WK9DISCUSSION8013 Education homework help

WK9DISCUSSION8013 Education homework help

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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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