Plate Tectonic Theory - PowerPoint Presentation

Slide1

Plate Tectonic Theory

One Theories Journey Slide2

Continental drift -- One theory's journey

Today, you might be laughed out of a geology course for questioning whether continents move through geologic time. But 75 years ago, only skeptics believed that continents could take a hike. Talk about conventional

unwisdom

: W.B. Scott, former president of the American Philosophical Society, even called the theory of continental drift "

utter damned rot

." (!) What's changed? Not the actual movement of continents, but our understanding of geology itself. Let's take a look at how the theory developed, and how the evidence began to favor it. Slide3

Antonio Snider-

Pellegrini

suggests that continents were linked during the Pennsylvanian period (325 million to 286 million years ago), because Pennsylvanian plant fossils from Europe and North America were similar. (See "carboniferous" on this

time line

.Slide4

Australian geologist Edward Seuss sees similarities between plant fossils from South America, India, Australia, Africa and Antarctica, and coins "Gondwanaland" for a proposed ancient super-continent with these land masses.Slide5

F.B.Taylor 1910

American physicist F.B. Taylor proposes concept of continental drift to explain formation of mountain belts.

Unfortunately no one remembers him or has his photo because he did not support his theory with

emperical

evidence!Slide6

Alfred Wegner 1912

German meteorologist

Alfred Wegener proposes theory of continental drift, based on evidence from geology, climatology and paleontology.

Wegener names one of the ancient super-continents "

Pangea

," and draws maps showing how the continents moved to today's positions.Slide7

How’d they move again?

Assorted arguments are used to debunk continental drift, most importantly the lack of a mechanism strong enough to move continents across or through ocean basins.Slide8

South African geologist Alexander du

Toit

maps out a northern super-continent,

"

Laurasia

," to explain coal deposits, which presumably indicate the remains of equatorial plants, in the Northern Hemisphere.Slide9

Paleomagnetism

Paleomagnetism: British scientists find that

magnetic fields recorded in rocks from Europe and North America indicate the rocks were formed in far different locations than their present positions

. The pattern of continental drift recorded by rocks show Europe and North America have drifted away from each other for more than 100 million years. This movement opened the Atlantic Ocean.Slide10

Paleomagnetism Slide11

Gondwana

Fossils of the plant genus

Glossopteris

occur on all five Gondwana continents. The seeds were too heavy to be carried by wind, and would have died quickly in salt water, indicating that the continents were once joined. Similarly, fossils of several reptiles occur on several continents.Slide12

Gondwana Slide13

Appalachian Mountain Chain

The Appalachian Mountain chain can be linked with mountains in Greenland, the United Kingdom and Norway,

indicating that these land masses were joined at the time the mountain chain formedSlide14

Marks left by glaciers on rocks in Africa, India, South America and Australia make no sense -- unless these continents were joined and arrayed around the South Pole

. Then, the glacial scars would have all pointed away from the Pole when they were made. Today, glaciers form near the poles and move away as they travel and eventually melt.Slide15

Ocean Floor

The oldest rocks on the ocean floor are younger than 220 million years, while the oldest terrestrial rocks are about 4 billion years old, indicating that the ocean floor is recycled back into the Earth.Slide16

Earth's magnetic field periodically changes polarity (your compass would point to the South Pole).

When magma solidifies at the mid-oceanic ridges, it records the polarity of the Earth's magnetic field. Bands of seafloor basaltic rocks paralleling the mid-oceanic ridges carry a record of this alternating polarity.Slide17

Geologists think convection cells in the mantle (the hot, plastic rock under the crust) power continental movements, overcoming an early objection to continental drift.Slide18

Convection Cells Slide19

Plate Boundries

Three kinds of boundaries

Continental drift -- or plate tectonics -- involves a lot of complicated motion at the plate boundaries. The tamest version generally occurs beneath the oceans, when plates move away from each other. At these

divergent

plate boundaries, molten magma rises and solidifies into solid rock, filling the gap formed as the two plates move apart. These

spreading centers

(see figure below or the graphic at the top of this page) form ridges on the seafloor.

As spreading continues and the rocks move away from the ridge, they cool and contract as they age. The movement usually occurs at a relatively constant rate of a few centimeters per year and tends not to produce large earthquakes. Slide20

What? Slide21

Either of the other two types of boundaries may be associated with large earthquakes.

At

transform plate boundaries

one plate moves laterally against and past another.

Some transform boundaries are also described as

strike slip faults

.

At a

convergent

boundary, one plate must either slip beneath the other (a

subduction

zone

) or the two plates must collide (a collision zone).

A classic

subduction

zone has a denser oceanic plate diving, or "

subducting

", beneath a less dense continental plate. Slide22

San Andres Fault in California Slide23

But

when a giant rock hits an immovable object -- when one tectonic plate moves suddenly against another

-- the havoc of a major earthquake can result. Lacking the

stress

relief of regular, minor earthquakes, strong rock gets stuck at the fault zone, allowing

strain

to build up. Slide24

Earthquake in Haiti Slide25

When the strain gets too great, it is relieved by the sudden movement causing a major earthquake

. The greater the strain, the larger the earthquake. Thus in a sense, earthquakes should be predictable if we know the strain and the strength of the rocks. Slide26

SeizmographSlide27

Unfortunately, despite that simple equation,

precise predictions of quakes are not possible now.

Beyond problems measuring the strength of rocks, we have only a foggy picture of the triggering mechanism. "How the slip on a fault starts is a fundamental problem in seismology," says Clifford Thurber, a geophysicist at the University of Wisconsin-Madison. "We don't really know the conditions and state that a fault is in when it starts moving." Slide28

The problem, simply, is inaccessibility.

Earthquakes start underground -- sometimes dozens or hundreds of kilometers deep, and "there is no direct way to detect conditions,"

as Thurber says. Indirect measurements may offer a guideline, but direct observations would be preferable.Slide29

That's one reason for a recent proposal to drill 3.5 kilometers into the San Andreas Fault to obtain direct measurements from that active region. Thurber notes that cores removed from the hole would be examined for strength and fluid content. Instruments in the hole itself would look at fluid pressure, which may be implicated in initiating quakes. "We want to get direct measurements of the physical properties for the first time," Thurber saysSlide30

The project would be a step toward the eventual -- and we stress

eventual

prediction of earthquakes. We'll get to that prospect shortly. Slide31

References

Earth Quakes @ Geology wise.

Edu

http://www.geology.wisc.edu/courses/g115/quake/5.html

The information in this presentation was taken almost in its entirety from Geology Wisconson.edu