BAESI
Plate Tectonics
Ellen Metzger
Director, Bay Area Earth Science Institue
SJSU Geology and Science Education Program
With thanks to Jon Hendricks, SJSU Geology Dept. for slides
Overview
• Plate tectonics and the Next Generation
Science Standards (see handout).
– Three components
• Science and Engineering Practices
• Disciplinary Core Ideas
• Crosscutting Concepts
• Links to the Common Core standards
Plate Tectonics Overview
 Have the positions of continents moved through
time?
 What is the history of the idea of continental
drift?
 How does plate tectonics work?
 How do plates interact at their boundaries?
Plate Tectonics
Plate Tectonics: The scientific theory that the surface of the Earth
(lithosphere) is divided into plates that move relative to one
another and that interact at their boundaries.
Slide shows an image of one
tectonic plate being
subducted beneath
another.
Image from USGS
Map of the World
Slide shows a satellite map of the world.
Image from NASA
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
the world map where
Glossopteris fossils
have been found.
Image from USGS
Image from NASA
Stars show places where Glossopteris fossils have been found.
Glossopteris Flora and Land Bridges?
Image from USGS
Slide shows a photo of a
Glossopteris fossil, as
well as locations on
the world map where
Glossopteris fossils
have been found. A
circle shows the
position of the ancient
land mass known as
“Gondwana”.
“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.

In 1915 presented evidence for a single
supercontinent, which he called Gondwana.

Slide shows a photograph
of Alfred
Wegener.
Early evidence presented by Wegener and
other workers (especially Alexander du Toit)
in support of continental drift:
 Continental fit.
 Rock sequences.
 Glacial flow directions.
 Distributions of fossils.
Image from USGS
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.
Image from USGS
Image from NASA
Fossil Evidence
Slide shows the
distributions of
Glossopteris,
Lystrosaurus, and
Mesosaurus fossils
when the southern
continents are
reunited as
Gondwana.
Image from USGS
Activity:
A Plate Tectonic Puzzle
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.
Interior of the Earth
 Inner core: mostly solid iron
 Outer core: mostly liquid iron
 Mantle: rocky material
 Crust:
Slide shows two figures
that detail the
different parts of
the interior of
the Earth
 Oceanic crust
 Continental crust
 Pressure increases with depth.
Images from USGS
Earth’s Magnetism
 Motion of iron-rich outer
core creates a magnetic
field.
Slide shows a
cartoon image
of Earth’s
magnetic field.
 Earth acts like giant bar
magnet with N and S
poles.
 Geographic and magnetic
poles offset.
Image from USGS
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.
Study of the Seafloor
The seafloor became
much better explored
during the 19401960’s.
WWII, sonar.
Complex topography.
Mid-oceanic ridges with
central furrow.
Volcanoes often associated
with ridges.
Slide shows an artist’s
painting of the MidAtlantic 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 photograph of
Harry Hess.
Image from
USGS
Thermal Convection
 Thermal convection is thought to be the
process driving the movement of plates.
 Earth is hotter (due to radioactive decay fission) in some portions of the deep mantle
than in others.
 This causes the formation of convection cells
that drag along overlying lithospheric plates acts like conveyor belts.
 Think about a container full of boiling water.
Slide shows two images. One
is a cross-section
through the Earth
showing how convection
cells in the mantle may
operate. The other
shows a container of
boiling water.
Images from USGS
Testing Hess’ Hypothesis
 How could one
test Hess’
hypothesis of
seafloor
spreading?
 What pattern
should one find
on either side of
mid-ocean ridge
systems if Hess’
hypothesis is
true?
Slide shows an image of the
Mid-Atlantic Ridge.
Image from Google Earth
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.
Test of Hess’ Hypothesis
 During the early 1960’s, it was
discovered that changes in
Earth’s magnetic polarity have
been recorded into rocks on
the seafloor (oceanic crust) as
they cooled.
 Symmetrical banding on each
side of mid-oceanic ridge
systems.
 Younger rock near ridge, older
away.
Slide shows two images that
illustrate magnetic
reversals on either side of
a mid-ocean ridge. These
reversals form
symmetrical patterns on
each side of the midocean ridge.
Images from USGS
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.
Ages of the World’s Ocean Basins
Slide has one image showing the ages of
different oceanic crust rocks in the
world’s ocean basins.
Image from USGS
Activity:
A Model of Seafloor Spreading
Major Plates of the World
Slide shows the major plates of the world.
Image from USGS
Plates Interact at Their Boundaries
Slide shows a figure that
details where recent
earthquakes have
occurred. Most
occurred near plate
boundaries.
Image from USGS; Earthquakes over last 30 days (http://neic.usgs.gov/neis/qed/)
Different Plate Boundaries
Three major types of plate boundaries:
1.Divergent - plates diverge from each other.
2.Convergent - plates converge toward each other.
 Oceanic-Continental - oceanic crust (denser)
subducts (goes under) beneath continental
crust.
 Continental-Continental - neither body of
continental crust subducts (equal density).
3.Transform - plates slide past each other.
Different Plate Boundaries
Slide shows a figure that provides an overview of the
three major types of plate boundaries: divergent plate
boundaries, convergent plate boundaries, and
transform plate boundaries.
Image from USGS
Divergent Plate Boundary
Mid-Atlantic Ridge
Slide shows two images.
The first shows a
picture that
illustrates a
divergent plate
boundary. The
second shows the
mid-Atlantic ridge
Image
system.
from
USGS
Image from Google 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 of the west coast of South
America, which is an oceaniccontinental convergent boundary.
Image
from
USGS
Image from Google Earth
Continental-Continental Convergent Plate Boundary
Himalaya Mountains, Asia
Image
Slide shows three images. One is an
illustration of a continental-continental
convergent boundary. The second is an
image of the Himalayan mountains,
which is an example of a continentalcontinental convergent boundary. The
is a cartoon that shows how India
fromthird
USGS
crashed into Asia.
Image from
USGS
Image from Google Earth
Transform Plate Boundary
Northwestern United States
Slide shows two images, both
of which illustrate the
positions of transform
plate boundaries near
the northwest coast of
the United States.
Image
from
USGS
San
Andreas
Fault
Image from Google Earth
Activity:
Edible Tectonics
Activity:
Exploring Plate Tectonics with
Google Earth
Activity:
This Dynamic Planet
Note:
Reconstructions of the arrangements of the continents at
various times in the past (and in the future) may be
viewed online at http://www.scotese.com/earth.htm