Chapter 2 Flashcards
Plate Tectonics
What was Wegener’s continental drift hypothesis? What was his evidence? Why didn’t other geologists agree?
continents once formed a single landmass (Pangaea) and drifted apart. His evidence included matching coastlines, fossil distribution, and similar rock formations across continents. Other geologists rejected it because he couldn’t explain the mechanism driving the movement.
How do apparent polar-wander paths show that the continents, rather than the poles, have moved?
Apparent polar-wander paths from different continents don’t align, implying that the continents have moved relative to the poles and to each other over time, rather than the poles themselves shifting.
Describe the hypothesis of sea-floor spreading.
Sea-floor spreading suggests that new oceanic crust forms at mid-ocean ridges and spreads outwards. This process drives the movement of oceanic plates and contributes to continental drift.
Describe the pattern of marine magnetic anomalies across a mid-ocean ridge. How is this pattern explained?
Marine magnetic anomalies form symmetric stripes of normal and reversed polarity on either side of a mid-ocean ridge. This pattern is explained by Earth’s magnetic field reversing over time as new crust forms.
How did drilling into the sea floor contribute further proof of sea-floor spreading
Drilling revealed that the age of oceanic crust increases with distance from the ridge, supporting sea-floor spreading
How did the sea-floor-spreading hypothesis explain variations in ocean floor heat flow?
Drilling revealed that the age of oceanic crust increases with distance from the ridge, supporting sea-floor spreading. The hypothesis explains that heat flow is highest near ridges where new crust is forming.
What are the characteristics of a lithosphere plate? Can a single plate include both continental and oceanic lithosphere?
Lithosphere plates are rigid and composed of both crust and the upper mantle. A single plate can include both continental and oceanic lithosphere.
How does oceanic lithosphere differ from continental lithosphere in thickness, composition, and density?
Oceanic lithosphere is thinner, denser, and primarily basaltic, while continental lithosphere is thicker, less dense, and primarily granitic.
How do we identify a plate boundary?
Plate boundaries are identified by the presence of seismic activity, volcanic activity, and distinct geological features like mid-ocean ridges, subduction zones, and transform faults.
Describe Divergent Plate Boundary
Plates move apart (mid-ocean ridges).
Describe Convergent Plate Boundary
Plates move towards each other (subduction zones, mountain formation).
Describe Transform Plate Boundary
Plates slide past each other (fault lines).
How does crust form along a mid-ocean ridge?
New oceanic crust forms as magma rises from the mantle at divergent boundaries, cools, and solidifies as it moves away from the ridge.
Why is the oldest oceanic lithosphere less than 200 Ma?
Oceanic lithosphere is continually recycled into the mantle at subduction zones, preventing it from becoming older than 200 million years.
Describe the major features of a convergent boundary.
Features include subduction zones, deep ocean trenches, volcanic arcs, and mountain ranges, where one plate is forced beneath another.
Why are transform plate boundaries required on an Earth with spreading and subducting plate boundaries?
Transform boundaries accommodate the horizontal motion between divergent and convergent boundaries, allowing plates to slide past each other.
What is a triple junction?
A triple junction is a point where three plate boundaries meet. Depending on the types of boundaries involved, it can be stable or unstable.
How is a hot-spot track produced, and how can hot-spot tracks be used to track the past motions of a plate?
A hot-spot track forms as a tectonic plate moves over a stationary mantle plume. The track records the direction and speed of the plate’s movement.
Describe the characteristics of a continental rift and give examples of where this process is occurring today.
A continental rift is where a continent is stretching and breaking apart, forming a rift valley. Examples include the East African Rift and the Basin and Range Province in the western U.S.
Describe the process of continental collision and give examples of where this process has occurred.
Continental collision occurs when two continental plates converge, causing the crust to thicken and form mountain ranges. An example is the collision of the Indian and Eurasian plates, forming the Himalayas.
Discuss the major forces that move lithosphere plates.
Major forces include slab pull, ridge push, and mantle convection. Slab pull occurs as a dense plate sinks into the mantle, while ridge push results from gravity acting on the elevated mid-ocean ridges.
Explain the difference between relative plate velocity and absolute plate velocity.
Relative plate velocity measures the movement of one plate relative to another, while absolute plate velocity measures a plate’s movement relative to a fixed point, like the mantle or a hotspot.
Why are the marine magnetic anomalies bordering the East Pacific Rise in the Pacific Ocean wider than those bordering the Mid-Atlantic Ridge?
The East Pacific Rise has a faster spreading rate, which creates wider magnetic anomaly stripes compared to the slower spreading Mid-Atlantic Ridge.
The North Atlantic Ocean is 3,600 km wide. Sea-floor spreading along the Mid-Atlantic Ridge occurs at 2 cm per year. When did rifting start to open the Atlantic?
The Atlantic began opening approximately 180 million years ago (3,600,000 meters ÷ 0.02 meters/year).
Mid-Ocean Ridges:
decompression of the mantle leads to partial melting, generating basaltic magmas and underneath that there’s sheeted dikes which are feeders to the volcanic eruptions
Plutonic Rocks vs Volcanic Rocks
Volcanic rocks erupt and crystallize on the surface of the Earth and plutonic rocks crystallize more slowly below the surface (magmas cooled below surface)
Full Spreading Rate
How fast two points on either side are moving away from each other;; hot, high melt supply
Half Spreading rate
rate of the new crust formation on one side of the ridge, cooler, lower melt supply
Convergent:
two plates, one of which is oceanic, move toward one another except the oceanic plate sinks down (subduction)
Wadati-Benioff Zone
band of earthquakes in a downgoing plate
Oceanic-Oceanic Convergent
Leads to the formation of island arcs (the pacific plate)
The Aleutians, Western Pacific
Oceanic Continental Convergent
always subducted at ocean-continent convergent boundaries; Cascade Volcanoes
Continental Continental Convergent
Lead to very thick crust, generate large mountain ranges
Formation of the Himalayas
Continental Crust
thicker crust, thicker lithosphere, less dense, older on average, more complex
Oceanic Crust
thinner crust, thinner lithosphere, more dense, younger, less complex
What are the general features of Subduction Zones?
Rising magma, partial melting near this in the asthenosphere. The lithosphere has a downgoing plate and an overriding plate. Then rising magma hits the crust and volcanic arcs form. From the oceanic side, trenches form.
What are the geologic features of Convergent Plate Boundary
Deep Earthquakes, Seismic Tomography of Subducting Slabs, Volcanic Arc
Volcanic Arc Formation
forms where oceanic plate subducts beneath the continental lithosphere then the volcanoes grow on the continent and form a continental volcanic arc
Why do Slabs Subduct?
they’re denser than the mantle because Mantle Lithosphere is colder and denser than mantle asthenosphere AND Slabs become denser with depth due to the growth of metamorphic minerals
what is an accretionary prism
a geological structure that forms at a convergent plate boundary. It is created when sediments and rocks from the ocean floor are scraped off the subducting plate and accumulate in a wedge-shaped mass at the edge of the overriding plate.
Mantle Melting Subduction Zones:
Mantle Melting due to the release of water from the slab; Magmas rise up to form overlying volcanoes. The Ring of Fire: Subduction zones around the Pacific lead to earthquakes and volcanism
Transform Plate Boundaries
ocean ridges exist in short segments that are offset laterally from each other called fracture zones
Fracture Zone
linear feature on the ocean floor that represents a fault or series of faults where tectonic plates have moved past each other. These zones typically occur perpendicular to mid-ocean ridges accomodating the horizontal movement of tectonic plates.
Transform Boundary (transform fault)
actively slipping segment of a fracture zone between two ridge segments. one plate slides sideways past another but no new plate forms and no old plate is consumed
Hot Spot Volcanoes
isolated points that are not a consequence of movement at a plate boundary
How are hotspot volcanoes formed?
active ones occur at the end of a chain of dead volcanic islands and seamounts. Heat sources for hot spots are mantle plumes, a column of very hot rock rising up through the mantle to the base of the lithosphere.
What happens during Continental Rifting?
linear belt in which continental lithosphere pulls apart. Lithosphere stretches horizontally and thins vertically and as continental lithosphere thins, the hot asthenosphere rises beneath the rift and starts to melt. Eruption of the molten rock produces volcanoes along the rift
Collision
process which two buoyant pieces of lithosphere converge and squeeze together
What Drives Plate Motion
model in which convection, ridge push, and slab pull all contribute to driving plates; it isn’t just a conveyor belt like system; Convection involvement
What is Convection?
Process of heat transfer in fluids: warmer, less dense regions of a fluid rise while cooler, denser regions sink, creating a circular motion known as a convection current.
Ridge-push Force
develops simply because the lithosphere of mid-ocean ridges lies at a higher elevation than that of the adjacent abyssal plains
Slab-pull force
force that subducting down-going plates apply to oceanic lithosphere at a convergent margin, arises simply because the lithosphere that was formed around 10 Ma is denser than the asthenosphere so it can sink into the asthenosphere
Relative Plate Velocity
refers to describing the movement of late A with respect to plate B
Absolute Plate Velocity
describing the movement of both plates relative to a fixed location in the mantle below the plates
Mantle Plume
an upwelling of abnormally hot rock within the Earth’s mantle. These are believed to originate deep within the mantle, potentially from the core-mantle boundary, and rise towards the Earth’s crust. They can lead to volcanic activity and the formation of large igneous provinces.
