WK9DISCUSSION8013 Education homework help
Methodology is the very specific process a researcher uses to conduct research. Certain steps and terms are important to all capstones. For this course, the most important issue is to ensure that the methodology is aligned with the problem statement, purpose, and, especially, the research question.
Plate Tectonics:
A Scientific
Revolution Unfolds
Chapter 5 Lecture
Natalie Bursztyn
Utah State University
Foundations of Earth Science
Eighth Edition
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1
Summarize the view that most geologists held prior to the 1960s regarding the geographic positions of the ocean basins and continents.
Focus Question 5.1
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Before 1960 geologists saw the positions of ocean basins and continents as fixed
Continental drift suggested but not agreeable
A new model of tectonic processes
A scientific revolution
Tectonic processes deform crust and create major structural features
From Continental Drift to Plate Tectonics
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List and explain the evidence Wegener presented to support his continental drift hypothesis.
Focus Question 5.2
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World maps in the 1600s suggested that South America and Africa fit together
In 1915, Alfred Wegener outlined the hypothesis of continental drift
Single supercontinent of all of Earth’s land: Pangaea
Fragmented ~200 mya and smaller landmasses drifted to their present positions
Continental Drift: An Idea Before Its Time
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Continental Drift: An Idea Before Its Time
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Similarity between coastlines on opposite sides of the Atlantic
Opponents argued that coastlines are modified through time by erosion and deposition
Continental shelf is a better approximation of the boundary of a continent
Evidence: The Continental Jigsaw Puzzle
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Identical fossils found in South America and Africa
Paleontologists agree: land connection necessary to explain fossil record
Evidence: Fossils Matching Across the Seas
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Evidence: Fossils Matching Across the Seas
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Mesosaurus
Small Permian aquatic freshwater reptile
Found in eastern South America and western Africa
Glossopteris
Seed fern
Africa, Australia, India, South America, and Antarctica
Opponents explain fossil patterns by rafting, oceanic land bridges, and island stepping stones
Evidence: Fossils Matching Across the Seas
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Evidence: Fossils Matching Across the Seas
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Rock types and geologic features match up
2.2 billion-year-old igneous rocks in Brazil and Africa
Mountain belts end at coastlines and reappear across oceans
Evidence: Rock Types and Geologic Features
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Evidence: Rock Types and Geologic Features
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Evidence for glaciation on continents now at tropical latitudes
Can be explained by supercontinent located near the South Pole
Evidence: Ancient Climates
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Wegener’s hypothesis of continental drift was met with criticism
Objections were based on lack of mechanism for continental drift
Wegener proposed that tidal forces moved continents and that sturdy continents broke through thin oceanic crust
The Great Debate
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List the major differences between Earth’s lithosphere and asthenosphere.
Explain the importance of each in the plate tectonics theory.
Focus Questions 5.3
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Oceanographic exploration increased dramatically following World War II
Discovery of global oceanic ridge system
Earthquakes at great depths in western Pacific ocean trenches
No oceanic crust older than 180 million years
Thin sediment accumulations in deep-ocean basins
Developments led to theory of plate tectonics
More encompassing theory than continental drift
The Theory of Plate Tectonics
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Lithosphere is the crust and uppermost (coolest) mantle
Oceanic lithosphere varies in thickness
Thin at ridges, up to 100 km thick in deep-ocean basins
Mafic composition
More dense than continental lithosphere
Continental lithosphere 150–200 km thick
Felsic composition
Responds to forces by bending or breaking
Rigid Lithosphere Overlies Weak
Asthenosphere
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Asthenosphere is the hotter, weaker mantle below the lithosphere
Rocks are nearly melted at this temperature and pressure
Responds to forces by flowing
Moves independently from lithosphere
Rigid Lithosphere Overlies Weak
Asthenosphere
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Rigid Lithosphere Overlies Weak
Asthenosphere
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Lithosphere is broken into irregular plates
Plates move as rigid units relative to other plates
7 major plates make up 94% of Earth
Earth’s Major Plates
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Earth’s Major Plates
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Interaction between plates at plate boundaries
Divergent boundaries (constructive margins)
Two plates move apart
Upwelling of hot material from mantle creates new seafloor
Convergent boundaries (destructive margins)
Two plates move together
Oceanic lithosphere descends and is reabsorbed into mantle
Two continental blocks create a mountain belt
Transform plate boundaries (conservative margins)
Two plates slide past each other
No lithosphere is created or destroyed
Plate Movement
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Plate Movement
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Sketch and describe the movement along a divergent plate boundary that results in the formation of new oceanic lithosphere.
Focus Question 5.4
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Most divergent plate boundaries are along the crests of oceanic ridges
New ocean floor is generated when mantle fills narrow fractures in oceanic crust
Also called spreading centers
Divergent Plate Boundaries and Seafloor Spreading
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Divergent Plate Boundaries and Seafloor Spreading
[insert Figure 5.11 here]
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Most divergent plate boundaries are associated with oceanic ridges
Elevated seafloor with high heat flow and volcanism
Longest topographic feature on Earth’s surface (covers 20% of surface)
Crest is 2 to 3 km higher than adjacent basin and can be 1000 to 4000 km wide
Rift valley is a deep canyon along the crest of a ridge resulting from tensional forces
Oceanic Ridges and Seafloor Spreading
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Oceanic Ridges and Seafloor Spreading
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Seafloor spreading is the process by which new seafloor is created along the ocean ridge system
Average spreading rate is ~5 cm/year
Up to 15 cm/year or as slow as 2 cm/year
New lithosphere is hot (less dense) but cools and subsides with age and distance from the ridge system
Thickness is dependent upon age
Oceanic Ridges and Seafloor Spreading
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Continental rifting occurs when divergent boundaries develop within a continent
Tensional forces stretch and thin the lithosphere
Brittle crust breaks into large blocks
Eventually become ocean basins
East African Rift demonstrates initial stage
Continental Rifting
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Divergent Plate Boundaries and Seafloor Spreading
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Compare and contrast the three types of convergent plate boundaries.
Name a location where each type can be found.
Focus Questions 5.5
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Convergent plate boundaries occur when two plates move toward each other
Convergence rate is equal to seafloor spreading
Characteristics vary depending on subducting crust
Subduction zones
Lithosphere descends into the mantle
Old oceanic crust is ~2% denser than asthenosphere
Continental crust less dense than asthenosphere
Convergent Plate Boundaries and Subduction
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Deep ocean trenches
Long, linear depressions
Result of subduction
Angle of subduction varies
Nearly flat to nearly vertical
Depends on density of crust
Older crust is cooler and denser
Convergent Plate Boundaries and Subduction
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Characteristics of convergent plate boundaries vary depending on type of crust being subducted
Oceanic + continental
Oceanic + oceanic
Continental + continental
Convergent Plate Boundaries and Subduction
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Convergent Plate Boundaries and Subduction
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Oceanic lithosphere + continental lithosphere = subduction of oceanic lithosphere
Continental lithosphere is less dense
Water from descending oceanic crust triggers partial melting of asthenosphere at ~100 km
Molten material is less dense and rises
Continental volcanic arcs
Oceanic-Continental Convergence
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Oceanic-Continental Convergence
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One slab subducts under another at
oceanic-oceanic convergent boundaries
Volcanism because of partial melting
Generates volcanic island arcs
Volcanic cones underlain by oceanic crust
Oceanic-Oceanic Convergence
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Oceanic-Oceanic Continental Crust
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Continental crust is buoyant
Neither plate subducts during continent-continent collisions
Folding and deformation of rocks
Mountain building
Continental-Continental Convergence
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Continental-Continental Convergence
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Describe the relative motion along a transform fault boundary.
Be able to locate several examples on a plate boundary map.
Focus Questions 5.6
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Transform plate boundaries form when two plates slide horizontally past one another
Transform faults
No lithosphere is produced or destroyed
Connect spreading centers and offsets oceanic ridges
Linear breaks in the seafloor are fracture zones
Fracture zones are inactive
Active faults occur between offset ridge segments
Transform Plate Boundaries
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Transform Plate Boundaries
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Transport oceanic crust to destruction site
Transform Plate Boundaries
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Few transform faults cut through continental crust
San Andreas Fault (California) and Alpine Fault (New Zealand) are exceptions
Transform Plate Boundaries
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Explain why plates such as the African and Antarctic plates are increasing in size, while the Pacific plate is decreasing in size.
Focus Question 5.7
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Total surface area of Earth is constant
Size and shape of individual plates changes
African and Antarctic Plates are growing
Surrounded by divergent boundaries
Pacific Plate is being consumed
Surrounded by convergent boundaries)
Plate boundaries move and change through time
How Do Plates and Plate Boundaries Change?
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New ocean basins were created during the breakup of Pangaea
The Breakup of Pangaea
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The Breakup of Pangaea
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Present plate motions can be used to predict future continental positions
Plate Tectonics in the Future
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53
List and explain the evidence used to support the plate tectonics theory.
Focus Question 5.8
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Evidence from Deep Sea Drilling Project
Collect sediment and oceanic crust
Date fossils in sediment
Sediment age increases with distance from ridge
Sediment is thicker with increased distance from the ridge
Oldest seafloor is 180 million years old
Evidence: Ocean Drilling
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Testing the Plate Tectonics Model
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Volcanoes in the Hawaiian Island-Emperor Seamount Chain increase in age with distance from the Big Island of Hawaii
A cylinder of upwelling hot rock (mantle plume) is beneath Hawaii
A hot spot is an area of volcanism, high heat flow, and crustal uplift above a mantle plume
A hot-spot track formed as the Pacific Plate moved over the hot spot
Evidence: Mantle Plumes and Hot Spots
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Testing the Plate Tectonics Model
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Today North and South magnetic poles align approximately with geographic North and South poles
Iron-rich minerals influenced by magnetic pole
Basalt erupts above the curie temperature, so magnetite grains are nonmagnetic
Grains align to magnetic field during cooling
Rocks preserve a record of the direction of magnetic poles at the time of formation
Paleomagnetism or fossil magnetism
Position of paleomagnetic poles appears to change through time because of continental drift
Evidence: Paleomagnetism
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Evidence: Paleomagnetism
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Evidence: Paleomagnetism
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Magnetic field reverses polarity during a magnetic reversal
Rocks with same magnetic field as today have normal polarity
Rocks with opposite magnetism have reverse polarity
Polarity of lava flows with radiometric ages was used to generate a magnetic time scale
Divided into chrons ~1 million years long
Finer-scale reversals within each chron
Vine and Matthews (1963) suggested stripes of normal and reverse polarity are evidence of seafloor spreading
Evidence: Paleomagnetism
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Evidence: Paleomagnetism
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Evidence: Paleomagnetism
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Evidence: Paleomagnetism
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Describe plate-mantle convection.
Explain two of the primary driving forces of plate motion.
Focus Questions 5.9
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Mantle is solid, but hot and weak enough to flow
Convection occurs as hot, less dense material rises and surface material cools and sinks
During slab pull, cold, dense oceanic crust sinks because it is denser than the asthenosphere
During ridge push, gravity causes lithospheric slabs to slide down the ridge
Drag in the mantle also affects plate motion
Forces That Drive Plate Motion
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Forces That Drive Plate Motion
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Forces That Drive Plate Motion
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Mantle convective flow drives plate motion
Subducting plates drive downward component of convection
Upwelling of hot rock at oceanic ridges drives upward component of convection
Convective flow is the heat transfer mechanism from Earth’s interior
Models of Plate-Mantle Convection
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Multiple models for convective flow:
Whole-mantle convection
Cold oceanic lithosphere sinks and stirs entire mantle
Subducting slabs sink to core-mantle boundary
Balanced by buoyant mantle plumes
Layer cake model
Thin, dynamic layer in upper mantle
Thick, larger, sluggish layer below
Subducting slabs do not sink past 1000 km
Models of Plate-Mantle Convection
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Models of Plate-Mantle Convection
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