Lecture 7: Plate Tectonics Flashcards
Father of Plate Tectonics
Alfred Wegener
Pangaea
Supercontinent
1915
The Origin of Continents and Oceans
Political Problem of Plate Tectonics
Alfred Wegener is German while most of the geosciences was based in US and Britain
Proofs of Continental Drift Theory
• Matching fossils in continents separated by large waters • Matching geological patterns • Karoo Glaciation found in South America, Africa, Australia, and Antarctica which can only happen (according to Wegener) if they were connected.
What moved the continents according to Alfred Wegener?
1The continents are icebergs on heavier SiMa crust
2Forces available to push the continents are
• Earth rotation effect
• Lunar and solar tidal forces
The proposal of Hess on sea floor spreading was called?
GEOPOETRY
Geopoetry was released on what year
1962
What are the ideas under Geopoetry?
•Ocean Ridges form new seafloor (from mantle material) while the old
seafloor is pushed and subducts to be reintroduced to mantle
• Movement is driven by mantle convection cells (remember the convection
currents where material goes up in ridges and goes down at trenches?)
• Continental crust does not descend but are lifted up as mountains – only
oceanic crust descends.
What year was magnetometer readings done to study ocean floor topography
1961
He noticed that the magnetic pattern is symmetric with respect to ocean ridges. The pattern showed alternating stripes of normal and reverse
magnetic polarity of rocks.
Vine
1963
Vine and Matthews (Vine’s Ph.D. supervisor) proposed
that the patterns are related to magnetic reversals.
The material from the mantle that rises up through the midocean ridge is______ that contains magnetite – a rock mineral that is highly magnetic and aligns with the magnetic field.
basalt
How are the stripes and magnetic reversal related?
• The material from the mantle that rises up through the midocean ridge is basalt that contains magnetite – a rock mineral that is highly magnetic and aligns with the magnetic field.
• As the material hardens, the magnetic field recorded gets “locked in” in the rock.
• Material is created again at the ridge, pushing out the
material before it
• When the earth’s magnetic field reverses, it is also recorded in the material. Hence the symmetry of magnetic record with respect to mid-ocean ridges)
Who is Morley?
Morley is also another geologist who came up
with the same idea independently from Vine and Matthews.
Who is John Tulzo Wilson?
Proponent of Mantle plume or Hot Spot
Hot Spot
• Spot where hot mantle material goes up through the
crust and up above the surface.
• It is non-moving and semipermanent
Hot Spots are found aroun the world mostly in ocean basins, name a hot spot found in a continent
Yellowstone hotspot
Oceanic ridges appear curved but are actually straight. The curve in the ridges are caused by offsets caused in turn by faults perpendicular to the ridge. What are these faults called?
Transform Fault
1965
Tuzo coined the term Transform Fault
At transform faults, plates
Move in opposite direction
Introduced the ideas of “plates” making up the crust.
John Tulzo-Wilson
1967
Plate maps are made
Name the Major plates (Top to bottom and Left to Right)
Eurasia, North America, Australia, Pacific, South America, Africa, India, and Antarctic
Plate Tectonic’s Rate of motion
1 – 10 cm/yr
The movement of Plate Tectonics
Translational and Rotational
Name the types of Plate Boundaries
Divergent, Convergent, Transfrom
Spreading boundaries
Dirvergent
Crustal material created by Diverging plates
mafic igneous rock – Basalt or Gabbro
How are Pillow Lavas form?
Pillow lavas are formed when the lava flows out into the water
How are Sheeted Dykes form?
Formed when the ridge opens up and a
column of magma cools in the crack.
3 types of convergent boundaries
- Ocean – Ocean
- Ocean – Continental
- Continental - Continental
Ocean Trenches form from what type of convergent boundary
Ocean-ocean
Which plate submerges during ocean-ocean convergence?
The Denser plate (older and colder)
How are chain of volcanic islands form?
During o-o convergence, Magma forms underneath (via flux melting*) and rises up
Which plate submerges during ocean-continent convergence?
Oceanic Platee
This is formed during o-c convergence when accumulated sediment on continental slope is thrust up
Accretionary Wedge
Happens when the plate has moved so much
Continental-continental Convergence
Why are 2 continental plates both lifted up during convergence?
Because no plate is denser
Plates slides across each other without
creation or destruction of crustal
material.
Transform Boundary
Upwelling of mantle causes newer and warmer
material to form that will in turn be pushing older and colder material away from the ridge.
Ridgepush
Older and colder plate segments at subduction zones
become colder and denser thus goes down further and pulls the rest of the plate attached to it
Slab pull
Which moves faster? Plates with or without subducting part?
With
Does bigger area means bigger traction velocity
No. Plate area is not related to its velocity
Main mechanisms of plate movement
1 Convection
2 Ridge push
3 Slab pull
3 magma forming plate-tectonic setting
- Divergent boundary (via decompression melthing)
- Convergent boundary (via flux melting)
- Mantle plume (via decompression melting)
Melting or partial melting of hot rock as
pressure on it is reduced (while temperature remains roughly the
same)
Decompression Melting
When water is added into a rock, it lowers the melting
point of the rock.
Flux Melting
Composite volcanoes form at ________ boundaries
Convergent
shield volcanoes are formed from
Mantle plumes, continental rifting
the elongation/stretching of the crust in
some areas resulting to its thinning. Since it is thin, it allows rising of mantle material. It forms shield volcanoes and cinder cones
Continental Rifting
This can be observed in mantle plumes, divergent
boundaries and ocean-ocean convergent boundaries.
Seafloor volcanism
What percent of utramafic mantle becomes mafic magma in the zone of partial melting>
~10%
rocks are buried deep (10-20km
typically), usually related to convergent boundaries, and spans thousands of km^2
Regional Metamorphism
a body of magma in the upper part
of crust can be the source of heat and metamorphose rock around it. The zone of contact metamorphism is a few meters up to tens of meters.
Contact Metamorphism
Resulting rock of Oceanic crust metamorphism on either side of the ridge
Greenstone and Greenschist
