BAESI

Plate TectonicsEllen Metzger

Director, Bay Area Earth Science InstitueSJSU 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?

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

Slide shows an image of one tectonic plate being subducted beneath another.

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.

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.

Stars show places where Glossopteris fossils have been found.

Image from USGS

Image from NASA

Glossopteris Flora and Land Bridges?

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”

Was sea level lower during late Paleozoic?

Image from USGS

Image from NASA

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.

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.

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.

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.

Slide shows a photograph of Alfred Wegener.

Image from USGS

Glacial Flow DirectionsSlide 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 TectonicsDespite 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.

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:

Oceanic crust Continental crust

Pressure increases with depth.

Slide shows two figures that detail the different parts of the interior of the Earth

Images from USGS

Earth’s Magnetism

Motion of iron-rich outer core creates a magnetic field.

Earth acts like giant bar magnet with N and S poles.

Geographic and magnetic poles offset.

Slide shows a cartoon image of Earth’s magnetic field.

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.

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 SeafloorThe seafloor became

much better explored during the 1940-1960’s.

WWII, sonar.Complex topography.Mid-oceanic ridges with

central furrow.Volcanoes often associated

with ridges.

Slide shows an artist’s painting of the Mid-Atlantic Ridge.

Image from USGS

Harry H. Hess & Seafloor Spreading

Hess’ Hypothesis of Seafloor Spreading:1962Continental and oceanic crust move together.New oceanic crust forms from rising magma at

mid-continental ridgesOceanic 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 ReversalsThe 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 mid-ocean 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 BoundariesThree 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

Slide shows two images. The first shows a picture that illustrates a divergent plate boundary. The second shows the mid-Atlantic ridge system.

Image from Google Earth

Image from USGS

Mid-Atlantic Ridge

Oceanic-Continental Convergent Plate Boundary

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 oceanic-continental convergent boundary.

Andes, South AmericaAndes, South America

Image from Google Earth

Image from USGS

Continental-Continental Convergent Plate Boundary

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 continental-continental convergent boundary. The third is a cartoon that shows how India

crashed into Asia.

Himalaya Mountains, AsiaHimalaya Mountains, Asia

Image from Google Earth

Image from USGS

Image from USGS

Transform Plate Boundary

Slide shows two images, both of which illustrate the positions of transform plate boundaries near the northwest coast of the United States.

Image from Google Earth

Northwestern United States

Image from USGS

San Andreas Fault

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