CHPTR 11/12 Plate Tectonics Flashcards
youngest part of the Earth
ocean floors
plumb line
rock on string - earliest method to map sea floor
old sonar
map time of the sonar wave to reach the bottom and then bounce back
modern sonar
multibeam - sonar obtains a profile of a narrow section of seafloor every few seconds
altimeter + how it works
measure the variation on sea surface elevation
mountains under sea will have extra gravitational pull and water would mound above it (anomaly)
geoid
round shape that models the Earth’s mean sea level and defines zero elevation.
Irregular surface
geoid low
mass deficit (valleys)
geoid high
excess mass (mountain)
continental margins + types
transition from continental crust to oceanic
passive and active
passive margin
change from continental crust to oceanic crust on same tectonic plate
active margin
plate boundary where one plate subduct below another
(change from continental crust to oceanic crust on different tectonic plates)
deep ocean basin
- trenches: formed by subduction
- abyssal plains: between trench and mid ocean ridge
- seamounts: extinct volcanoes
Parts of a passive continental margin - in order of increasing depth
continental shelf
continental slope
continental rise
(then the abyssal plane but thats not part of the margin)
continental shelf
flooded extension of the continental crust
continental slope
boundary between continental and oceanic crust
continental slope angle
about 5 degrees
continental rise + formation
accumulated sediment at base of continental slope
formed by rifting as continents split apart
continental rise angle
about 0.3 degrees
deep sea fan definition and location
turbidity current produce turbidite deposits
forms in the continental rise
accretionary wedge
wedge formed as the top plate scrapes sediment off of the subducting plate
active continental margin trench
trench between the subducting crust and the continental crust
not always present - full of sediment
active continental margin order from greatest depth
oceanic crust
accretionary wedge
continental crust
abyssal plain
large flat region of the ocean floor
mid-ocean ridge
location where the plate moves apart; creates new ocean floor
(divergent plate boundary)
What percentage of the Earth is made up of mid-ocean ridges
20%
origin of mid-ocean ridges
volcanic
sea-floor spreading
the divergent plates move apart
magma rises to meet the water
the magma cools and forms new seafloor
what rocks are formed at mid-ocean ridges
intrusive gabbro
extrusive basalt
sheeted dikes location
formed at mid ocean ridges
rift zone
a deep rift valley where magma from beneath the Earth’s crust wells up, cools, and is pushed away from the ridge’s flanks
unique (dynamic) features of mid-ocean ridges (5)
- frequent Earthquakes
- hydrothermal circulation
- formation of mineral deposits
- volcanic eruptions
- sulpher based ecosystems
what controls the sea water chemistry at mid ocean ridges
hydrothermal circulation
ophiolite
when ocean basin closes, oceanic crust gets scraped off and placed on land
moho
boundary between Earth’s crust and its mantle
Explain the magnetic anomaly present at mid ocean ridges
- rocks become magnetized when it cools from magma to lava
- It is magnetized to the magnetic field that was present at the time
- Earth’s magnetic field reverses N/S periodically
- You end up with stripes of alternating magnetism at mid ocean ridges
atolls formation
- volcano sinks as plates moves away from the mid-ocean ridge + loses its source of magma
- coral reefs form on the volcano
- volcano continues to sink but reef keeps growing; replacing where the volcano initially was
- this leaves a lagoon surrounded by coral
Why does the seafloor get deeper as you move away from mid-ocean ridges
The plates cool and contract
VMS
volcanogenic massive sulphide deposit; important mineral formed at mid-ocean ridges
terrigenous sediments
sand sized particles that originated on the continents but are now sea-floor sediments
biogenous sediments
shells and skeletons of plankton living near the surface of the ocean but are now sea-floor sediments
manganese nodules
form on abyssal plains; economically valuable
Wegener proof the Pangea once existed (4)
- fit of coastlines
- fossil distribution
- ancient mountain belt locations
- past glaciation pattern
Where do coastlines fit best
at the continental shelf
What did Wegener think of land bridges + other name
stupid. Isthmian links where used to explain how similar fossils could be on continents separated by the ocean
tillites
distinct rock type formed by glacier movement
Wegener’s proof from glaciation patterns
- tillites found in tropical climates
- scratches showed glaciers moving away from coastline (impossible)
- You could not argue glaciers were flowing from other locations because glaciers do not flow across oceans
Polar fleeing
Wegener’s original mechanism for the movement of the continents
Centrifugal forces were responsible.
It was wrong and the reason his theory was initially abandoned
What years was Wegener’s theory abandoned
1930s
What years was Wegener’s theory revived and by what
1950s
exploration of ocean floor
paleomegnetism
paleomagnetism
“fossil magnetism”
Explain how paleomagnetism supported Wegener’s theory
Rocks become magnetized as they solidify.
You get different angles of magnetism at different location.
The angle of magnetism did not match what the expected angle was.
Curie Temperature
If above; rocks are no longer magnetized
If below; rocks are magnetic
580 degrees C
What angle will a compass be at the North Pole
vertical
what angle will a compass be at the Equator
Horizontal
changing pole theory
the theory that the reason rocks where having magnetism directions that did not match their location was because of a changing magnetic pole
polar wandering path support for Wegener
- look at rocks with diff. ages + see they travel along a path
- path is different at every continent (disproves changing pole theory)
- if was allow continents to move, the pole does not change (proves moving plates)
Seafloor Spreading hypothesis
plates move apart at ridges
upwelling mantle creates new seafloor
trenches are where material sank back into the interior
This cycle is mantle convection
Harry Hess
proposed and found proof for the seafloor spreading hypothesis
found mid-ocean ridges
geomagnetic reversal
Earth’s magnetic direction changes chaotically
How does geomagnetic reversal support Wegener’s theory
- magnetic patterns were measured across sea floor
- they had a strong/weak pattern
- both sides of a mid-ocean ridge were a symmetric patter (evidence for sea floor spreading)
When was the last geomagnetic reversal
780 000 years
Why did researches find that the seafloor has a strong/weak magnetic pattern
Rocks with normal polarity appeared as strong magnetic intensity
Rocks with reverse polarity appeared as weak magnetic intensity (formed when the Earth’s magnetism was flipped)
Expanding Earth Theory
Incorrect. Earth is getting bigger which causes continents to crack and get pulled apart
Continental Drip Theory
Incorrect/Joke: The continents are somewhat upside down water droplet shape. Continents are dripping northwards on the globe
are continental plates felsic or mafic
felsic
are oceanic plates felsic or mafic
mafic
Plate tectonics theory
Lithosphere is strong outer layer
asthenosphere is weak under layer
lithosphere is broken into a set of rigid plates that move
plates contain both oceanic and continental crust
mid-ocean ridge speed
1-20cm per year
continental rift
separating continental crust;
will eventually split into two plates
then fill with water and turn into a linear sea
What is the oldest seafloor
180 million years
failed rift
a continental rift does not finish separating (we don’t know why)
Keweenawan Rift
failed rift extending from Lake Superioir to Kansas
Examples of oceanic-continent convergent plate boundaries
Cascadia, Andes
Examples of oceanic-oceanic convergent plate boundaries
Western Aleutians (Alaska)
Examples of continent-continent convergent plate boundary
Tibetan Plateau
Depth at which water released from the subducting oceanic plate causes the formation of volcanoes
100km
How does angle of subduction angle affect how far into the continental crust volcanoes are formed
When the subducting oceanic plate reaches 100km, volcanoes form.
If the angle is greater, the volcanoes will form closer to the subduction trench
If the angle is lesser, the volcanoes will form farther from the subduction trench
What is the thickness of the crust at continent-continent convergent boundaries
> 80km
What do continent-continent convergent plates form
- thickened crust and major mountain belt
- no volcanoes because of no subduction occurring
what causes continent-continent convergent boundaries to have partial melt
- higher quantities of radioactive materials that release heat
This partial melt causes flow
George Airy
If you have much mass above the Earth’s surface, the mantle underneath will have a higher density which counteracts any gravitational effects of the large mass.
Examples of transform plate boundaries
San Andreas Fault, North Anatolian Fault
Relationship between transform faults and mid-ocean ridge
transform faults connect mid-ocean ridges.
How do Earthquakes support the Plate Tectonic Model
Divergent - occur in a narrow and only along the plate boundary area
Convergent - occur in a broad area along the subducting plate
How does the age of the seafloor support the plate tectonic model
As you move away from the mid-ocean ridge, the seafloor gets older
They used fossils to discover this
How do hotspots/mantle plumes support the plate tectonic model
as the plume moves, volcanoes are created and alive while the plume is beneath them
when the plate moves, the plume moves with it and the existing volcano loses its magma source
How does GPS support the plate tectonic model
we can measure the real movement of plates
What drives plate motion
mantle convection
mantle convection
magma rises because is it hot then falls when it cools
what mechanisms (based on mantle convection) produce forces that move the plates
mantle traction/drag
ridge push/sliding
slab pull
mantle traction/drag
convection current carries plate as it flows
ridge push/sliding
mid-ocean ridge creates a slight hill that plates fall down
slab pull
as the plat subducts, it pull the entire plate
volcanic arcs are associated with
ocean-ocean convergent boundaries
