EESA06 READING #2 CHAPTER 2 â PLATE TECTONICS What are ...
EESA06 READING #2 CHAPTER 2 – PLATE TECTONICS What are plate tectonics? Tectonics – the study of the origin and arrangement of the broad structural features of the earth’s surface including folds, faults, mountain belts, continents, and earthquake belts Plate Tectonics – the Earth’s surface is divided into a few large, think plates that move slowly and change in size Plate boundaries – plates move away from one another, past one another, or towards one another Eight large plates/smaller plates – makes up the outer shell of the Earth (the crust and upper part of the mantle) Plate tectonic theory developed by: Continental drift – continents move freely over the Earth’s surface, changing their positions relative to one another Sea floor spreading – A hypothesis that the sea floor forms at the crest of mid-oceanic ridges, then moves horizontally away from the ridge crest toward an oceanic trench. – Two sides of the ridge are moving in opposite directions How did plate tectonics theory evolve? The Early Case for Continental Drift Alfred Wegener • Meteorologist • Continental drift • Examined continents that have identical late Paleozoic rocks and fossils Pangea – continents form a giant supercontinent Laurasia –northern supercontinent (North America) Gondwanaland – southern supercontinent Paleoclimatology – study of ancient climate – Examined ancient sedimentary rocks he discovered that paleoclimatic reconstructions suggested polar positions very different to those at present Skepticism about Continental Drift – Believes that the less dense continents drifted through oceanic
crust, crumpling up mountain ranges on their leading edges as they pushed against the oceanic crust (DIAGRAM PAGE 25) Renewed Interest in Continental Drift –
Study of the Sea Floor o Samples of rocks and sediments can be taken from the sea floor by: 1. Rock Dredge – open steel container dragged over the ocean bottom at the end of a cable 2. Corer – a weighted steel pipe dropped vertically into the mud and sand of the ocean floor 3. Sea-floor drilling – drilling a hole in the deep sea floor o Submersibles – small research submarines for geologists o Single-bean echo sounder – measures water depth and draws profiles of submarine topography 1. A sound is sent downward from the ship and bounces off the sea floor and returns to the ship 2. Water depth = amount of time interval o 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
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Geophysical Research EXPLAIN (PAGE 28) o Polar wandering Magnetic poles are located close to the geographic poles Magnetic poles move from year to year o Magnetism of old rocks can be measured to determine the direction and strength of the magnetic field in the past o Paleomagnetism – study of ancient magnetic fields
Recent Evidence for Continental Drift • Refined rock matches between now-separated continents (EX. South America and Africa) • Rocks are similar in type, structure, fossils, age, etc. What is sea-floor spreading? Harry Hess The Sea floor might be moving too like continental drift Sea floor spreading – the sea floor moves away from the mid-oceanic ridge as a result of mantle convection
Subduction – the sliding of the sea floor beneath a continent or island arc Convection – is a circulation pattern driven by the rising of hot material and the sinking of cold material Hot materials have a lower density so it rises Cold materials have a higher density so it sinks Andesitic volcanism – it is produced by the sea floor moving downward into the mantle along a subduction zone. This interaction between the moving sea-floor rock and the stationary rock causes these volcanoes and even earthquakes. • Formed on the edge of a continent or an island arc How old is the sea floor? • Young sea floor is being formed by basalt eruptions at the ridge crest • Basalt is then carried sideways by convection and is subducted into the mantle at an oceanic trench • Old sea floor is being destroyed and new sea floor is being formed What are plates and how do they move? Continental Drift and Sea floor spreading Formed Plate tectonic theory Plate – is a large, mobile slab of rock that is part of Earth’s surface • Made up of sea floor, continental or oceanic rock Lithosphere – rigid outer shell of earth that includes rocks from the crust and the uppermost mantle • Continental lithosphere is thicker (DIAGRAM PAGE 33) Asthenosphere – below the lithosphere, which is a zone of low-seismicwave velocity that behaves plastically because of increased temperature and pressure • Allows the plates to move • Below this is mantle rock Plates made up of sea floor – subducted down into the mantle forming oceanic trenches Plate leading edge made up of continental rock –NOT subduct because continental rock is less dense than oceanic rock Three types of plate boundaries: 1. Divergent plate boundary – plate boundary moving APART 2. Convergent plate boundary – plates moving TOWARDS each other 3. Transform plate boundary – move HORIZONTALLY past each other
How do we know that plates move? Paleomagnetic Evidence Magnetic reversals – changes in the polarity of the magnetic field, when the north and south magnetic pole exchange places Normal polarity – magnetic lines flow from the South Pole to the North Pole and our compass needles point to the north Reversed polarity – magnetic lines run from North Pole to South Pole and our compass needles point to the South Paleomagnetism – study of magnetic fields recorded in rocks helps us understand the magnetic fields in the past Magnetic polarity time scale – this is constructed by using stacked continental lava flow to record the pattern of magnetic reversals over time Magnetometer – an instrument that measures the strength of the Earth’s magnetic field. It is taken over land surface or sea.
Marine Magnetic Anomalies • •
Magnetic anomalies on the sea floor are arranged in bands that lie parallel to the rift valley of the mid-oceanic ridge Positive and negative anomalies form a stripelike pattern parallel to the ridge crest
The Morley-Wine-Matthews Hypothesis • The pattern of magnetic anomalies at sea matches the pattern of magnetic reversals of lava flows on the continents 1. Opening of cracks within the rift valley on the mid-oceanic ridge crest 2. They are filled by basaltic magma 3. Cools and forms dikes i. Cools ii. Magnetized iii. Forms new dikes How fast do plates move? • On each side of the ridge the sea floor is moving away from the ridge crest 1cm to 10cm per year Predicting Sea-Floor Age • Predict the age of the igneous rock of the sea floor by measuring the magnetic anomalies at the sea surface o Matching the measured anomaly pattern with the known
pattern of anomalies Another Test: Fracture Zones and Transform Faults • Studying the seismicity of fracture zones • Transform Fault – the portion of a fracture zone between two offset portions of ridge crest o Earthquakes o Rock is moving away from the ridge crest What happens at plate boundaries? Divergent Plate Boundaries – plates move AWAY • To create and open new ocean basin • Found in middle of a continent or ocean 1. Rifting – continental crust is stretched and thinned 2. Basaltic volcanism – (Graben – down-dropped fault block that acts as a path way for basaltic magma) 3. Uplift – caused by the upwelling of hot mantle beneath the crust 4. Crust separates and creates an ocean 5. Plates continue to diverge and widening the sea 6. Thermal uplift – creates a mid-oceanic ridge EX. The Red River (occurred), Atlantic Ocean (happening) Mid-Oceanic Ridges Mid Ocean Ridges – giant undersea mountain ranges that extend around the world Rift Valley – runs down the crest of each ridge (Absent in the Pacific Ocean) Geologic Activity on Ridges READ OVER Shallow-focus earthquakes – below the sea floor Basalt eruptions – occurs in and near the rift valley on ridge crests Transform Boundaries – plate’s slide HORIZONTALLY • Shallow-focus earthquakes • Transform faults means that the fault ends or transform into another kind of displacement o Divergent o Convergent o Trench to Ridge Convergent Plate Boundaries – plates move TOWARD each other Three situations of convergence: 1. Oceanic crust vs. Oceanic crust – one plate subducts under the other
2. Oceanic crust vs. Continental crust – Dense Oceanic crust subducts under the continental plate 3. Continental crust vs. Continental crust – both plate collide and crumple Ocean-Ocean Convergence Oceanic trench – is a narrow deep trough parallel to the edge of a continent or an island arc (around many continents) Island Arc – magma erupts, a curved line of volcanoes that form a string of islands parallel to the oceanic trench • Trench position change with time Ocean-Continent Convergence Magmatic Arc – formed by magma that is created by ocean-continent convergence, a term used for both island arcs and for belts of igneous activity on the edges of continents. Continent-Continent Convergence • Collision of two continents forms a young mountain belt in the interior if a new, larger continent • EX. India and Asia formed the Himalayas Backarc Spreading • Regional extensions in the overlying plate of a subduction zone that are able to tear an arc in two or move two halves in opposite directions • Can split continents • EX.Japan How do mountain ranges form? Orogenies and Plate Convergence Orogeny (Mountain/Formation) – is an episode of mountain building, intense deformation of the rocks in a region. •
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Arc-Continent Convergence o When an island arc collides with a continent it destroys oceans by subduction o Flipping Subduction Zone - Direction of the new subduction is opposite to the direction of the original subduction Continent-Continent Convergence o Mountain belts form when an ocean basin closes and continents collide o EX. Appalachian Mountains o DIAGRAM PAGE 51 Ocean-Continent Convergence
o EX. South America plate overriding the Nazca plate How do plate change over time? Plate Boundaries • Nothing is fixed in plate tectonics (boundaries, subduction, etc.) California’s San Andreas Fault • A transform boundary • Trying to jump inland again Plate • • •
Size Plate’s sizes can change (smaller, larger, etc.) Nazca Plate = Smaller North America Plate = Larger
What • • •
causes plate motions? Convection in the mantle Gravity Movement of mantle rock beneath the plate
Why do plates diverge and sink? 1. Ridge push – as plates move away from a divergent boundary it cools and thickens, plates are pushed apart and slide downhill on the sloping boundary between the lithosphere and asthenosphere 2. Slab-pull – dense leading edge of a subducting plate sinks down into the asthenosphere 3. Trench-suction – plates fall into the mantle at angles steeper than their dip and then trenches and overlying plates are pulled horizontally
How are mantle plumes and hot spots related? Mantle plumes – narrow columns of hot mantle rock that rise through the mantle from thermal boundary layers at the base of the mantle • Plumes are able to move plates • Continental breakup caused by mantle plume (DIAGRAM PAGE 58) • Plumes may form hot spots o Yellowstone o Iceland o Hawaii Seamounts, Guyots, and Aseismic Ridges Seamounts – cone-shaped undersea mountains that rise 1,000m or
more above sea floor (DIAGRAM PAGE 60) • Rocks from seamounts are mostly basalt • Most on crest of mid-oceanic ridge Guyots – flat-topped seamounts found mostly in the western Pacific Ocean • Flat top is because of wave actions • Below sea level (DIAGRAM PAGE 58/60) Aseismic Ridges – submarine ridges that are not associated with earthquake Why is it important to understand plate tectonics? Plate Tectonics Cause: • Volcanoes • Earthquakes • Sea floor • Seamount • Hot spots • Mountain belts • Pangea/Rodinia Canadian Rocky Mountain • Formed by erosion and uplift of rock strata deformed by plate tectonic collisions Mount Yamnuska • Steep cliffs above the thrust are made of Cambrian limestone The Wilson Cycle – Page 54 Plate tectonics and Earthquakes Risk in Southern Ontario – Page 61
crust, crumpling up mountain ranges on their leading edges as they pushed against the oceanic crust (DIAGRAM PAGE 25) Renewed Interest in Continental Drift –
Study of the Sea Floor o Samples of rocks and sediments can be taken from the sea floor by: 1. Rock Dredge – open steel container dragged over the ocean bottom at the end of a cable 2. Corer – a weighted steel pipe dropped vertically into the mud and sand of the ocean floor 3. Sea-floor drilling – drilling a hole in the deep sea floor o Submersibles – small research submarines for geologists o Single-bean echo sounder – measures water depth and draws profiles of submarine topography 1. A sound is sent downward from the ship and bounces off the sea floor and returns to the ship 2. Water depth = amount of time interval o 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
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Geophysical Research EXPLAIN (PAGE 28) o Polar wandering Magnetic poles are located close to the geographic poles Magnetic poles move from year to year o Magnetism of old rocks can be measured to determine the direction and strength of the magnetic field in the past o Paleomagnetism – study of ancient magnetic fields
Recent Evidence for Continental Drift • Refined rock matches between now-separated continents (EX. South America and Africa) • Rocks are similar in type, structure, fossils, age, etc. What is sea-floor spreading? Harry Hess The Sea floor might be moving too like continental drift Sea floor spreading – the sea floor moves away from the mid-oceanic ridge as a result of mantle convection
Subduction – the sliding of the sea floor beneath a continent or island arc Convection – is a circulation pattern driven by the rising of hot material and the sinking of cold material Hot materials have a lower density so it rises Cold materials have a higher density so it sinks Andesitic volcanism – it is produced by the sea floor moving downward into the mantle along a subduction zone. This interaction between the moving sea-floor rock and the stationary rock causes these volcanoes and even earthquakes. • Formed on the edge of a continent or an island arc How old is the sea floor? • Young sea floor is being formed by basalt eruptions at the ridge crest • Basalt is then carried sideways by convection and is subducted into the mantle at an oceanic trench • Old sea floor is being destroyed and new sea floor is being formed What are plates and how do they move? Continental Drift and Sea floor spreading Formed Plate tectonic theory Plate – is a large, mobile slab of rock that is part of Earth’s surface • Made up of sea floor, continental or oceanic rock Lithosphere – rigid outer shell of earth that includes rocks from the crust and the uppermost mantle • Continental lithosphere is thicker (DIAGRAM PAGE 33) Asthenosphere – below the lithosphere, which is a zone of low-seismicwave velocity that behaves plastically because of increased temperature and pressure • Allows the plates to move • Below this is mantle rock Plates made up of sea floor – subducted down into the mantle forming oceanic trenches Plate leading edge made up of continental rock –NOT subduct because continental rock is less dense than oceanic rock Three types of plate boundaries: 1. Divergent plate boundary – plate boundary moving APART 2. Convergent plate boundary – plates moving TOWARDS each other 3. Transform plate boundary – move HORIZONTALLY past each other
How do we know that plates move? Paleomagnetic Evidence Magnetic reversals – changes in the polarity of the magnetic field, when the north and south magnetic pole exchange places Normal polarity – magnetic lines flow from the South Pole to the North Pole and our compass needles point to the north Reversed polarity – magnetic lines run from North Pole to South Pole and our compass needles point to the South Paleomagnetism – study of magnetic fields recorded in rocks helps us understand the magnetic fields in the past Magnetic polarity time scale – this is constructed by using stacked continental lava flow to record the pattern of magnetic reversals over time Magnetometer – an instrument that measures the strength of the Earth’s magnetic field. It is taken over land surface or sea.
Marine Magnetic Anomalies • •
Magnetic anomalies on the sea floor are arranged in bands that lie parallel to the rift valley of the mid-oceanic ridge Positive and negative anomalies form a stripelike pattern parallel to the ridge crest
The Morley-Wine-Matthews Hypothesis • The pattern of magnetic anomalies at sea matches the pattern of magnetic reversals of lava flows on the continents 1. Opening of cracks within the rift valley on the mid-oceanic ridge crest 2. They are filled by basaltic magma 3. Cools and forms dikes i. Cools ii. Magnetized iii. Forms new dikes How fast do plates move? • On each side of the ridge the sea floor is moving away from the ridge crest 1cm to 10cm per year Predicting Sea-Floor Age • Predict the age of the igneous rock of the sea floor by measuring the magnetic anomalies at the sea surface o Matching the measured anomaly pattern with the known
pattern of anomalies Another Test: Fracture Zones and Transform Faults • Studying the seismicity of fracture zones • Transform Fault – the portion of a fracture zone between two offset portions of ridge crest o Earthquakes o Rock is moving away from the ridge crest What happens at plate boundaries? Divergent Plate Boundaries – plates move AWAY • To create and open new ocean basin • Found in middle of a continent or ocean 1. Rifting – continental crust is stretched and thinned 2. Basaltic volcanism – (Graben – down-dropped fault block that acts as a path way for basaltic magma) 3. Uplift – caused by the upwelling of hot mantle beneath the crust 4. Crust separates and creates an ocean 5. Plates continue to diverge and widening the sea 6. Thermal uplift – creates a mid-oceanic ridge EX. The Red River (occurred), Atlantic Ocean (happening) Mid-Oceanic Ridges Mid Ocean Ridges – giant undersea mountain ranges that extend around the world Rift Valley – runs down the crest of each ridge (Absent in the Pacific Ocean) Geologic Activity on Ridges READ OVER Shallow-focus earthquakes – below the sea floor Basalt eruptions – occurs in and near the rift valley on ridge crests Transform Boundaries – plate’s slide HORIZONTALLY • Shallow-focus earthquakes • Transform faults means that the fault ends or transform into another kind of displacement o Divergent o Convergent o Trench to Ridge Convergent Plate Boundaries – plates move TOWARD each other Three situations of convergence: 1. Oceanic crust vs. Oceanic crust – one plate subducts under the other
2. Oceanic crust vs. Continental crust – Dense Oceanic crust subducts under the continental plate 3. Continental crust vs. Continental crust – both plate collide and crumple Ocean-Ocean Convergence Oceanic trench – is a narrow deep trough parallel to the edge of a continent or an island arc (around many continents) Island Arc – magma erupts, a curved line of volcanoes that form a string of islands parallel to the oceanic trench • Trench position change with time Ocean-Continent Convergence Magmatic Arc – formed by magma that is created by ocean-continent convergence, a term used for both island arcs and for belts of igneous activity on the edges of continents. Continent-Continent Convergence • Collision of two continents forms a young mountain belt in the interior if a new, larger continent • EX. India and Asia formed the Himalayas Backarc Spreading • Regional extensions in the overlying plate of a subduction zone that are able to tear an arc in two or move two halves in opposite directions • Can split continents • EX.Japan How do mountain ranges form? Orogenies and Plate Convergence Orogeny (Mountain/Formation) – is an episode of mountain building, intense deformation of the rocks in a region. •
•
•
Arc-Continent Convergence o When an island arc collides with a continent it destroys oceans by subduction o Flipping Subduction Zone - Direction of the new subduction is opposite to the direction of the original subduction Continent-Continent Convergence o Mountain belts form when an ocean basin closes and continents collide o EX. Appalachian Mountains o DIAGRAM PAGE 51 Ocean-Continent Convergence
o EX. South America plate overriding the Nazca plate How do plate change over time? Plate Boundaries • Nothing is fixed in plate tectonics (boundaries, subduction, etc.) California’s San Andreas Fault • A transform boundary • Trying to jump inland again Plate • • •
Size Plate’s sizes can change (smaller, larger, etc.) Nazca Plate = Smaller North America Plate = Larger
What • • •
causes plate motions? Convection in the mantle Gravity Movement of mantle rock beneath the plate
Why do plates diverge and sink? 1. Ridge push – as plates move away from a divergent boundary it cools and thickens, plates are pushed apart and slide downhill on the sloping boundary between the lithosphere and asthenosphere 2. Slab-pull – dense leading edge of a subducting plate sinks down into the asthenosphere 3. Trench-suction – plates fall into the mantle at angles steeper than their dip and then trenches and overlying plates are pulled horizontally
How are mantle plumes and hot spots related? Mantle plumes – narrow columns of hot mantle rock that rise through the mantle from thermal boundary layers at the base of the mantle • Plumes are able to move plates • Continental breakup caused by mantle plume (DIAGRAM PAGE 58) • Plumes may form hot spots o Yellowstone o Iceland o Hawaii Seamounts, Guyots, and Aseismic Ridges Seamounts – cone-shaped undersea mountains that rise 1,000m or
more above sea floor (DIAGRAM PAGE 60) • Rocks from seamounts are mostly basalt • Most on crest of mid-oceanic ridge Guyots – flat-topped seamounts found mostly in the western Pacific Ocean • Flat top is because of wave actions • Below sea level (DIAGRAM PAGE 58/60) Aseismic Ridges – submarine ridges that are not associated with earthquake Why is it important to understand plate tectonics? Plate Tectonics Cause: • Volcanoes • Earthquakes • Sea floor • Seamount • Hot spots • Mountain belts • Pangea/Rodinia Canadian Rocky Mountain • Formed by erosion and uplift of rock strata deformed by plate tectonic collisions Mount Yamnuska • Steep cliffs above the thrust are made of Cambrian limestone The Wilson Cycle – Page 54 Plate tectonics and Earthquakes Risk in Southern Ontario – Page 61
