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Lecture 2-The Dynamic Earth.pdf

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This document provides information about plate tectonics and the dynamics of the Earth. It discusses key topics such as continental drift, Earth's layered structure, types of plate boundaries including convergent, divergent and transform boundaries, seafloor spreading, plate motions, and forces driving plate tectonics. The objectives are to understand concepts like continental drift, Earth's structure, plate margins, seafloor spreading, how plates move on the surface of the sphere, and driving forces. Diagrams and figures are included to illustrate these topics.

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1

Where is Earth in the Universe?? What is the earth made out off???

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2. The Dynamics of Earth Sub-Unit 2. The Dynamics of Earth:  Continental drift  Earth’s structure  Types of Plate Margins  Seafloor spreading  Plate motion on the surface of the sphere  Forces driving plate tectonic motions

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2. The Dynamics of Earth Objectives  Know what is a continental drift and the factors causing it  Identify different layers that made up the spherical earth  Identify different types of Plate Margins  Understand Seafloor spreading  Know how plates are moving on the surface of the sphere  Categories forces driving plate tectonic motions

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• Is a dynamic planet, endlessly changing both internally And externally Processes of transforming the surface of earth 1. endogenic processes (i.e., of internal origin)- by volcanism and tectonism, metamorphism , earthquakes , crustal warping , folding and faulting . The Earth

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2. Exogenic processes (i.e., of external origin) - such as weathering, erosion, transportation, deposition, denudation. • Also, Earth surface is deformed by geological processes at rates that are extremely slow in human terms, cm per year weathering, erosion, transportation deposition Pacific Plate - North American Plate – 5cm/year Mt William Wara Simbu Purari

6

Hypotheses to explain the underlying mechanisms causing dynamics of earth Hypothesis of a Contracting Earth (late nineteenth) - shrinking cause mountains like wrinkles • Propose Southern landmass combine into one great continent in Palaeozoic times called Gondwanaland. (Africa, Antarctica, Arabia, Australia, India and South America). In the late nineteenth century - Austrian geologist Eduard Suess • In 1912 - German meteorologist Alfred Wegener proposed the idea of Super Continent called Pangea (Greek: All the Earth) Formation of Nappe structures in the Alps

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• Alexander Du Toit, a South African geologist, (1937) proposed that “northern continents” are all combine as Laurasia. (North America (including Greenland), Europe and most of Asia) Laurasia and Gondwanaland split apart in the Early Mesozoic Super Continent of Pangae Land is supported by the proposed Laurasia and Gondwanaland The hypothesis of Displacement was Born!

8

1.2.2 Continental drift Continental Drift was proposed by “Displacement hypothesis” • crustal blocks having continental dimensions are making large-scale horizontal displacement continental drift, • The displacements take place slowly over long time intervals. Wegener’s reconstruction of Pangaea

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Computer-assisted reconstructions Pangaea Reconstruction The traces of opposite continental margins were matched by an iterative procedure They digitized the continental outlines • approximately 50 km intervals for different depth • (900 m) depth was used for matching

10

Geographical and Magnetic Poles Earth magnetic field (Geomagnetic Field) • points vertically downward in the North Pole • Points vertically upward in the South Pole • Forms earths magnetosphere • Can change polarity over time • caused by electric currents in the liquid outer core Palaeomagnetism and continental drift

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Palaeomagnetism and continental drift Palaeomagnetism - study of the past record of the direction and intensity of Earth's magnetic (geomagnetic field) field in rocks by oxides of iron (magnetite, hematite, maghemite) when they form. • Magnetic Pole Changes with respect to time

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(a) In hot lava flows, thermal energy exceeds atomic magnetic exchange energy and the magnetic moments (movements) are randomized. (b) When the lava cools, magnetic moment aligns parallel to the ambient geomagnetic field. (c) When magnetic particles are eroded from a source rock and are deposited as a sediment, the particles may align with the geomagnetic field to give rise to a depositional remanent magnetization (DRM). How rocks acquire a remanent magnetization?

13

Virtual geomagnetic pole (VGP) position –calculation of the position of the magnetic pole at the time in which rocks get magnetised • By connecting mean VGP positions of different ages for sites on the same continent apparent polar wander path is obtain Conclude that poles are not moving but continents are!! Apparent polar wander (APW) -The appearance that the pole has shifted with time • retrace the relative motion of continents, • formation and break-up of supercontinents

14

Earth structure Early in the twentieth century it became evident from the study of seismic waves • interior of the Earth has a radially layered structure • boundaries are marked by abrupt changes in seismic velocity or velocity gradient • Each layer is characterized by composition, pressure and temperature in the layer • Four main layers are the crust, mantle and the outer and inner cores

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• S waves cannot pass through fluid while P waves can • shadow zone -angular distances of 104 to 140 degrees from a given earthquake that does not receive any direct P waves • Only refracted P waves arrive at position opposite the epicentre How seismic is used to determination of the properties of the Four Layer P and S Waves

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Depths of the most important seismic discontinuities giving boundaries of layer Inner Core Solid; Radius of Earth 1. Crust 2. Upper Mantle • Lithosphere • Asthenosphere 3. Lower Mental • Mesosphere 4. Outer Core 5. Inner Core

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Lithospheric plates • Lithosphere- crust and uppermost mantle are rigid, forming a hard outer shell • Asthenosphere- weaker layer may be able to flow over long periods of time like a viscous liquid or plastic solid, • The convection current in the mental causes relative motions of the overlying lithospheric plates • when strongly stressed, fracture happens causing earthquake. Ridge Lithosphere Asthenosphere Trench Trench Ridge This action • resulted in the plate movements • Seismic zones of the earth Earthquakes – is a violent release of elastic energy due to sudden displacement on a fault plane

18

Seismic Belts: (a) the circum-Pacific “ring of fire”; (b) a sinuous belt running from the Azores through North Africa and the Alpine–Dinaride–Himalayan mountain chain as far as S.E. Asia; and (c) the world-circling system of oceanic ridges and rises. Seismic Zones of the Earth

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Tectonic plates Tectonic plates - fragmented slabs of lithosphere that visible trough boundaries called tectonic plate boundaries. • Sizes range: 1000 - 10,000 sq. km (e.g., the Pacific plate and Philippines plate) • Movement – mm-cm/year • Twelve major plates - (Antarctica, Africa, Eurasia, India, Australia, Arabia, Philippines, North America, South America, Pacific, Nazca, and Cocos) and several minor plates (e.g., Scotia, Caribbean, Juan de Fuca). The seismic zones subdivide the lithosphere laterally into tectonic plates

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Lecture 2-The Dynamic Earth.pdf

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2. Divergent - two tectonic plates move away from each other Three types of plate boundary (Margins) 3. Transform - Two plates sliding past each other 1. Convergent - two plates come together, Constructive plate margins. Destructive plate margin Conservative plate margin,

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There are 3 types of Convergent Boundaries… Type 1: Oceanic plate-Continental plate collision • oceanic is more dense and sub duct beneath continental plate • Hot more buoyant material rises • Magmatic arc formation Subduction: The process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle at a convergent plate boundary.

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VOLCANOES occur at subduction zones Mountains form at subduction zones

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Type 2: Ocean plate - Ocean plate collision The less dense plate slides under the more dense plate creating a subduction zone called a TRENCH Oldest will subduct

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Type 3: Continental plate - Continental plate collision  Neither subduct due to buoyancy of crusts  A place where folded and thrust faulted mountains form.

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• May form Mountain Ranges. These are Folded Mountains, like the Himalayas

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Owen Stanley Ranges

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 Two plates move apart  Run through ocean basins Divergent Plate Boundaries

29

Mid-oceanic ridges, transform faults and rift valleys, and the formation of the ocean floor are characteristics associated with divergent plate boundaries. Magma

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Transform Plate Boundary Plate 1 Plate 2 Transform Plate Boundary -two plates slide past one another The fracture zone that forms a transform plate boundary is known as a transform fault

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San Andreas Fault

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Sea-Floor Spreading The process by which molten material adds new oceanic crust to the ocean floor In 1961 R. Dietz term this process “sea-floor spreading” for the ridge process

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Seafloor spreading formed Mid-Ocean Ridge Mid-Ocean Ridge:  The undersea mountain chain where new ocean floor is produced;  Cause of divergent plate boundary Mid-Ocean Ridge

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Mid Ocean Ridge

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What evidence did scientists find for sea- floor spreading in the 1960s?  Evidence from molten material  Evidence from magnetic stripes  Evidence from drilling samples

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The Vine–Matthews–Morley hypothesis “When hot lava solidifies and its temperature cools below the Curie temperature of its magnetic minerals, the basalt becomes strongly magnetized in the direction of the Earth’s magnetic field at that time.” Evidence of Magnetic strip

37

Along an active spreading ridge: • basalt form symmetrically on opposite sides of the spreading center, • each carrying the magnetic imprint of the field in which it formed. • Sea-floor spreading can persist for many millions of years at an oceanic ridge. • changes in polarity of different time -ve +ve

38

Evidence From Drilling Samples  When scientists sampled the rocks, they found that the further away from the ridge the rocks were older  The younger rocks were always in the center of the ridges

39

Rates of sea-floor spreading • the opposite sides are moving away from the ridge at equal speeds • increasing slowly at a mean rate of 17mm /yr

40

Computation of half-rates of sea- floor spreading at different spreading centers by measuring the distances to anomalies with known radiometric ages Total spreading rate is twice the half-spreading rate

41

2. Constructive plate margin- two tectonic plates move away from each other Plate Margins 3. Conservative plate margin - Two plates sliding past each other 1. Destructive plate margin - two plates come together Eg. Divergent boundaries Eg. Convergent boundaries Eg. Transform boundaries

42

Hotspots refer to a long-lasting center of surface volcanism and locally high heat flow rooted in the mantle below the lithosphere • more common in the ocean basins • hot, rising plume • forming a volcanic island • Seamount The motion of the plate transports the island away from the hotspot and the volcanism becomes extinct

43

Example • radiometric dating of basalt samples from islands and seamounts along the Hawaiian Ridge portion of the Hawaiian–Emperor chain

44

Plate motion on the surface of a sphere Earth's surface is a sphere • Made up of Earth's tectonic plates • Plates move continuously with respect to each other, Mathematically speaking, the most general way to describe the motion of rigid plate on a sphere is through rotation about a fixed pole This is known as Euler geometrical theorem

45

This is described in the Euler geometrical theorem, which shows that every displacement of a plate from one position to another on the Earth's surface can be regarded as a simple rotation of that plate about a suitably chosen axis, known as an Euler pole or pole of rotation, which passes through the centre of the Earth. b A Euler pole or pole of rotation

46

Transform Fault

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Forces driving plate tectonic motions There are forces acting on lithospheric plates that causing the movement of plates • Forces can be driving and resistive Upper mantle convection forces • mantle drag force (𝑭𝑫𝑭) – caused by flow of material beneath a plate on the base of the plate. If convection is fast compare to plate movement. The plate is dragged. Opposite is true. • Continental drag force (𝑭𝑪𝑫) – caused by greater weight of the plate

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• ridge push force (𝑭𝑹𝑷) – gravitational sliding towards trenches Force at spreading ridges Forces at transform faults • transform force (𝑭𝑻𝑭) - frictional resistance in the contact zone

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• slab pull force (𝑭𝑺𝑷) – downward pull force of the mantle to the subducting slab • slab resistance force (𝑭𝑺𝑹) – resistance the slab experience as it tries to move further down into mantle Forces at subduction zones Forces at Subduction Zone

50

Forces at Plate collisions • “trench suction” (𝑭𝑺𝑼) – force caused by pull force on the lower plate that pulls the upper plate towards the trench. • collision resistance force (𝑭𝑪𝑹) – opposite motion of colliding plates cause resistance

51

Forces at hotspots • hotspot force (𝐹𝐻𝑆) – caused by transfer of mantle material to the lithosphere

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Driving Forces are: • slab pull, • slab suction, • ridge push and • trench pull force on the upper plate In Conclusion Resistive Forces are: • slab resistance, • collision resistance, and • transform fault forces.

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END

More Related Content

Lecture 2-The Dynamic Earth.pdf

  • 1. Where is Earth in the Universe?? What is the earth made out off???
  • 2. 2. The Dynamics of Earth Sub-Unit 2. The Dynamics of Earth:  Continental drift  Earth’s structure  Types of Plate Margins  Seafloor spreading  Plate motion on the surface of the sphere  Forces driving plate tectonic motions
  • 3. 2. The Dynamics of Earth Objectives  Know what is a continental drift and the factors causing it  Identify different layers that made up the spherical earth  Identify different types of Plate Margins  Understand Seafloor spreading  Know how plates are moving on the surface of the sphere  Categories forces driving plate tectonic motions
  • 4. • Is a dynamic planet, endlessly changing both internally And externally Processes of transforming the surface of earth 1. endogenic processes (i.e., of internal origin)- by volcanism and tectonism, metamorphism , earthquakes , crustal warping , folding and faulting . The Earth
  • 5. 2. Exogenic processes (i.e., of external origin) - such as weathering, erosion, transportation, deposition, denudation. • Also, Earth surface is deformed by geological processes at rates that are extremely slow in human terms, cm per year weathering, erosion, transportation deposition Pacific Plate - North American Plate – 5cm/year Mt William Wara Simbu Purari
  • 6. Hypotheses to explain the underlying mechanisms causing dynamics of earth Hypothesis of a Contracting Earth (late nineteenth) - shrinking cause mountains like wrinkles • Propose Southern landmass combine into one great continent in Palaeozoic times called Gondwanaland. (Africa, Antarctica, Arabia, Australia, India and South America). In the late nineteenth century - Austrian geologist Eduard Suess • In 1912 - German meteorologist Alfred Wegener proposed the idea of Super Continent called Pangea (Greek: All the Earth) Formation of Nappe structures in the Alps
  • 7. • Alexander Du Toit, a South African geologist, (1937) proposed that “northern continents” are all combine as Laurasia. (North America (including Greenland), Europe and most of Asia) Laurasia and Gondwanaland split apart in the Early Mesozoic Super Continent of Pangae Land is supported by the proposed Laurasia and Gondwanaland The hypothesis of Displacement was Born!
  • 8. 1.2.2 Continental drift Continental Drift was proposed by “Displacement hypothesis” • crustal blocks having continental dimensions are making large-scale horizontal displacement continental drift, • The displacements take place slowly over long time intervals. Wegener’s reconstruction of Pangaea
  • 9. Computer-assisted reconstructions Pangaea Reconstruction The traces of opposite continental margins were matched by an iterative procedure They digitized the continental outlines • approximately 50 km intervals for different depth • (900 m) depth was used for matching
  • 10. Geographical and Magnetic Poles Earth magnetic field (Geomagnetic Field) • points vertically downward in the North Pole • Points vertically upward in the South Pole • Forms earths magnetosphere • Can change polarity over time • caused by electric currents in the liquid outer core Palaeomagnetism and continental drift
  • 11. Palaeomagnetism and continental drift Palaeomagnetism - study of the past record of the direction and intensity of Earth's magnetic (geomagnetic field) field in rocks by oxides of iron (magnetite, hematite, maghemite) when they form. • Magnetic Pole Changes with respect to time
  • 12. (a) In hot lava flows, thermal energy exceeds atomic magnetic exchange energy and the magnetic moments (movements) are randomized. (b) When the lava cools, magnetic moment aligns parallel to the ambient geomagnetic field. (c) When magnetic particles are eroded from a source rock and are deposited as a sediment, the particles may align with the geomagnetic field to give rise to a depositional remanent magnetization (DRM). How rocks acquire a remanent magnetization?
  • 13. Virtual geomagnetic pole (VGP) position –calculation of the position of the magnetic pole at the time in which rocks get magnetised • By connecting mean VGP positions of different ages for sites on the same continent apparent polar wander path is obtain Conclude that poles are not moving but continents are!! Apparent polar wander (APW) -The appearance that the pole has shifted with time • retrace the relative motion of continents, • formation and break-up of supercontinents
  • 14. Earth structure Early in the twentieth century it became evident from the study of seismic waves • interior of the Earth has a radially layered structure • boundaries are marked by abrupt changes in seismic velocity or velocity gradient • Each layer is characterized by composition, pressure and temperature in the layer • Four main layers are the crust, mantle and the outer and inner cores
  • 15. • S waves cannot pass through fluid while P waves can • shadow zone -angular distances of 104 to 140 degrees from a given earthquake that does not receive any direct P waves • Only refracted P waves arrive at position opposite the epicentre How seismic is used to determination of the properties of the Four Layer P and S Waves
  • 16. Depths of the most important seismic discontinuities giving boundaries of layer Inner Core Solid; Radius of Earth 1. Crust 2. Upper Mantle • Lithosphere • Asthenosphere 3. Lower Mental • Mesosphere 4. Outer Core 5. Inner Core
  • 17. Lithospheric plates • Lithosphere- crust and uppermost mantle are rigid, forming a hard outer shell • Asthenosphere- weaker layer may be able to flow over long periods of time like a viscous liquid or plastic solid, • The convection current in the mental causes relative motions of the overlying lithospheric plates • when strongly stressed, fracture happens causing earthquake. Ridge Lithosphere Asthenosphere Trench Trench Ridge This action • resulted in the plate movements • Seismic zones of the earth Earthquakes – is a violent release of elastic energy due to sudden displacement on a fault plane
  • 18. Seismic Belts: (a) the circum-Pacific “ring of fire”; (b) a sinuous belt running from the Azores through North Africa and the Alpine–Dinaride–Himalayan mountain chain as far as S.E. Asia; and (c) the world-circling system of oceanic ridges and rises. Seismic Zones of the Earth
  • 19. Tectonic plates Tectonic plates - fragmented slabs of lithosphere that visible trough boundaries called tectonic plate boundaries. • Sizes range: 1000 - 10,000 sq. km (e.g., the Pacific plate and Philippines plate) • Movement – mm-cm/year • Twelve major plates - (Antarctica, Africa, Eurasia, India, Australia, Arabia, Philippines, North America, South America, Pacific, Nazca, and Cocos) and several minor plates (e.g., Scotia, Caribbean, Juan de Fuca). The seismic zones subdivide the lithosphere laterally into tectonic plates
  • 21. 2. Divergent - two tectonic plates move away from each other Three types of plate boundary (Margins) 3. Transform - Two plates sliding past each other 1. Convergent - two plates come together, Constructive plate margins. Destructive plate margin Conservative plate margin,
  • 22. There are 3 types of Convergent Boundaries… Type 1: Oceanic plate-Continental plate collision • oceanic is more dense and sub duct beneath continental plate • Hot more buoyant material rises • Magmatic arc formation Subduction: The process by which oceanic crust sinks beneath a deep-ocean trench and back into the mantle at a convergent plate boundary.
  • 23. VOLCANOES occur at subduction zones Mountains form at subduction zones
  • 24. Type 2: Ocean plate - Ocean plate collision The less dense plate slides under the more dense plate creating a subduction zone called a TRENCH Oldest will subduct
  • 25. Type 3: Continental plate - Continental plate collision  Neither subduct due to buoyancy of crusts  A place where folded and thrust faulted mountains form.
  • 26. • May form Mountain Ranges. These are Folded Mountains, like the Himalayas
  • 28.  Two plates move apart  Run through ocean basins Divergent Plate Boundaries
  • 29. Mid-oceanic ridges, transform faults and rift valleys, and the formation of the ocean floor are characteristics associated with divergent plate boundaries. Magma
  • 30. Transform Plate Boundary Plate 1 Plate 2 Transform Plate Boundary -two plates slide past one another The fracture zone that forms a transform plate boundary is known as a transform fault
  • 32. Sea-Floor Spreading The process by which molten material adds new oceanic crust to the ocean floor In 1961 R. Dietz term this process “sea-floor spreading” for the ridge process
  • 33. Seafloor spreading formed Mid-Ocean Ridge Mid-Ocean Ridge:  The undersea mountain chain where new ocean floor is produced;  Cause of divergent plate boundary Mid-Ocean Ridge
  • 35. What evidence did scientists find for sea- floor spreading in the 1960s?  Evidence from molten material  Evidence from magnetic stripes  Evidence from drilling samples
  • 36. The Vine–Matthews–Morley hypothesis “When hot lava solidifies and its temperature cools below the Curie temperature of its magnetic minerals, the basalt becomes strongly magnetized in the direction of the Earth’s magnetic field at that time.” Evidence of Magnetic strip
  • 37. Along an active spreading ridge: • basalt form symmetrically on opposite sides of the spreading center, • each carrying the magnetic imprint of the field in which it formed. • Sea-floor spreading can persist for many millions of years at an oceanic ridge. • changes in polarity of different time -ve +ve
  • 38. Evidence From Drilling Samples  When scientists sampled the rocks, they found that the further away from the ridge the rocks were older  The younger rocks were always in the center of the ridges
  • 39. Rates of sea-floor spreading • the opposite sides are moving away from the ridge at equal speeds • increasing slowly at a mean rate of 17mm /yr
  • 40. Computation of half-rates of sea- floor spreading at different spreading centers by measuring the distances to anomalies with known radiometric ages Total spreading rate is twice the half-spreading rate
  • 41. 2. Constructive plate margin- two tectonic plates move away from each other Plate Margins 3. Conservative plate margin - Two plates sliding past each other 1. Destructive plate margin - two plates come together Eg. Divergent boundaries Eg. Convergent boundaries Eg. Transform boundaries
  • 42. Hotspots refer to a long-lasting center of surface volcanism and locally high heat flow rooted in the mantle below the lithosphere • more common in the ocean basins • hot, rising plume • forming a volcanic island • Seamount The motion of the plate transports the island away from the hotspot and the volcanism becomes extinct
  • 43. Example • radiometric dating of basalt samples from islands and seamounts along the Hawaiian Ridge portion of the Hawaiian–Emperor chain
  • 44. Plate motion on the surface of a sphere Earth's surface is a sphere • Made up of Earth's tectonic plates • Plates move continuously with respect to each other, Mathematically speaking, the most general way to describe the motion of rigid plate on a sphere is through rotation about a fixed pole This is known as Euler geometrical theorem
  • 45. This is described in the Euler geometrical theorem, which shows that every displacement of a plate from one position to another on the Earth's surface can be regarded as a simple rotation of that plate about a suitably chosen axis, known as an Euler pole or pole of rotation, which passes through the centre of the Earth. b A Euler pole or pole of rotation
  • 47. Forces driving plate tectonic motions There are forces acting on lithospheric plates that causing the movement of plates • Forces can be driving and resistive Upper mantle convection forces • mantle drag force (𝑭𝑫𝑭) – caused by flow of material beneath a plate on the base of the plate. If convection is fast compare to plate movement. The plate is dragged. Opposite is true. • Continental drag force (𝑭𝑪𝑫) – caused by greater weight of the plate
  • 48. • ridge push force (𝑭𝑹𝑷) – gravitational sliding towards trenches Force at spreading ridges Forces at transform faults • transform force (𝑭𝑻𝑭) - frictional resistance in the contact zone
  • 49. • slab pull force (𝑭𝑺𝑷) – downward pull force of the mantle to the subducting slab • slab resistance force (𝑭𝑺𝑹) – resistance the slab experience as it tries to move further down into mantle Forces at subduction zones Forces at Subduction Zone
  • 50. Forces at Plate collisions • “trench suction” (𝑭𝑺𝑼) – force caused by pull force on the lower plate that pulls the upper plate towards the trench. • collision resistance force (𝑭𝑪𝑹) – opposite motion of colliding plates cause resistance
  • 51. Forces at hotspots • hotspot force (𝐹𝐻𝑆) – caused by transfer of mantle material to the lithosphere
  • 52. Driving Forces are: • slab pull, • slab suction, • ridge push and • trench pull force on the upper plate In Conclusion Resistive Forces are: • slab resistance, • collision resistance, and • transform fault forces.
  • 53. END