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11 Plate Tectonics GLY 2010 – Summer 2014 - Lecture 15.

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Presentation on theme: "11 Plate Tectonics GLY 2010 – Summer 2014 - Lecture 15."— Presentation transcript:

1 11 Plate Tectonics GLY 2010 – Summer 2014 - Lecture 15

2 22 Subduction Zones Plots of earthquake foci over time delineate the position of subducting plates The plate which is subducted is always denser than the plate which remains on the surface Earthquakes associated with convergent plate margins where one plate is subducting are of shallow, intermediate and deep focus Deep focus earthquakes are not known except at subduction zones

3 33 Shallow Subduction Angle Plates near the spreading center will be much warmer  They will be only slightly denser than surface plate, and the subduction angle will be shallow

4 44 Steep Subduction Angle Plates far from the spreading center will be relatively cold, and therefore dense  They will subduct at a steep angle

5 55 Breakup of Subducting Slabs Slabs break off, and are no longer attached to the subducting plate Lack of attachment stops their movement, and they no longer generate earthquakes

6 66 Accumulation of Slabs Broken slabs are now known to accumulate within the mantle, stacking up like pancakes

7 77 Oceanic Trenches Subducting plate drags part of the surface with it Creates large oceanic trenches, which also serve to mark the top of the subduction zones

8 88 Continental Volcanic Arcs Plates subducted under continents partially melt, creating long chains of stratovolcanoes on the continents  Cascades and Andes are examples

9 99 Volcanic Island Arc Plates subducted under oceanic plates create chains of oceanic islands  Japan, the Philippines, and Indonesia are examples

10 10 Plate Motions Two plates move relative to each other  Convergent - Plates move toward each other, often a head-on collision  Divergent - Plates move away from each other  Sideways (transform) - Plates move past each other along transform faults

11 Convergent Movement 11

12 Divergent Motion 12

13 Transform Motion 13

14 14 Plate Types At any given point, a plate is either oceanic or continental Interactions between plates are thus:  Ocean-ocean (O-O)  Ocean-continent (O-C)  Continent-continent (C-C)

15 15 Plate Interactions

16 Summary of Plate Interactions 16

17 17 Hydrothermal Vents Spreading centers are marked by vents which spew hydrothermal fluids as hot as 650  C Fluids contain dissolved metals which precipitate when they hit cold ocean water, encrusting basalt - vents are called “black smokers” for this reason

18 18 Hydrothermal Vent – Black Smoker Black Smoker along the Juan de Fuca Ridge Temperature: 648ºC Courtesy PBS Station WGBH Click to play video

19 19 Alvin With room enough for only one pilot, a cameraman, and a massive IMAX camera, Alvin dove to depths of 12,000 feet (3,700 meters) during an ambitious effort to film hydrothermal vents on the Mid- Atlantic Ridge Alvin photographed from research vessel Atlantis

20 20 Hydrothermal Vent – White Smoker “White smokers” release water that is cooler than their cousins’ and often contains compounds of barium, calcium, and silicon, which are white Hydrothermal vents are believed to play an important role in the ocean’s temperature, chemistry, and circulation patterns Click to play video

21 21 Vent Biology - Tubeworms A tube worm colony near hydrothermal vents Click to play video

22 22 Vent Biology – Vent Crabs The vent crab is typically found among dense clusters of tubeworms at an average depth of 1.7 miles and can tolerate a temperature gradient that ranges from 77°F in the tubeworm clumps, to 36°F, which is the temperature of the water surrounding the vent sites Click to play video

23 23 Vents of the World Some known vent localities on the ocean floor – note association with mid-ocean ridges

24 24 Earth’s Magnetic Field Earth has a strong magnetic field It is dipolar, with the poles being called north and south

25 25 Earth’s Magnetic Polarity Present north magnetic pole is located near the south geographic pole South magnetic pole is located near the north geographic pole

26 26 Rock Magnetism Rocks often become magnetized because magnetic mineral grains (usually magnetite) are aligned Rock’s magnetic field is fixed at the time magma cools below the Curie point for igneous rocks, or at the time of lithification for sedimentary rocks Magnetism of older rocks is called “paleomagnetism”

27 Magnetic Inclination and Declination 27

28 28 Magnetic Stripes In the early 1960’s oceanographic research uncovered a curious phenomenon, called magnetic stripes Measurements of the earth’s magnetic field show small variations from place to place

29 29 Magnetic Anomalies Magnetic Anomaly = Average regional magnetic field of the earth - magnetic field at a point Plotting magnetic anomalies lead to a curious pattern of “stripes”, first seen in the Atlantic, later in the Pacific

30 30 Explanation of Stripes The first to propose an explanation that was scientifically accepted was L. Wilson Morley, a Canadian geoscientist Morley sent his paper to the British journal Nature in January, 1963 Nature rejected the article, and it was not published until more than a year later in another journal 30

31 31 Morley’s Idea Sea-floor spreading - new magma emerging at a MOR and hardening into rock, which then spread away from the ridge with time (Hess-Dietz hypothesis) Polarity reversals - the North and South magnetic poles changing position suddenly

32 Polarity Reversal 32

33 33 Vine-Matthews Working completely independently of Morley, and unaware of his idea, D.H. Matthews and his graduate student, Frederick Vine, formulated an explanation often called the Vine-Mathews theory 33 Frederick J. Vine

34 34 Vine-Matthews Publication Published “Magnetic Anomalies Over Ocean Ridges (in Nature!) in September, 1963 Matthews was a Research Fellow of King's College, Cambridge 34

35 35 Vine-Matthews-Morley Theory If we assume sea-floor spreading is occurring, the magnetic field of the rock is fixed, in alignment with the earth’s field, at the time the rock cools The measured field above such rocks equals the earth’s field plus the rock’s field (because they are aligned)

36 36 After Polarity Reversal If the earth’s field reverses, the field of the previously created rock will be aligned against the earth’s field, slightly decreasing it A second reversal will again align the field of the rock and earth

37 37 Magnetic Stripes As magma rises, it hardens and its magnetic field matches the present field of the earth - after a polarity reversal, it will be aligned against the earth’s field

38 38 Sea Floor Magnetism Animation

39 39 Anomalies When the rock’s field and the earth’s field are aligned, the field at a point will be greater than the regional average - a positive anomaly When the rock’s field and earth’s field are in opposite directions, the field at a point will be less than the regional average - a negative anomaly

40 40 Anomaly Diagram Anomalies are really just regions of high and low magnetic intensity

41 41 Magnetic Stripe Theory By postulating a series of irregular (in time) polarity reversals, with continuous eruption of magma at the spreading center, Morley and later Vine-Mathews offered an explanation for magnetic stripes After several years of discussion, this explanation was accepted by most earth scientists in a series of conferences in 1967- 68

42 42 Magnetic Reversal Record Recent magnetic field data from lava samples of known age

43 43 Rate of Plate Movement Once Plate Tectonics was accepted, it became necessary to determine how fast plates move Three methods have been used  Hot spots  Satellite Tracking  Magnetic reversal

44 44 Hot Spots Hot spots, which generate magma in the asthenosphere, below the moving lithospheric plates, may be used as a reference since they are effectively stationary relative to lithospheric plate

45 45 Hot Spots “Movement” Hot spots produce volcanoes, like the Hawaiian Islands or many seamounts or guyots (mountains that made it above sea-level, then were flattened by wave erosion)

46 46 Hot Spot Diagram Diagram showing creation of several Hawaiian Islands Age of islands should be progressively older as they move away, and this is observed

47 Hot Spot Animation 47

48 48 Pacific Ocean Near Hawaii

49 49 Satellite Tracking Lasers bounce beams off satellite in geosynchronous orbit Time necessary to make the round-trip journey can be used to measure the distance of the laser station from the satellite The amount this changes from year to year can be used to determine plate motions

50 50 GPS Receiver Receivers such as this receive signals from satellites…using a computer, the distance to each satellite is computed The position on the earth’s surface is then calculated

51 51 Magnetic Reversal Times The age of the rock at each reversal can be dated The distance from the spreading center, together with the age, can be used to calculate a velocity (units of which are distance/time)

52 52 Building Continents Craton - The original nucleus of the continent - broken into two categories  Continental shield - Broad areas of exposed crystalline rocks that have not changed in more than a billion years. Often exposed by glaciation  Continental platform - Areas surrounding the shield, where layers of younger rock cover the shield

53 53 Formation of Early Landmasses Early islands probably collided to form larger landmasses, and heat and pressure of collision may have metamorphosed some of the rock The mafic/ultramafic rocks might have heated to the point where low-melting minerals, usually felsic, formed magmas Intermediate to felsic magmas cooled to form the first continental crust

54 54 Early Atmosphere and Weathering Volcanic gases modified the early atmosphere Weathering and erosion would have created sediment to fill in sedimentary basins

55 55 Accretion As more continental land masses formed, collisions between them became inevitable Subduction followed, leading to further differentiation of felsic/intermediate magma by partial melting at 100-150 km

56 Addition of Terranes 56

57 57 Identification of Exotic Terranes The displaced, or exotic, terranes can be distinguished from surrounding rock based on:  Their age  Fossil assemblages  Stratigraphy  Paleomagnetic data

58 58 Ophiolites Accretion also adds ophiolite materials to continents Ophiolites are rocks that comprise the oceanic crust They are mafic, but often include serpentinite, formed by metasomatitism of the mafic rock Serpentinite is soft, green, and often contorted into a snake-like appearance

59 59 Formation of Ophiolite Ophiolite sequences are formed near spreading centers, and include evidence of metasomatism 59

60 60 Ophiolite Sequence 60

61 61 Suture Zone A boundary between colliding continents, in which the continental crust if thickened as one continent slides under the other 61 The Great Lakes Tectonic Zone which bounds gneisses to the south and Greenstones to the north

62 62 Exotic Terrane USGS video about Hells Canyon Idaho region

63 63 Continental Collisions As the continents accreted, some joined to become supercontinents Southern supercontinent = Gondwana Northern supercontinent = Laurasia

64 64 Formation of Pangaea By 225 mybp, at the end of the Paleozoic era, Pangaea was a vast supercontinent, which included essentially all land on earth, and stretched from pole to pole

65 Animation of Pangaea Breakup 65

66 66 Rifting, Stage 1 Plumes of hot magma originating beneath the lithosphere penetrate the lithosphere and cause triangular cracks

67 67 Rifting, Stage 2 Mantle currents usually separate and widen two of the cracks These cracks may flood with seawater

68 68 Rifting, Stage 3 The other crack often becomes inactive The inactive branch is often called a failed arm

69 69 Rift Rivers The Benue Trough is occupied by the Benue River, a tributary of the Niger River

70 70 Modern Rifting The Red Sea, Gulf of Aden, and the East African Rift Zone are a modern day example of a rifting event

71 71 Current Rifting in North America The New Madrid earthquake may represent release of pent-up stress along the failed arm Currently, rifting is occurring  In the Rio Grande area between Colorado and N. Mexico  In the Basin and Range of Arizona, Nevada, and Utah

72 72 Active Branches The active branches are marked by high heat flow, normal faulting, frequent shallow-focus earthquakes, and widespread basaltic volcanism The great basalt plateaus of the continents (Columbian, Deccan) mark current or ancient rift zones

73 73 Formation of Graben Valleys Rifts widen, crating grabens The grabens are invaded by the sea The Gulf of Aden and the Red Sea form an example The Great Rift Valley of East Africa, where many believe man originated, is believed to be the inactive arm

74 74 Incomplete Rifting Rifting may start, with a great outpouring of mafic lavas, and then cease This happened in N. America about 1100 MYBP around Lake Superior

75 75 The Last 600 MY

76 Plate Boundary Process Correlation 76


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