Plate Tectonics
Bay Area Earth Science Institute
Ellen Metzger
January 23, 2010
Acknowledgements
• Chevron: funding for Saturday workshops 2009-2010
and a new online component for BAESI
• Intel: Stipends
• This Dynamic Planet maps courtesy of funding from the
House Family Foundation
• Fault maps provided by the California Geological Survey
• Thanks to Jonathan Hendricks (SJSU Geology) for
providing some of the slides in this presentation
It’s BAESI’s 20th anniversary!
BAESI’s Web Site
• www.baesi.org
• “One-stop shopping”
Plate tectonics in the news:
January 2010
• Small earthquakes in San Jose area
• Magnitude 6.5 earthquake near Eureka
• Magnitude 7.0 in Haiti
The Action Is At The Plate Boundaries!
Download a complete PowerPoint presentations
about plate tectonics, hazards, volcanoes and
other Earth science topics at:
Your Planet Earth
http://www.earth4567.com/
History of Plate Tectonics
Slide shows two images
made by
geographer
Antonio Snider-
Pellegrini, 1858.
One shows the
continents
separated. The
other shows North
and South America
connected with
Europe and Africa.
Maps by geographer Antonio Snider-Pellegrini, 1858
Glossopteris – “Seed Fern”
Slide shows a
photo of a
Glossopteris
fossil, as well
as locations on
Image from USGS the world map
where
Glossopteris
fossils have
been found.
Image from NASA
Stars show places where Glossopteris fossils have been found.
Glossopteris Flora and Land Bridges?
Slide shows a
photo of a
Glossopteris
fossil, as well
as locations on
Image from USGS the world map
where
Glossopteris
fossils have
been found. A
circle shows
the position of
“Gondwana”
the ancient
land mass
known as
“Gondwana”.
Image from NASA
Was sea level lower during late Paleozoic?
Alfred Wegener (1880-1930)
German meteorologist who proposed idea of
“continental drift”: idea that continents moved
(and continue to move) horizontally over the
surface of the Earth.
Slide shows a
In 1915 presented evidence for a single photograph
of Alfred
supercontinent, which he called Gondwana. Wegener.
Early evidence presented by Wegener and other
workers (especially Alexander du Toit) in
support of continental drift:
Continental fit.
Rock sequences. Image from USGS
Glacial flow directions.
Distributions of fossils.
Glacial Flow Directions
Slide shows two images.
One shows a
picture of scratch
marks on rocks
caused by glacial
flow. The second
shows ancient
glacial flow
directions on the
modern world
map. The third
shows that the
glacial flow
directions in the
southern
continents only
make sense if the
continents were
once connected.
Images from USGS
Fossil Evidence
Slide shows the
distributions
of
Glossopteris,
Lystrosaurus,
and
Mesosaurus
fossils when
the southern
continents are
reunited as
Gondwana.
History of Plate Tectonics
Despite the extensive evidence that the positions of the
continents have changed through time, most geologists
rejected the idea of continental drift.
This was because there was no known mechanism that could
produce such change.
New evidence was to come from study of the seafloor.
Study of the Seafloor
The seafloor became much better explored
during the 1940-1960’s.
WWII, sonar.
Slide shows an
Complex topography. artist’s painting
Mid-oceanic ridges with central furrow. of the Mid-
Volcanoes often associated with ridges. Atlantic Ridge.
Image from USGS
Harry H. Hess & Seafloor Spreading
Hess’ Hypothesis of Seafloor Spreading: 1962
Continental and oceanic crust move together.
New oceanic crust forms from rising magma at mid-
continental ridges
Oceanic crust moves away from ridge as it cools.
Mechanism: thermal convection.
Slide shows a
photograp
h of Harry
Hess.
Image from
USGS
Thermal Convection
Thermal convection is thought to
be the process driving the
movement of plates.
Slide shows two
images. One is
a cross-section
Earth is hotter (due to radioactive through the
Earth showing
decay - fission) in some portions of how convection
cells in the
the deep mantle than in others. mantle may
operate. The
other shows a
container of
This causes the formation of boiling w\ater.
convection cells that drag along
overlying lithospheric plates - acts
like conveyor belts.
Think about a container full of
boiling water.
Images from USGS
Earth’s Magnetism
Motion of iron-rich outer
core creates a magnetic Slide shows a
cartoon
field. image of
Earth’s
magnetic
Earth acts like giant bar field.
magnet with N and S poles.
Geographic and magnetic
poles offset.
Image from USGS
Magnetic Reversals
The polarity of Earth’s magnetic field has
“flipped” many times throughout the geologic
past.
The reason(s) why are not at all clear.
Durations of “normal” and “reversed” polarity
highly variable in length.
Magnetism is Recorded in Rocks
Some rocks contain iron minerals.
These minerals align themselves to Earth’s
magnetic field as the rock forms.
Iron particles in sedimentary rock align as they fall
out of suspension from water.
Iron particles in magma (igneous rocks) align
before the magma cools.
“Frozen” orientations preserve record of the
ancient orientations of Earth’s magnetic field.
Testing Hess’ Hypothesis
How could one test
Hess’ hypothesis of
seafloor spreading?
Slide shows an image
of the Mid-
What pattern Atlantic Ridge.
should one find on
either side of mid-
ocean ridge
systems if Hess’
hypothesis is true?
Image from Google Earth
Hess’ Hypothesis Was NOT Falsified
Enough support has since been provided for
plate tectonics that the idea is now accepted as
a unifying theory for geology.
Simple idea with great explanatory power.
Major Plates of the World
Slide shows the major plates of the
world.
Image from USGS
What are tectonic plates made of?
• Plates are
made of rigid
lithosphere.
The lithosphere is
made up of the
crust and the
upper part of the
mantle.
Source: Your
Source: Your Planet Earth
The Crust
• This is where we live!
• The Earth’s crust is made
of:
Continental Crust Oceanic Crust
- thick (10-70km) - thin (~7 km)
- buoyant (less dense - dense (sinks under
than oceanic crust) continental crust)
- mostly old - young
Source: Your Planet Earth
Plate Movement
“Plates” of lithosphere are moved around by
the underlying hot mantle convection cells
Source: Your Planet Earth
Three types of plate boundary
• Divergent
• Convergent
• Transform
Source: Your Planet Earth
Divergent Boundaries
• Spreading ridges
As plates move apart new material is erupted to
fill the gap
Source: Your Planet Earth
Divergent Plate Boundary
Mid-Atlantic Ridge
Slide shows two
images. The
first shows a
picture that
illustrates a
divergent
Image plate
from boundary. The
USGS second shows
the mid-
Atlantic ridge
system.
Image from Google Earth
Age of Oceanic Crust
Courtesy of www.ngdc.noaa.gov
Convergent Boundaries
• There are three styles of convergent plate
boundaries
Continent-oceanic crust collision (subduction)
Ocean-ocean collision (subduction)
Continent-continent collision
Continent-Oceanic Crust Collision
• Called SUBDUCTION
Source: Your Planet Earth
Oceanic-Continental Convergent Plate Boundary
Andes, South America
Slide shows two images.
On is an illustration of
a oceanic-continental
convergent boundary.
The second is an Image
image of the west from
coast of South USGS
America, which is an
oceanic-continental
convergent boundary.
Image from Google Earth
Ocean-Ocean Plate Collision
• When two oceanic plates collide, one runs over the
other which causes it to sink into the mantle forming a
subduction zone.
• The subducting plate is bent downward to form a very
deep depression in the ocean floor called a trench.
• The worlds deepest parts of the ocean are found along
trenches.
E.g. The Mariana Trench is 11 km deep!
Continent-Continent Collision
• Forms mountains, e.g. European Alps, Himalayas
Continental-Continental Convergent Plate Boundary
Himalaya Mountains, Asia
Slide shows three
images. One is an
illustration of a
Image from USGS continental-
continental
convergent
boundary. The
second is an image
of the Himalayan
mountains, which
is an example of a
continental-
continental
convergent
boundary. The
third is a cartoon
Image from
that shows how USGS
India crashed into
Asia.
Image from Google Earth
Transform Boundaries
• Where plates slide past each other
Above: View of the San Andreas
transform fault
Source: Your Planet Earth
Volcanoes and Plate Tectonics…
Source: Your Planet Earth
Pacific Ring of Fire
Volcanism is
mostly
focused at
plate
margins
Source: Your Planet Earth
Volcanoes are formed by:
- Subduction - Rifting - Hotspots
Source: Your Planet Earth
Pacific Ring of Fire
Source: Your Planet Earth
Hotspot
volcanoes
What are Hotspot Volcanoes?
• Hot mantle plumes breaching the
surface in the middle of a tectonic plate
The Hawaiian island chain are
examples of hotspot volcanoes.
Photo: Tom Pfeiffer / www.volcanodiscovery.com
Source: Your Planet Earth
The tectonic plate moves over a fixed hotspot
forming a chain of volcanoes.
The volcanoes get younger from one end to the other.
Source: Your Planet Earth
Earthquakes and Plate Tectonics…
• As with volcanoes, earthquakes are not
randomly distributed over the globe
Figure showing
the distribution of
earthquakes
around the globe
• At the boundaries between plates, friction
causes them to stick together. When built up
energy causes them to break, earthquakes
occur.
Source: Your Planet Earth
Where do earthquakes form?
Figure showing the tectonic setting of earthquakes
Source: Your Planet Earth
California Tectonics: Present
Source: USGS
California Tectonics: Past
California Tectonics: Past
Cartoon of the subduction zone present on the West Coast
100 million years ago showing position of the Franciscan
accretionary complex. Source: National Park Service
Rocks of the Franciscan Complex
An accretionary wedge
http://www.nps.gov/prsf/naturescience/images/Subduction-animation_1.gif
Source: Phil Stoffer, Rocks and Geology in the San Francisco Bay Region
Source: Phil Stoffer, Rocks and Geology in the San Francisco Bay Region
A Tale of Two Earthquakes
Loma Prieta Haiti
Year 1989 2010
Magnitude 7.1 7.0
Fault San Andreas Enriquillo-Plantain
(strike-slip) Garden (strike-slip)
Number 63 Tens of thousands
killed
Magnitude 7.0 HAITI
Tuesday, January 12, 2010 at 21:53:09 UTC
A powerful earthquake hit the impoverished country of Haiti on
Tuesday, collapsing the presidential palace and numerous other
critical government buildings and raising fears of substantial
casualties in what a witness called “a major, major disaster.”
NBC News
Before and After Earthquake
Presidential Palace in Port-au-Prince, Haiti
Magnitude 7.0 HAITI
Tuesday, January 12, 2010 at 21:53:09 UTC
The earthquake occurred about 10 miles west of the capital
of Haiti, Port-au-Prince, and caused extreme shaking. USGS Shaking Intensity
Mike Blanpied of the US
Geological Survey said that,
based on the location and size
of the quake, about three
million people would have
been severely shaken by its
impact.
Perceived
Modified Mercalli Intensity Shaking
Extreme
Violent
Severe
Very Strong
Strong
Moderate
Light
Weak
Not Felt
Magnitude 7.0 HAITI
Tuesday, January 12, 2010 at 21:53:09 UTC
Left: Photo taken prior to
the earthquake. Low
income housing-
unreinforced masonry.
Below: After
Further complicating
the situation, many
people live in structures
that are vulnerable to
earthquake shaking.
BBC
Magnitude 7.0 HAITI
Tuesday, January 12, 2010 at 21:53:09 UTC
Carel Pedre via Twitter
BBC
"Thousands of people were feared dead today
after a powerful earthquake struck Haiti's
capital, leaving tens of thousands homeless
and buried beneath rubble....Thousands of
people gathered in public squares late into the
night, singing hymns and weeping, with many
seriously injured people sitting in the streets
pleading for doctors."
The Gazette, U.K.
Magnitude 7.0 HAITI
Tuesday, January 12, 2010 at 21:53:09 UTC
Aftershocks
This earthquake was
followed by five powerful
aftershocks within the first
two hours after the
devastating quake.
USGS
Left: Aftershocks (yellow)
In the first eleven hours after
the earthquake there have
been 32 aftershocks greater
than magnitude 4.
Google Earth
Magnitude 7.0 HAITI
Tuesday, January 12, 2010 at 21:53:09 UTC
Earthquake and Historical Seismicity
This earthquake (star), plotted with
regional historical seismicity, occurred
on the transform plate boundary
between the Caribbean and North
American plates.
As expected for an earthquake on a
transform boundary, the depth of the
event was quite shallow at about 10
km.
The depth and proximity to the
population center contributed to the
destruction.
This powerful earthquake was the
largest magnitude this region has
USGS seen in two centuries.
Magnitude 7.0 HAITI
Tuesday, January 12, 2010 at 21:53:09 UTC
Regional Tectonics
This map shows the rates and directions of motion of the Cocos, Pacific, and
Caribbean plates with respect to the North American Plate. The small arrows on
the Caribbean Plate show that it moves eastward at a rate of about 20 mm/yr
(2 cm/year) with respect to the North American Plate. This is a fairly slow rate of
transform motion between the Caribbean and North American plates. For
comparison, the rate of transform motion across the San Andreas transform fault
between the North American and Pacific plates is about 50 mm/yr (5 cm/yr).
Divergent
Plate
Boundaries North American Plate
(red lines)
Caribbean Plate
Pacific
Transform
Plate Cocos Plate Plate
Boundaries
(yellow lines)
Magnitude 7.0 HAITI
Tuesday, January 12, 2010 at 21:53:09 UTC
Haiti occupies the Mann
western part of the
island of Hispaniola.
At the longitude of the
earthquake, motion
between the
Caribbean and North
American plates is
partitioned between
two major east-west
trending, strike-slip
fault systems -- the
Septentrional fault
system in northern The location and focal mechanism of the
Haiti and the earthquake are consistent with the event
Enriquillo-Plantain having occurred as left-lateral strike slip
Garden fault system faulting on the Enriquillo-Plantain Garden
in southern Haiti. fault system. This fault system
accommodates about 7 mm/y, nearly half
USGS Centroid the overall motion between the Caribbean
Moment Tensor plate and North America plate.