed are carried along on the lithospheric plates. •  Thus, with Bme a volcano becomes separated from its magma source and becomes exBnct. •  When this happens, a new volcano forms over the mantle plume. •  The combined moBon of the plate and the fixed posiBon of the plume produces a chain of volcanoes all except one of which are exBnct. •  Only the volcano at the end is acBve. •  If the plate carrying the volcanoes is oceanic, it cools as it ages and sinks. •  Since the exBnct volcanoes it carries are no long growing taller, they ulBmately sink below the ocean's surface and become seamounts. •  ParBcularly, old hot spot chains may be a combinaBon of seamounts and subaerial volcanoes. •  Other chains are comprised enBrely of seamounts, i.e. submarine volcanoes. The moBon of the lithospheric plate above a hot spot's mantle plume causes the resultant volcano to ulBmately be cut off from its magma source causing it to go exBnct. Hot Spots •  The recogniBon of hot spot chains is important because they can provide informaBon about plate spreading direcBon and rate. •  The spreading direcBon is simply parallel the direcBon of the chain. •  Spreading rate can be determined by daBng the volcanism at a given volcanic center and measuring the distance from that center to the beginning of the change. •  Dividing the distance by the Bme interval yields the spreading rate. •  By determining the spreading rate for a number of centers along a chain, it is possible to determine if the spreading rate was constant or changed with Bme. Hot Spots •  An prominent feature of the Emperor seamount-­‐Hawaii island hot spot chain is a prominent change in direcBon at the island of Midway. •  Such bends or kinks in a hot spot chain indicate that the direcBon of spreading changed during the Bme the chain was formed. Each segment of the chain represents a different spreading episode characterized by a different spreading direcBon. The direcBon of spreading for each episode is simply parallel the trend of the chain. In addiBon, the number of kinks plus one indicates the spreading episodes. Hot Spots •  Using each segment of a hot spot chain that trends at a different angle, it is also possible to calculate a spreading rate for each spreading episode. •  For the most recent spreading episode, i.e. that nearest the acBve volcano, the technique is similar to that described earlier. •  For other segments, the procedure is to date the volcanism of an island and measure its distance from the kink in the chain. •  By subtracBng the age of this volcano from the age of the volcano at the kink, the duraBon of the spreading can be obtained. •  This is then divided into the distance to get the spreading rate. •  In this manner, the spreading rate for each segment can be determined. •  By performing this calculaBon for two or more volcanic centers within a segment, it is possible to check to see if the spreading rate was constant or variable during a spreading episode. Hot Spots •  Hot spots are regions of localized volcanism that are persistent for tens of millions of years. •  They are believed to be formed by decompression melBng of a rising mantle plume at the base of the lithosphere. •  The resultant magmas ascend through the lithosphere to erupt on the surface form a localized volcanic center oGen 100-­‐200 km in diameter.