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Plate Tectonics
Definition
Plate:
Rigid, rocky outer
surface of the Earth
Tectonic:
To build or destroy
The processes by which the
rocky plates of the planet move
and interact with each other
Internal Structure of the Earth
lithosphere
plates
inner core
solid iron
mantle
hot dense
rock
outer core
liquid iron
Lithospheric Plates
Oceanic:
Continental:
thin, heavy,
mostly lava flows
thick, light,
mostly granite
Asthenosphere
Weak, plastic upper mantle
Lithospheric Plates
Diverging Plate Boundaries
Where plates pull apart from each other
Stretching & thinning
High heat flow
(New Crust Created)
Diverging Plate Boundaries
Key landform: mid-oceanic ridges
volcanic mt. ranges
axial valley
1000’s miles long
1-2 miles tall
Converging Plate Boundaries
Where plates collide
Subduction zone: collision between
oceanic and continental lithosphere
(Crust Destroyed)
Subduction zones
Key landforms:
trenches & volcanic arcs
Transform Plate Boundaries
Where plates slide side by side
Key landform: faults
(Crust neither destroyed or created)
Driving Force
thermal convection /
convection currents
pushes up at ridges
plates spread away
hot rock
cold rock
plate sinks and subducts
Geologic activity at plate margins
Volcanism: C, D
Earthquakes: C, D, T
Rock Deformation: C, D, T
Evidence: shape of continents
Africa
South
America
The margins of
some continents
look like they
could fit together
First
proposed by
Alfred Wegner
Fossil Evidence
Evidence: volcanoes
Nonrandom occurrence of volcanic
activity
Evidence: earthquakes
Nonrandom occurrence of earthquakes
Shallow at diverging margins
x
x x
x
x x
x
xx
x
deep at converging
margins
Evidence: hot spots
Stationary upwellings of heat away
from plate margins
Evidence: hot spots
As plates move, the volcanoes are
carried away from the hot spot
Only volcanoes near the hot spot are
are active
Can determine direction and
speed of the plate motion
Evidence: age of seafloor rocks
Youngest rocks are found at the
mid oceanic ridges
Oldest rock are found
farthest from the ridge
Evidence: paleomagnetism
Earth has a strong magnetic field
Iron rich volcanic rocks record
Earth’s magnetic field at the time
they form
seafloor paleomagnetic record
Bands of alternating polarity
Symmetrical about the mid oceanic ridges
seafloor paleomagnetic record
lava forming at the ridges picks up the
current magnetism, then gets pushed out
of the way as newer lava forms
Records of the growth
and spreading of the
seafloor
seafloor paleomagnetic record
The alternating polarity
represents complete
reversals of Earth’s
magnetic field
continental paleomagnetic record
The location of magnetic poles on
separate continents appear to wander
with time
If the poles haven’t moved
significantly, then the continents did
Use overlapping poles to reconstruct
the past positions of the continents
Plate Tectonics in the PNW
Mt Meager
M t C ay le y
Mt G ar ibal di
Mt B ak er
G la cie r P ea k
an
u
J
de
Fu
ca
North American
Mt Rai nier
Mt St Helen s
Mt Adams
Mt Hood
Mt Jefferson
Thr ee
S is te r s
C ra ter La ke
Pacific
Mt M c Loughin
M t S has ta
L as s e n P e ak
Sa
nd
nA
0
r ea
20 0
a
sF
km
Cascadia
subduction
zone
ult
Plate Tectonics in the PNW
Cascades volcanic arc
Cross section of the Cascadia Subduction zone
East
West
150 miles
Coast
Range
Pacific
ocean
Willamette
valley
su bdu
cti ng
oc ean
ic
Western
Casc ades
High
Cascades
volcanoes
Desc hutes
basin
60 miles
pl ate
partial
melting
Plate Tectonics in the PNW
Mt Meager
M t C ay le y
Mt G ar ibal di
Mt B ak er
G la cie r P ea k
an
u
J
de
c
Fu
a
North American
Mt Rai nier
Mt St Helen s
Mt Adams
Mt Hood
Mt Jefferson
Thr ee
S is te r s
C ra ter La ke
Pacific
Mt M c Loughin
M t S has ta
L as s e n P e ak
Sa
nd
nA
0
re a
20 0
a
sF
km
Juan de Fuca &
Gorda mid ocean
ridges
ult
Plate Tectonics in the PNW
Mt Meager
M t C ay le y
Mt G ar ibal di
Mt B ak er
G la cie r P ea k
North American
Mt Rai nier
Ju
an
de
Fu
ca
Mt St Helen s
Mt Adams
Mt Hood
Mt Jefferson
Thr ee
S is te r s
C ra ter La ke
Mt M c Loughin
Pacific
M t S has ta
L as s e n P e ak
Sa
nd
nA
0
re a
20 0
a
sF
km
Blanco &
Medocino
fracture zones
ult
Plate Tectonics in the PNW
“exotic terranes
islands carried by the plates
Attached to the North
America continent
seafloor paleomagnetic record
Bands of alternating polarity
Symmetrical about the mid oceanic ridges
Evidence: paleo-environments
glacial
deserts
desert
coral reefs
deserts
tropics
desert
glacial
glacial
Evidence: distribution of mountains
Importance of Plate Tectonics