Plate Tectonics: The Grand Unifying Theory of Geology

Plate Tectonics
The Grand Unifying Theory of Geology
Earth’s lithosphere is divided into a
number of pieces, called plates
These plates of rigid lithosphere ride
over the softer, ductile asthenosphere
Motion of these plates cause nearly all
geologic features we observe on the
earth’s surface

Plate Tectonics
• Plate tectonic theory is powerful.
• It provides a unified mechanism explaining:
– Igneous, sedimentary, and metamorphic rocks.
– The distribution of earthquakes and volcanoes.
– The origin of continents and ocean basins.
– The distribution of fossil plants and animals.
– The genesis and destruction of mountain chains.
– Continental drift.

Elevations and Lithospheric Loading
• The weight of the lithosphere causes depression of
the asthenosphere and bending in the lithosphere

Buoyancy
• First described by Archimedes more than 2.2 ka.
• Floating solids displace water equal to their mass.
• An iceberg “sinks” until the mass of water it displaces
is equal to the total mass of the iceberg.
• This concept applies to lithospheric plates.
– Continental – Floats higher.
– Oceanic – Sinks lower.

Why Are Continents Higher?
• Continental Crust - granite – density ~2.6 g/cm 3
• Oceanic Crust – basalt – density ~3.0 g/cm 3
:: Review Hypsograph Exercise ::

The Major Plates of Earth’s Lithosphere
- Most plates contain both oceanic and continental crust.
A few contain only oceanic crust. Thus a plate ≠ a continent
- Not all boundaries between continents and oceans are plate
boundaries

Plate boundaries are zones of many earthquakes
- Earthquakes clearly outline major plate boundaries.
- Thus active continental margins ARE plate margins.
Passive continental margins are NOT plate margins.
- So what happens at passive margins?

Active & Passive Margins
• Margin: boundary
between a continent and an
ocean
• Active Margin: A
continental shoreline that
IS a plate boundary
• Passive Margin: A
continental shoreline that
IS NOT a plate boundary
• Continental Shelf: Thick
sediment cover, gentle
slope, shallow to moderate
depth
• Abyssal Plain: nearly flat
in slope, deep depth,
overlies oceanic crust
Cross-section of a typical passive margin

The Major Plates of Earth’s Lithosphere
- Geologists define only three types of plate boundaries
1- Divergent
2- Convergent
3- Transform

Type 1 - Divergent Plate Boundary
• Divergent - Two plates
that pull away or separate
from each other.
• Produce new crust
• Examples: Mid Atlantic
Ridge, East Pacific Rise
• Effect of movement – Sea
Floor Spreading
:: Divergent Animation ::

Type 2 - Convergent Plate Boundary
• Convergent - Two plates
that move towards or
collide with each other.
• Consume old crust
• Example, India into Asia,
NW Coast of U.S. SW
coast of South America
• Effect of movement –
Subduction or Collision
:: Convergent Animation ::

Type 3 - Transform Plate Boundary
• Transform - Two plates that
slide horizontally past each
other.
• Do NOT consume or create
crust
• Example, North Anatolian
Fault (Turkey), Dead Sea
Fault (Israel, Jordan), San
Andreas Fault
• Effect of movement –
Transform Faulting
:: Transform Animation ::

Divergent Boundaries
• Sea-floor spreading causes plates to move apart.
• Magma wells up to fill the gap.
• Magma cools, adding material to each plate.

Divergent Plate Boundaries: Sea Floor Spreading

New Oceanic Crust at Mid-Ocean Ridges Intrudes from Dikes
Dike: A steep (~vertical) cracks filled with magma

Mid-Ocean Ridges
• Linear mountain ranges in Earth’s ocean basins.
• Example: The Mid-Atlantic Ridge
– Snakes N-S through the entire Atlantic Ocean.
– Elevated ridge 1,500 km wide and 2 km above abyssal plains.

Why Are Mid-Ocean Ridges Elevated?
Mid-ocean ridges are high above most of the seafloor
This is because new oceanic crust is warmer and less
dense, and very little lithosphere has been added,
close to the ridges.
Further from the ridges, the crust is cool and more
dense, and the thicker lithosphere adds even more
weight, pulling the whole lithosphere down into the
asthenosphere

Mid-Ocean Ridge Activity:
Black Smokers & Pillow Basalts
• Magma heats water and it jets out of these
chimney-like vents. The water is black
because of dark colored minerals. Strange
critters live here!
• When lava erupts underwater,
instead of making coherent
flows, it forms rounded bloblike
pillows. Commonly
called pillow basalts

Pillow
Pillows In The Field
Pillow
Pillow
Pillow
www.erictwelker.com/greenscreek.htm
Pillow

Age of the Sea Floor & Divergent Plate Boundaries

Convergent Plate Boundaries: Subduction
When two plates collide (if one is oceanic), one oceanic plate bends and
sinks downward into the asthenosphere. This is called subduction.
Why does the oceanic plate sink?
Because oceanic lithosphere is more dense than the asthenosphere – it
wants to sink. Why is it more dense? Its colder!
Slab sinking rates range 10-15 cm/yr.

Features of Convergent Plate Boundaries: Subduction
(Subduction animation)

Convergent Boundaries
• Accretionary prisms – Deformed sediment
wedges.
– Sediments scraped off subducting plates are smeared
and welded onto the overriding plates.
– These contorted sediments can be pushed above sea.
• Washington’s Olympic Peninsula.
• Taiwan.

Convergent Boundaries
• Volcanic arc – Volcanic belt on an overriding plate.
– The descending plate contains fluids, squeezed out at ~ 150
km depth. Causes the overlying plate to partially melt.
– Magmas burn upward, fueling volcanic eruptions.
– A curved Earth dictates that volcanic belts are curved.
• Arc type depends on
overriding plate.
– Continental crust –
Continental arc.
– Oceanic – Island arc.

The Wadati-Benioff Zone
• How do we know where
the subducting plate is?
• Mega Thrust
Earthquakes occur along
the interface between the
upper and lower plate
• The pattern of
earthquakes outlines the
location and shape of the
lower plate
• The deepest and largest
earthquakes in the world
occur at subduction zones
• What is the fate of the
sinking plate?

• The current
hypothesis about
the fate of
subducted plates
suggests that they
may sink all the
way to the lower
mantle near the
core/mantle
boundary
• Geophysical
techniques and
models may help
figure this out
Subducted Plate Graveyard

Subduction and Island Arcs
Island Arc: Chain of volcanic islands that form behind a subduction
zone.
• Island arcs may eventually get swallowed by the subduction zone.
• What happens when a continent collides into another continent?
• What happens when the subducting oceanic plate runs out?

Continent-Continent Collision
• Eventually, a subduction zone may swallow all of the dense oceanic crust
• Two plates of continental material (?) cannot subduct because they are
not dense enough.
(collision animation)

Transform Boundaries
• Lithosphere slides past; not created or destroyed.
– Most transforms offset spreading ridge segments.
– Some transforms cut through continental crust.
• Characterized by…
– Earthquakes.
– Absence of volcanism.

Oceanic Transforms
• The MOR axis is offset by transform faults.
– Offset of linear MOR is geometric necessity on a sphere.
– Transforms provide strong evidence of sea-floor spreading.
• Abundant earthquakes common between ridge segments.
• Earthquakes vanish past ridge segment overlaps.

Transform Plate Boundaries – No Net Loss or Gain of Lithosphere
Transforms indicate where plates slide past each other; Most are fairly short
Most occur as fracture zones adjacent to mid-ocean ridges
Oceanic transforms were first properly described recognized by J.T. Wilson
Incorrect!
Transform Animation

Continental Transform Plate Boundaries
Some transform boundaries occur on the continents – San Andreas Fault is a transform
• The San Andreas
Transform moves with a
right-lateral sense of
motion (paper demo)
• Accommodates motion
between the spreading
center in Baja in the
south, and in the north
the SAF ends at a triple
junction
(So what is a triple junction?)

SAF Aerial View

Triple Junctions
• Triple Junction – Where three plates
meet.
• Named by the three types of boundaries
that meet at the junction, e.g. ridgeridge-ridge,
or transform-transformtrench

Hot Spots
• Not all volcanoes on Earth are
related to plate boundaries
• Some occur at hot spots;
locations where hot molten
rock rises through tectonic
plates
• Hot spots arise from hot mantle
plumes that are stationary
through time and buoyantly rise
to through the crust, e.g.
Hawaii, and Yellowstone
(hot spot animation)

Hawaiian Hot Spot
• The islands of Hawaii exist because
of a hot spot
• The hot spot is currently under the big
island of Hawaii
• The pacific plate is moving to the
NW
• So, Oahu (where Honolulu is) is older
not currently active
• Highest volcanoes are on Hawaii and
the islands get more eroded (because
they are older) to the NW
• So, the orientation of the chain of
islands can tell us what the plate
motion is!

The orientation
of the Hawaiian
archipelago
changed at
40Ma
This tells
geologists that
the motion of the
pacific plate
changed at
40Ma.
Hawaii Hot Spot Track

Other Hot Spot Tracks
• There are lots of hot spots on Earth
• Iceland is unique: A hot spot and a divergent
plate boundary all above sea level.

The Birth of a Divergent boundary
• How are divergent
boundaries
formed?...Rifting
• If continental material
(granite) is being rifted,
then it is sometimes
called continental rifting
• Stretching causes
thinning of the crust
(think of taffy or silly
putty)
• (rifting animation)

Continental Rifting Today?
• Eastern Africa is currently
being rifted apart.
• Called the East African Rift
• Associated Earthquakes,
Volcanism, and Faulting.

• The western U.S.
– most of Nevada,
part of Utah,
California,
Arizona, Oregon,
and Idaho are
being stretched
apart
• Called the Basin
and Range
province
• Maybe this will
evolve into a rift.
Rifting in the U.S.?

Death of a Subduction Zone
• Eventually, a subduction zone may swallow all of the dense oceanic crust
• Two plates of continental material (?) cannot subduct because they are
not dense enough.
(collision animation)

What Drives Plate Motions?
• The old hypothesis was that
the asthenosphere contains
convection cells, which
drive plate motions. This is
only partly true…
• These cells bring hot molten
material up in some regions
(divergent) and pulls cold
material down in other
regions (subduction)
• Scientists tested this
hypothesis and found that
they could not replicate the
motions of plates with
simple convection cells.
• So, convective cells
probably do exist, but they
are not the main driver of
plate motions

Ridge-Push and Slab-Pull forces
• Divergent boundaries are
driven by a ridge-push force.
• Ridge-Push is driven by
gravity
• Slab-Pull force drives
subduction (also from gravity)
• Dense lithosphere sinks and
once it starts to sink the
density difference pulls the
rest of the plate with the
sinking slab

• Thanks to GPS and other
geodetic techniques, we
can measure the motions
of the Earth’s plates on a
up to the minute basis.
Velocities of the Plates
A permanent
GPS station

GPS
Motions
in Los
Angeles,
CA

Relative Plate Velocities
Ignore red arrows! They are absolute motions.
Black arrows show relative motions between plates.

Plates and Continents Over Geologic Time
• The evidence for plate tectonics is
overwhelming.
• The theory of plate tectonics
illustrates how the Earth’s surface
changes through time
• This theory also highlights the
fact that our planet is constantly
changing its surface by
– Addition of molten material
– Recycling of old oceanic crust