L5: Plate Tectonics Flashcards
What is seafloor spreading?
A theory proposed during the 60s in which suggesting lateral movement of the oceanic crust away from mid-ocean ridges
What occurs during seafloor spreading?
- The release of pressure at mid-ocean ridges as the plates move apart, which allows hot magma from the asthenosphere to reach the ocean floor.
- There it cools and crystallizes to form new lithosphere
- The system of mid-ocean ridges along divergent plate boundaries forms linear submarine mountain chains
- Youngest rocks are located closest to the ocean ridges; progressively older as you travel further
Divergent plate margins.
- Fractures in the lithosphere where 2 plates move apart
- AKA spreading centers
- Can occur in plates capped by either continental or oceanic crust
Convergent plate margins
- Occur where 2 plates are moving towards each other
- Can occur in oceanic, continental, or both
- Because of their very different properties, the results of the convergence will be fundamentally different
Transform fault plate margins.
Fractures in the lithosphere where 2 plates slide past each other, grinding and abrading their edges as they do so. Can occur in both oceanic or in continental crust
Ocean crust is _____ than continental crust, hence the reason for very old ________.
- Stable
- Continents
Define continental drift.
Slow movement of continents across the face of earth
Define continental-rift zone.
Rifting and spreading zones on continents are characterized by rift valley, volcanism, and earthquakes
What is continental rifting?
Mantle material also rises from near the core-mantle boundary in the form of thermal plumes
What are thermal plumes?
- Occurs in continental rifting
- When a plume nears the lithosphere with overlying continent, it dreads laterally, doming the overlying plate and moving the rifted segments outward from the central area
- Uplifts results in fractures that eventually can open up to form narrow oceanic tracts
Describe the process rifting/rifting Africa (?)
The dissection of a continent and potential birth of a plate
Huge continental masses act as thermal blankets; they slowly heat up and expand
Notes bc that shit LONG
What is the Wilson Cycle?
- The opening of new ocean basins along divergent zones, the expansion of the basin as seafloor spreading continues, and the ultimate closure of the basin as plates converge
- Occurs bc spreading is not continuous; eventually needs to close
List the forms of plate margins.
- Divergent boundaries
- Convergent boundaries
- Transform boundaries
- Hot spots
Define ocean-continent boundaries.
- Continental lithosphere is relatively low in density and buoyant
- Fe rich mantle
Describe the process of ocean-continent boundaries.
Oceanic lithosphere is more similar in density to the underlying asthenosphere, so it’s easily forced under the continental material when the plates converge. (subduction)
When _____ lithosphere meets _______ lithosphere, the _______ lithosphere is subducted, and a _______ ________ belt is formed at the __________ __________.
- Oceanic
- Continental
- Oceanic
- Volcanic mount
- Continental margin
What is subduction?
When 2 plates capped by oceanic crust converge, one of the plates will undergo subduction and descend into the mantel underneath the other plate; the sinking of old, cold oceanic lithosphere into the asthenosphere
What happens to mid-ocean ridges created in new lithosphere?
- Subduction zones of the world balance seafloor creation, so these ridges would be consumed in subduction
- Buoyant continents cannot be subducted, so they’re preserved hence the old pieces of crust on each continent.
How does subduction work?
- The subducted plate is heated, melted, and re-incorporated into the asthenosphere
- Volcanoes form where the melted material rises up through the overlying plate to the surface
- Meanwhile the continental crust is compressed, folded, and faulted leading to mountains forming
Describe ocean-ocean convergence.
- Where oceanic lithosphere meets oceanic lithosphere
- On plate gets subducted under the other
- A deep sea trench and a volcanic island arc are formed
- The older, more dense plate sinks below the younger, less dense plate, which leads to the formation of “island arcs”
Continental Shield
Assemblage of ancient cratons and orogens
What is a supercontinent?
The assemblage of cratons into large continental complexes
Define icostasy
- The property whereby the lithosphere maintains floatation balance
- This means that his mass is stable w/ respect to the underlying asthenosphere and is neither rising nor sinking
- Icostasy is what prevents “ from sinking
Define mid-ocean ridge transform fault.
Mid-ocean ridges are typically offset by transform faults
Define continental transform fault.
At the transform faults, plates slip horizontally past each other
What occurs at a spreading ridge?
- The rift is not a long continuous crack
- Actually forms numerous short segments where 2 opposite plates scrape past each other
- These transform faults have a lot of stress leading to earthquakes
What are hot spots?
- Arise when isolated plumes of magma (smalled versions of those in rifting) rise from deep within the asthenosphere to the surface
- These plumes are thought to be fixed in the mantle
- As plates move over it there becomes a progressive trail of volcanoes at the Earths surface
Fill out the table
Notes
Orogens
- Bands of mountains
- Younger along current active margins
- Ancient that are remnant mountains (often just the roots), some welding cratons together
Craton
The oldest part of a continent, formed, evolved, and altered over billions of years (stable)
How are mountain belts formed?
- Collision results in large zones of deformation where continental crust is involved in subduction
- Typically form high mountain ranges (orogens) causing crust to thicken = rocks within orogens are crumpled (deformed)
- Plates are not perfectly rigid!
Continent-continent convergence.
- Where 2 continents converge, the crust crumples and thickens, creating high mountains and a wide plateau
- 2 landmasses come together to form a mountain range bc both plates are too buoyant to sink into the mantle
- @ buoyant plates lead to the thickening of the crust and the resultant formation of a mountain chain
List convergent boundaries.
- Ocean-ocean convergence
- Ocean-continent
