Plate Tectonics
Chapter 3
How Does The Earth Change With
Depth
• Earth’s Crust – Outermost layer.
– Continental Crust - 35-40 km in thickness
– Oceanic Crust – exists beneath oceans.
Average of 7 km thick.
– Mantle – extends from base of crust down
2,900 km.
– Outer Core – Molten iron and nickel.
• 2900 to 5150 km
– Inner Core – Solid iron and nickel.
Layers of the earth
Some Layers Are Stronger Than
Others
• Earth’s layers are defined by how easily
they are affected by force.
• Lithosphere vs. Asthenosphere
– Lithosphere – Uppermost portion of mantle
and crust. Relatively strong layer
• From the Greek word, stone
– Asthenosphere – Lower mantle the soft, weak
zone over which the lithosphere moves.
• From the Greek word, not strong
Some Layers Are Stronger than
Others
Oceanic crust
Lithosphere
(stronger)
Continental crust
Uppermost mantle
Asthenosphere (weaker)
Asthenosphere: hot and weak; mostly solid
01.03.b1
Why Are Some Regions Higher
In Elevation than Others?
Observe the relationship between the height of each block
and its thickness relative to other blocks
Thick blocks higher
than thin blocks
Dense materials
are lower
Relationship between crustal thickness
and elevation: isostasy
01.03.m1
The Plate Tectonic Theory
• Describes the formation, motion and
interaction of the plates.
• Continents are embedded in
lithospheric plates.
• As plates move, continents move.
Divergent Boundaries
• 2 plates separating.
• Caused the
formation of the
Mid-Ocean Ridge.
Continents rift apart
Initial uplift from rising mantle
Stretching and faulting form rift
Can lead to seafloor spreading
and new ocean basin
Ocean widens with spreading
03.04.b1-4
Sketch a mid-ocean ridge
Narrow trough, or rift
Ridge high (hot rocks and
thin lithosphere)
Magma erupts
or solidifies at
depth; forms
new oceanic
crust
Magma rises
through fractures
Asthenosphere
rises and melts
03.04.a
Convergent Boundaries
• When 2 plates move
toward each other
• 2 types
– Subduction – When an
oceanic plate plunges
beneath another plate
– Collision – 2
continental plates
collide.
• Forms mountain ranges
Subduction Boundaries
Ocean-Ocean subduction
• When 2 oceanic
plates converge
and one plunges
below the other..
– Causes formation
of volcanic
islands.
Subduction boundaries
Ocean-Continental
• Form when oceanic
plate collides with
continental plate.
– Oceanic plate ALWAYS
plunges because it’s
more dense.
– Forms a trench.
• Due to friction.
– Forms volcanic
mountains inland.
Transform Boundaries
• 2 plates sliding
against each other.
– Causes earthquakes
– Transform boundaries
link other types of
plate boundaries such
as a mid-ocean ridge
and a trench.
Evidence of Plate Tectonics
• Location of earthquakes, volcanoes and
mountain ranges.
• Magnetism and age of ocean floor.
– The change in magnetic poles is recorded in
igneous rock.
• The shape of the continents
• Mesosaurus fossils in S. America and
Africa.
03.03.a1
Compare the distribution of earthquakes (yellow),
volcanoes (orange), and high elevations (brown)
EQ, volcanism,
or mtn. building =
tectonic activity
Belts of tectonic
activity divide
lithosphere into
tectonic plates
03.03.a1
Magnetism and the age of the
earth’s floor
• Mid-Ocean Ridge (MOR)a long chain of volcanic
mountains on the ocean
floor with a deep central
valley.
– Scientists found rock on
either side of MOR were
identical.
– The center of the ridge
always showed current
magnetic orientation.
Middle
Mesozoic
(140 m.y ago)
Late
Mesozoic
(100 m.y ago)
Present
Evolution of South
America
Observe the
evolution of
South America,
beginning with
continental rifting
away from South
America
Shape of the Continents/
Mesosaurus
How Do Plates Move?
• Movement requires a
driving force
overpowering a
resisting force.
– Question: What forces
drive tectonic plates?
• Ridge Push
• Slab Pull
Ridge Push/Slab Pull
Ridge push
Slab pull
Other forces, such as
convection in mantle
Ridge Push/Slab Pull
• Ridge Push – MOR is higher than the ocean
floor because the lithosphere near the ridge is
thinner and hotter and gravity causes the plate
to slide away and outward.
• Slab Pull – Subducting oceanic lithosphere is
denser than the surrounding asthenosphere.
Gravity pull the plate downwards into the
asthenosphere.
– **Strongest driving force**
03.08.c
Lines of islands
and seamounts
Plate moves over a hot spot
Plate subsides
as cools, so
islands become
seamounts
Volcano
forms
over hot
spot
Volcanism shuts
off as area
moves away
Observe the
features
around South
America
Envision a
cross section
from west of
South
America
to the MidAtlantic
Ridge
03.09.a1
Compare this cross section with the one
you envisioned
Andes over subduction zone;
trench offshore
Subduction beneath
western edge
Spreading along
mid-ocean ridge
Eastern edge of
continent not a
plate boundary
03.09.b1
Earthquakes
• An earthquake occurs when
mechanical energy is released and
transmitted through rock as vibrations
called seismic waves.
• Hypocenter (focus) – point where
earthquake is generated.
–Usually depths less than 100 km.
Earthquakes continued
• Epicenter – point on Earth’s
surface directly above
hypocenter. (p. 332)
Epicenter
Hypocenter
Causes of Earthquakes
• Caused by movement along faults.
• Three types (p. 332)
• Normal Faults – Rocks above fault
(hanging wall) move down against the
lower rocks (foot wall).
• Occurs mostly at divergent plate
boundaries.
Causes of Earthquakes
• Reverse and Thrust Faults – The
hanging wall moves up against
the foot wall.
• Happens at convergent
boundaries.
Causes of Earthquakes
• Strike-Slip Faults – The two sides
slip horizontally past each other.
• Occurs at transform boundaries.
Build Up and Release of Stress
• A build up of stress caused by
friction created by sliding plates is
released when the stress equals
the fault strength. (P. 335)
• The stress is immediately
released and decreases to the
original level.
Earthquake Waves
• Earthquakes travel in seismic waves.
• Seismic is the Greek word for earthquake.
• There are 2 types.
– Body waves – travel inside the earth.
– Surface waves – travel on the surface of
the earth.
• Body waves become surface waves.
Wave Shape
• Crest – the top of a wave
• Trough – the bottom of the wave
Surface Waves
• Vertical Surface Wave – Travel up
and down.
• Horizontal Surface Waves –
Travel perpendicular to the
direction of travel.
Body Waves
• Two types
• Primary (p-waves) – Travel in the
same direction in which it starts.
(like a slinky)
–Travels through liquids and
solids because the particles are
close enough.
–Faster!
Body Waves
• Secondary (s-waves) – Travel up
and down, they sheer the rock.
(like a snake)
–Cannot travel through liquids
because they’re not rigid. Only
through solids.
Surface waves
Vertical surface wave
compresses materials
Horizontal
surface wave
shears material
Primary body
wave (P-wave)
compresses
material; fastest
Body waves
Secondary body
wave (S-wave)
shears material
12.04.a
Recording Seismic Waves
• Seismometers are
used to record ground
motion during
earthquakes
• They also calculate
the hypocenter and
magnitude.
Reading a Seismograph
• Seismogram plots vibrations vs.
time. Time is marked at regular
intervals to determine arrival of first
p and s waves. (p. 339)
Waves on seismogram
Seismogram
Amplitude vs. Period
• Seismic waves are characterized by wave
amplitude – How much the ground moves
• And…the time it takes for the wave to
pass by (the period)
Determining Location
• Seismograph stations in different
locations record the P-S interval.
• The difference in P-S interval shows
the distance of the earthquake from
the seismograph station.
– The longer the interval, the farther the
earthquake.
p. 340 in your book
Estimating Station Distance
• The P-S interval is mathematically related
to the distance from the epicenter using a
time-travel curve. (p. 340)
• Takes into account the materials the wave
passes through.
Triangulating the Epicenter
• Distance from each station can be
compared graphically to find the epicenter.
• Circle is drawn around each station with a
radius equal to the distance calculated
from the time-travel curve
• Intersection of circles is the
epicenter
Measuring Magnitude
• Richter Scale – Based off distance and
amplitude.
Mercalli Scale
• The modified
Mercalli
intensity scale
(MMI)
• Describes the
effects of an
earthquake.
• On a scale from
I to XII.
12.07.a
Charleston, 1886
Hebgen Lake, 1959
Alaska, 1964
San Francisco, 1906
Northridge, 1994
Mexico City, 1985