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An In-depth Look at Earthquakes
at divergent boundaries – shallow only, usually weak
at translational boundaries – shallow only, often strong
at convergent boundaries – often strong
Plate Tectonics II
continent-continent – shallow and intermediate
subduction zones – shallow, intermediate, and deep, in that
order, moving away from trench toward overriding plate
Boundary characteristics and
Driving Forces
How and why do the plates move?
Faults at divergent boundaries
normal faulting in central rift
valley and between blocks
Tension pulls apart basalt
blocks
Blocks in rift valley fall down
relative to others
Volcanism in rift valley creates
seamounts
transform faulting on both sides
of rift valley
Rift valley not continuous but is
punctuated by transform faults
Two plates slide past each
other at transform faults
Fracture zones are inactive
extensions of transform faults
Earthquakes at divergent bdry’s
Shallow focus, low magnitude quakes
most occur at transform faults
some occur in central rift valley
Central Rift Valley
(new oceanic crust is formed here)
Plate #1
Transform Faults have strike-slip
motion created by two blocks sliding past
one another; these faults are prone to
earthquake activity due to the frictional
stresses that build up along the faults.
Plate #2
Transform Fault
(active seismicity)
X X X X X X X X X
Plate #1
Fracture Zone
(inactive seismicity)
Plate #2
Fracture Zone
(inactive seismicity)
X
X = prone to earthquakes
X
X
X
Plate #1
1
Faults at subduction zones
Reverse faulting in
subduction zones at
deep-sea trench
Subduction zone examples
Oceanic-continental Convergent Boundary
ocean-continent
ocean-ocean: more dense
plate subducts under less
dense plate; get island arc
ocean-continent: ocean
crust subducts under
continental crust; get
volcanic mountain chain
Andes Mtns.
(volcanic mtn. range)
Accretionary wedge at trench
Composed of sediment
scraped off down-going plate
Can be uplifted eventually if
subduction leads to continentcontinent collision
Peru-Chile Trench
narrow shelf dropping off
quickly to the trench
(“active” cont. margin)
broad coastal plain
& continental shelf
(“passive” continental
margin)
Earthquakes at subduction zones
Subduction zone examples
ocean-ocean
Earthquake focus can be shallow, intermediate, or deep
Often very high magnitude
Can actually see downgoing plate by location of earthquake foci
Benioff Zone – intermediate and deep earthquakes occur in
this inclined zone, tilted away from trench toward volcanic arc,
extends downward up to 700 km
active magmatic arc
(volcanic island arc)
accretionary prism
backarc basin
notice the island arcs and
volcanic mountain ranges
(e.g., Japan) landward of
the trenches
1991 eruption of
Mt. Pinatubo
rising bodies
of magma
oceanic
lithosphere
x
x
x
x x
partial melting
of subducting
plate near base
of lithosphere
Mariana Islands
(volcanic island arc)
trench
f ore-arc basin
x
x
xx
x
x
x
sea level
x
x
oceanic
crust
oceanic
lithosphere
Benioff Zone
x
x
x
Mariana Trench
x
xx
x
x
x
x = earthquake foci
2
Continent-continent collisions
thrust faulting at continent-continent collisions
a sort of extreme form of reverse faulting
major mountain building regions
another site of strong earthquakes, shallow to deep foci
India-Asia is classic example
deep-sea sediments
& oceanic crust
squeezed between
the two continents
deformed magmatic
arc & suture zone
(massive mountain chain)
x
continental
crust
old
magmatic
arc
x x
x
x
x
x
x
x
x
xx x
x
x
xx
x
x
x x x
x
continental
crust
x
continental
lithosphere
oceanic
lithosphere
x
x
oceanic
crust
x = earthquak e foci
Earthquakes at translational bdry’s
transform faults, or “strike-slip” faults exist where one plate
slides past another
ocean crust neither created nor destroyed at these
boundaries
San Andreas Fault through California is the classic example
other long transform faults on northern and southern edges
of the Scotia Plate and the Caribbean Plate
x
x x
x
x
x
x
continental
lithosphere
Himalaya &
Tibetan Plateau
continent-continent
collision
Faults at translational bdry’s
Shallow focus but can be strong …
Pacific Northwest – 3-in-1
The Pacific Northwest combines all 3 plate
boundary types in one region …
Divergent: Juan de Fuca Ridge
Convergent: Cascadia Subduction Zone
Translational: Mendocino Fault (northwest
extension of San Andreas Fault)
Mt. Rainer
3
The Wilson Cycle (J. Tuzo Wilson)
Wilson Cycle refers to the
sequence of events leading
to the formation, expansion,
contracting and eventual
elimination of ocean basins.
Hotspots – a whole ‘nother story
Stages in basin history are:
Embryonic - rift valley forms as
continent begins to split.
Juvenile - sea floor basalts
begin forming as continental
fragments diverge.
Mature - broad ocean basin
widens, trenches eventually
develop and subduction begins.
Declining - subduction
eliminates much of sea floor
and oceanic ridge.
Terminal - last of the sea floor is
eliminated and continents
collide forming a continental
mountain chain.
What drives this plate motion?
oceanic
crust
spreading
center
km
oceanic
crust
spreading
center
trench
Lithosphere
Pushing force
Pulling force
800
ne
zo
600
n
io
400
continental
crust
sea level
t
uc
bd
su
200
magmatic arc
(volc. mtn. chain)
landward of trench
Asthenosphere
upper Mantle (ductile)
intraplate volcanism (within a plate) like Hawaii, or located on a
spreading ridge like Iceland
linear chains of islands, seamounts, or ridges form
due to plate moving over stationary hot spot
hot spots are surface expressions of magma plumes rooted deep
in the mantle
over time, plate continues to move over hot spot, resulting in linear
chain of volcanoes
as plate moves, volcanoes move off hotspot, become seamounts
Convection connections
There are competing hypotheses as to how and where
mantle convection occurs.
The whole mantle model has convective
flow throughout the entire mantle.
According to this model, subducted slabs
of oceanic lithosphere sink through the
660-km boundary between the
asthenosphere and lower mantle, all the
way down to the core-mantle boundary,
where they melt.
The layered mantle model has two
separate zones of convection, one in
the asthenosphere and the other in the
lower mantle.
According to this model, there is very
little mixing between the two layers, and
slabs of lithosphere either melt or pile
up at the bottom of the asthenosphere.
Mantle (rigid)
1000
Convection in the asthenosphere
and/or lower in the mantle partly
drives movement of the plates.
In addition, the leading edges of
suducting plates are pulled down by
gravity, while plates at spreading
ridges are pushed apart.
Convection can basically be defined as a
process in which hot, less dense material rises
(such as magma that feeds a spreading ridge),
and cold, more dense material sinks (such as
old oceanic lithosphere that is being subducted
into the mantle).
4
Plate Tectonics Puzzler
ocean-ocean
collision:
oceanic
magmatic arc
crust
(island arc)
km
200
400
sea level
hot spot island
and linear chain
of seamounts
(see facing page)
oceanic
crust
spreading
center
ocean-continent
collision:
magmatic arc
(volcanic mtn.
chain)
trench
1
continental
crust
continental
margin
Lithosphere
trench
2
3
Plate Tectonics summary fig.
4
Asthenosphere
(ductile)
oceanic
crust
spreading
center
continental
crust
The figure below is a theoretical schematic of the different
types of plate boundaries, rather than a representation of an
actual, present-day location on Earth. Enjoy!
continental
margin
5
600
800
1000
1200
1400
1600
Mantle
1800
2000
Quick question: How many
plates are shown here?
Hint: Look for boundaries.
2200
There are 5 plates.
2400
2600
2800
3000
source of hot spot?
Liquid Outer Core
5