Chapter 8 - Plate Tectonics
http://sos.noaa.gov/datasets/Land/paleo.html
Animations

Plate Tectonics: the rocky history of an idea
Ever notice that the shore lines of
South America and Africa seem to fit together?

Antonio Snider-Pellegrini's Illustration of the closed and opened Atlantic Ocean ( 1858 ).

Continental Drift: a theory
In 1912, Alfred
Wegener proposed the idea of
“continental drift.”
The continents were once part of a single land mass and they drifted their present locations.
to

Continental Drift
Wegener named the supercontinent
Pangea ( meaning
“all lands” in Greek )
The vast ocean that surrounded Pangea was named
Panthalassa.

Evidence used to support the
Continental Drift Theory
The continents shapes match
The plants & animals (fossils) match
The rocks match
The ice matches
The climates don’t match

The shorelines of the continents appear to fit together like a jigsaw puzzle.

The plant & animal fossils match
Fossils of the same kind were found on several different continents.
Ex : Mesosaurus & Glossopteris

The rock matches
Rocks of the same age, same types and structure are found on widely separated continents

The rock matches
When the continents are reassembled, the mountain chains form a continuous belt.
Ex : The Appalachian and Caledonian mountain ranges align.

The ice matches

The ice matches

The ice matches
Glacial striations on rocks show that glaciers moved across continents
Scratches from a glacier.

Climate Change
The discovery of fossils of tropical plants in the form of coal deposits in Antarctica.
This frozen land must have been situated closer to the equator in the past, in a warmer climate where, swampy vegetation could grow.

Climate Change
Glacial till & striations found in deserts.
Moraine and till

St. Mary Lake, Glacier National Park.

New Zealand's Franz
Joseph glacier
Bridal Veil Falls in Yosemite
National Park flowing from a hanging valley

Yeager Rock, a 400 ton erratic sitting on top of glacial till on the Waterville Plateau, Washington.

Although plenty of evidence supports
Wegener’s Continental Drift theory, it was not
accepted by the scientific world at the time.
Pretend you are a skeptical scientist.
Would Wegener’s evidence be enough to convince you that the continents are moving?
What could you argue against this theory?
What is Wegener missing?

The mystery…
How does it work?
What causes the motion?
Do continents just plow through the ocean?
These questions weren’t answered until decades later after Wegener’s death.

…solved
Beginning in the 1950’s new evidence emerged to support the old theory and helped develop a new one…

Imagine an Earth without water

The Earth’s solid surface is broken up into pieces called plates that “float” on top of the gooey part of the mantle.

Plate Tectonics: a beginning
To understand the relatively new idea of Plate Tectonics, we have to review the layers of the
Earth.

Crust – 2 types
Ocean crust
Thin, dark in color
More dense
Mostly made of Basalt
Continental crust
Thick, light in color
Less dense
All rock types

Inside Earth:

Tectonic Plates are made from the Earth’s broken lithosphere.
The lithosphere consists of the crust and the upper solid part of the mantle.

The lower pliable part of the mantle is the
asthenosphere.
The lithospheric plates move over the slow flowing asthenosphere.

New Evidence used to develop
Plate Tectonics
Ocean floor mapping led to the discovery of…
Seafloor spreading
Magnetic Reversals
Locations of Earthquakes and Volcanoes

Ocean Floor Map

Mid-Ocean Ridges
Mid-Ocean Ridges are chains of volcanic mountains on the ocean floor with a deep central valley called a rift
zone.
Mid-ocean ridges are the sights of Seafloor
Spreading.

Seafloor Spreading occurs where oceanic plates are moving apart.
Magma rises through the rift and spreads out at the surface building new sea floor.

As magma cools & hardens it pushes ocean floor away from the ridge forming new ocean crust.
Old rock is destroyed in a trench.

Age of rocks increase away from the rift zone http://www.wwnorton.com/college/geo/egeo/animations/ch2.htm

More evidence…
Magnetic reversals recorded in rocks along the mid-ocean ridges support Seafloor Spreading and
Plate Tectonics.
The direction of the Earth’s magnetic field are preserved in the new rock formed at the ridges.
The poles switch and are seen in a pattern of bands paralleling mid-ocean ridges.
http://www.wwnorton.com/college/ geo/egeo/animations/ch2.htm

Earthquakes mark plate boundaries

Locations of Earthquakes &
Volcanoes
Do not occur randomly.
Actually mark the locations of plate boundaries.
Occur when plates are pushing towards, pulling away, or sliding past each other.

Remember…convection is the motion of boiling water

What drives the plates?
Mantle Convection

Plate Boundaries…are where two plates meet
3 types of motions
Divergent
Convergent
Transform Fault http://www.wwnorton.com/college/geo/egeo/animations/ch2.htm

Divergent Boundaries
Plates are moving apart and new crust is created.
Land or Ocean crust splits.
A rift valley over land continental crust splits. forms when
A mid-ocean ridge forms when crust splits.
ocean

Divergent – Continent splits

Divergent – Rift Valley
Example : East
African Rift Zone

Divergent Boundaries
Ex: Mid-Atlantic Ridge & Eastern Pacific Rise

Divergent Boundaries

Convergent Boundaries
Plates are moving towards each other.
Ocean  Continent
Ocean  Ocean
Continent  Continent

Ocean

Continent
Subduction occurs when one plate dives under another forming a trench.
Ocean crust is being carried down into the mantle, where it begins to melt.
The magma rises to form volcanoes along the edge of the continent.

O

C
Ex : Andes in South America
Cotopaxi is considered one of the world's highest active volcanoes and is at 5897m in Ecuador.

Ocean

Continent
Ex : Cascades in Oregon & Washington
Plumes of steam, gas, and ash often occurred at
Mount St. Helens in the early 1980s. On clear days they could be seen from Portland, Oregon, 50 miles
(81 kilometers) to the south. The plume photographed here rose nearly 3,000 feet (1,000 meters) above the volcano's rim. The view is from
Harrys Ridge, five miles (8 kilometers) north of the mountain.
Mount Rainier over Tacoma, Washington,
USA.

Ocean

Ocean
The deep sea trench is the site of a subduction zone.
The subducted ocean crust melts and rises to the surface forming an
island arc (volcanic islands).

Ocean trenches – Volcanic mtns. / islands

Ocean

Ocean
Ex : Japan
Ex : Mariana Trench
Challenger Deep - the deepest surveyed point in the oceans, with a depth of about 11,000 meters (about 36,000 feet).
If you cut Mount Everest off at sea level and put it on the ocean bottom in the Challenger Deep, there would still be over a mile of water over the top of it.

Ocean

Ocean
Ex : Aleutian Islands
Chain of volcanic islands off the coast of Alaska

Continent

Continent
Continental crust collides with and slides over other continental crust, forming high mountain ranges.

C

C
The continuous collision of India and Asia, which began about 45 million years ago, produced the
Himalayas.

Continent

Continent
Himalayas - Mt. Everest
View from space station

Transform Fault
Ocean or Continental crust slide past each other.
Forms an offset crack in the ocean or land.

Transform Fault
Ex : The San Andreas Fault in California.

Transform Fault
Ex: The San
Andreas Fault in
California.

Transform
Fault
Ex : Found at right angles to the mid-ocean ridges.

World Relief Map

Type
Divergent
Ocean
Land
Convergent
O-C
O-O
C-C
Transform
Fault
Motion Features Examples

The main features of Plate
Tectonics are:
The Earth's surface is covered by a series of lithospheric plates.
The ocean floors are continually moving, spreading from the center, sinking at the edges, and being regenerated.
Convection currents beneath the plates move the plates in different directions.
Visualizations