Download Topic 11A: Plate Tectonics, Part III Online Lecture: Types of

11A_3 – Slide 1
Topic 11A:
Plate Tectonics, Part III
Online Lecture:
Types of Plate Boundaries
○ Boundaries:
– Divergent
– Convergent
– Transform Faults
○ Hotspots
Why do the Plates Move?
11A_3 – Slide 2
○ rising magma (lava) forces
the plates apart
○ friction with the convection cells
in the mantle drag the plates
away from MOR
○ The higher density, cold side
of the plate is pulling its end of
the plate down. (“Slab Pull”)
Continental Lithosphere
3.10 g/cm3
Old Oceanic Lithosphere 3.28 g/cm3
Upper Mantle
3.25 g/cm3
Plate motion has been
observed using satellites &
instruments on the ground
over the last few decades:
a few inches per year, on
average, consistent with
sediment layer thickness,
radiometric dating, etc.
11A_3 – Slide 3
The MOR: Where Plates Move Apart I
○ plates move away from the MOR
“Diverging”
– lava comes up & cools: adding to plate (gets larger)
○ sink down a bit into the mantle as they get older,
because their density gets higher:
– cooler (contracts / shrinks)
This is why the MOR
is higher than the rest
of the ocean floor.
– more and more sediments pile up
on top of the plates, making them heavier
Higher Density Ends Sink
Lower Density Ends Rise
Calcareous Ooze
Red Clay
MOR
Ocean Crust
Cr
Ocean
ust
Ocean
Crust
Origin of a MOR feature
11A_3 – Slide 4
#1
Notice that there are
no trenches next to
either continent;
continental lithosphere
can be pushed by
oceanic lithosphere
(they can be part of
the same plate).
#2
#3
#4
Continents Splitting:
Where Plates Move Apart II
11A_3 – Slide 5
Red
Sea
○ Convection cells can cause
continental lithosphere to split (“rift”):
– basalt lava comes up in the gap,
making new oceanic crust
– eventually the gap could
become an entire ocean,
like the Atlantic
ri
Af
ia
ud
Sa abia
Ar
Iceland
ca
Subduction Zones: Where Plates
Come Together I
Oceanic Crust
meets Oceanic Crust
Volcanic
Islands
“Converge”
Mid-Ocean
Ridge
Volcanic
Mountains
Trench
Continental
Lithosphere
Ocean Litho.
s
Cell
Convection
Cell
e
ak
Older crust
more
dense than
Convection
Younger crust
Basalt more dense
than Granite
qu
th
Ea
rt h
Continental Crust
meets Oceanic Crust
r
Ea
qu
ak
es
Trench
11A_3 – Slide 6
Continental Collisions:
Where Plates Come Together II
11A_3 – Slide 7
○ When 2 plates with continental lithosphere come together,
the continental crust does not dive down, because
its density is lower than the density of the mantle.
– The material has to go somewhere, so it goes
both up and down, thickening the crust:
results in rising mountains
when the plates move towards
each other during earthquakes
#1
#2
Structure of the
Mid-Ocean Ridge:
Transform Faults
earthquakes
noes
volca
The Mid-Ocean Ridge (MOR)
is “broken” into “pieces.”
volca
noes
earthquakes
“Gaps” / “Jumps” are
“Transform Faults”
earthquakes
noes
volca
Mid-Ocean Ridge
Transform Fault
“Side-swipe”
Car
Car
11A_3 – Slide 8
Lithosphere
Mantle
Trench
11A_3 – Slide 9
The
San
Andreas
Fault
We have earthquakes
in southern California
because we live along
the mid-ocean ridge!
n
Sa
s
ea
dr
An
Fa
t
ul
Where plates run into and grind against one another, they are “squeezed” or
“bent.” The pressure to move builds up, eventually overcomes friction, and the
plate(s) “slip” forward and “snap” back into shape (an earthquake happens).
11A_3 – Slide 10
Hotspots
○ Hotspots are special places where magma from deep
inside the Earth is rising up, melting the bottom of a plate.
– lava keeps coming up, forming a volcano
Hotspots can also occur
under a continent (“Old
Faithful” in Yellowstone
Nat. Park) or at the midocean ridge (Iceland).
IslandSeamount
Chain
11A_3 – Slide 11
Hotspots & Island-Seamount Chains
○ The plate is moving,
so the volcano gets carried off the hotspot.
○ More volcanoes grow over the hotspot, then get carried off:
– creates a “chain ”
of islands & seamounts
– best example are the islands
of Hawaii (part of a chain
reaching all the way to Alaska)
1
2
3 4
5
Hawaiian
Islands
Hotspots: From Islands to Seamounts
11A_3 – Slide 12
○Where
Volcano
stops growing when it moves off the hotspot,
do you think the hotspot is in
because
more lava
the
Hawaiianno
Islands?
Whycomes
there? up
○ Volcano begins to sink,
Which direction
is the Pacific
because
it cools:
plate moving? How can you tell?
– contracts =
becomes smaller
– more dense =
volcano & the plate
sink into the mantle a bit
(do not float as high anymore)
○ Evidence for sinking islands:
– flat-topped seamounts
(eroded by waves)
– coral atolls
Plate Motion
#1
Mantle Plume
Corals
Grow
Upward