COS 7: Theory of Plate Tectonics Notes

Introduction to plate tectonicsPlate tectonics

1. the scientific theory that attempts to explain the movements of the Earth's lithosphere

2. that have formed the landscape features we see across the globe today. 3. By definition the word "plate" in geologic terms means a large slab of solid rock. 4. "Tectonics" is a part of the Greek root for "to build" 5. together the terms define how the Earth's surface is built up of moving plates.6. Geology and Cartography

The theory of plate tectonics 1. Says that the Earth's lithosphere is made up individual plates that are broken

down into over a dozen large and small pieces of solid rock.2. These fragmented plates ride next to each other on top of the Earth's more fluid lower mantle to create different types of plate

boundaries that have shaped the Earth's landscape over millions of years.History of Plate Tectonics

1. Plate tectonics grew out of a theory that was first developed in the early 20th century by the meteorologist Alfred Wegener.2. In 1912, Wegener noticed that the coastlines of the east coast of South America and the west coast of Africa seemed to fit

together like a jigsaw puzzle.3. Further examination of the globe revealed that all of the Earth's continents fit together somehow and Wegener proposed an idea

that all of the continents had at one time been connected in a single supercontinent called Pangaea. 4. He believed that the continents gradually began to drift apart around 300 million years ago - this was his theory that became

known as continental drift.5. Continental Drift

Main problem 1. Wegener's initial theory was that he was unsure of how the continents moved apart from one another. 2. Throughout his research to find a mechanism for continental drift, Wegener came across fossil evidence that gave support to his

initial theory of Pangaea.3. In addition he came up with ideas as to how continental drift worked in the building of the world's mountain ranges.4. Wegener claimed that the leading edges of the Earth's continents collided with each other as they moved causing the land to

bunch up and form mountain ranges. 5. He used India moving into the Asian continent to form the Himalayas as an example.6. Eventually Wegener came up with an idea that cited the Earth's rotation and its centrifugal force toward the equator as the

mechanism for continental drift. 7. He said that Pangaea started at the South Pole and the Earth's rotation eventually caused it to break up, sending the continents

toward the equator. 8. This idea was rejected by the scientific community and his theory of continental drift was dismissed as well.

Thermal Convection: 1. In 1929, Arthur Holmes, a British geologist, introduced a theory

of thermal convection to explain the movement of the Earth's continents.

2. He said that as a substance is heated its density decreases and it rises until it cools sufficiently to sink again.

3. According to Holmes it was this heating and cooling cycle of the Earth's mantle that caused the continents to move.

4. This idea gained very little attention at the time.5. By the 1960s, Holmes' idea began to gain more credibility as

scientists increased their understanding of the ocean floor via mapping, discovered its mid-ocean ridges and learned more about its age.

6. In 1961 and 1962, scientists proposed the process of sea floor spreading caused by mantle convection to explain the movement of the Earth's continents and plate tectonics.

7. Ridge push – weight of an uplifted ridge pushes an oceanic plate toward the trench.8. Ridge push and the horizontal flow at the top of the convection current could create drag on the lithosphere and cause plate

motion9. Slab pull – weight of a subducting plate pulls the trailing lithosphere into the subduction zone10. This sinking region of mantle convection could suck in an oceanic plate into the subduction zone.

Principles of Plate Tectonics Today1. Scientists today have a better understanding of the make-up of the tectonic plates, the driving forces of their movement, and the

ways in which they interact with one another.

COS 7: Theory of Plate Tectonics Notes

2. Modern evidence of plate movement 3. A tectonic plate itself is defined as a rigid segment of the Earth's lithosphere that moves separately from those surrounding it.

Three main driving forces (for the movement of the Earth's tectonic plates.)1. mantle convection,

a. Mantle convection is the most widely studied method of tectonic plate movement and it is very similar to the theory developed by Holmes in 1929.

b. There are large convection currents of molten material in the Earth's upper mantle. c. As these currents transmit energy to the Earth's asthenosphere (the fluid portion of the Earth's lower mantle below the

lithosphere) new lithospheric material is pushed up toward the Earth's crust. d. Evidence of this is shown at mid-ocean ridges where younger land is pushed up through the ridge, causing the older land

to move out and away from the ridge, thus moving the tectonic plates.2. gravity,

a. Gravity is a secondary driving force for the movement of the Earth's tectonic plates. b. At mid-ocean ridges the elevation is higher than the surrounding ocean floor. c. As the convection currents within the Earth cause new lithospheric material to rise and spread away from the ridge,

gravity causes the older material to sink toward the ocean floor and aid in the movement of the plates. 3. Earth's rotation,

a. The Earth's rotation is the final mechanism for the movement of the Earth's plates but it is minor in comparison to mantle convection and gravity.

b. As the Earth's tectonic plates move they interact in a number of different ways and they form different types of plate boundaries.

Plate Boundaries: 1. Divergent boundaries

a. are where the plates move away from each other and new crust is created.

b. Mid-ocean ridges are an example of divergent boundaries. c. Form ridges in ocean d. Rift valley on land (East Africa)e. Rift valley may form new ocean basin

2. Convergent boundaries a. are where the plates collide with one another causing

the subduction of one plate beneath the other. b. 3 types

i. Ocean crust to ocean crustii. Ocean crust to continental crust

iii. Continental crust to continental crustc. Subduction – when one of 2 plates moves beneath

the otherd. O to O: Subduction plates descends into mantle and

melts (recycling)e. O to C: Denser oceanic plate gets subductedf. C to C: colliding edges are crumpled and uplifted

(mountains)3. Transform boundaries are the final type of plate boundary and at these locations the no new crust is created and none is

destroyed. a. Instead the plates slide horizontally past one another.

COS 7: Theory of Plate Tectonics Notes

No matter the type of boundary though, the movement of the Earth's tectonic plates is essential in the formation of the various landscape features we see across the globe today.

How Many Tectonic Plates Are on Earth?There are seven major tectonic plates

1. North America, 2. South America, 3. Eurasia, 4. Africa, 5. Indo-Australian, 6. Pacific 7. Antarctica8. Several microplates such as the Juan de Fuca plate near the United States' state of Washington.

COS 7: Theory of Plate Tectonics Notes

Please use the following colors for the plates.

1- Green -dark2- Yellow3- Red4- Green - light5- Purple6- Orange7- Blue - dark8- Pink

9- Teal10- Violet11- Blue – Light12- Brown 13- Black14- Orange-Red15- Gray/Silver

COS 7: Theory of Plate Tectonics Notes

Seafloor Spreading1. Until the mid -1900s people thought the ocean floor was flat2. They also thought that the ocean crust was unchanging and older than the continental crust3. Bill Nye - Seafloor Spreading

Technology1. Sonar- echo-sounding methods

a. Uses sound waves to measure water depthb. Sonar Basics

2. Magnetometera. Detects small changes in magnetic fieldsb. Mapping the ocean floor

3. False-color photo Ocean Floor a. A false-color image is an image that depicts an object in colors that differ from those a photograph (a "true-color"

image) would show.b. Ocean Ridges – underwater mountain chainsc. Deep sea trenches – narrow elongated depressions in the sea floor with very steep sidesd. Mariana Trench – Earth

Ocean Rocks and Sediments:Collection of ocean rocks and sediments led to 2 important discoveries

1. Ages of rocks that make up seafloor spreading vary in different placesa. Changes are predictableb. Age of oceanic crust consistently increases with the distance from a ridgec. Rock samples oldest part is about 180 million years old

2. Sediments thickness is much less than expecteda. Sediments thickness increases with distance from an ocean ridge

Magnetism: 1. Paleomagnetism – study of earth’s magnetic history2. Basalt

a. Iron rich rockb. formed by volcanism c. Provides accurate record d. Basaltic lava cools and the earth’s magnetic field is imprinted

within the rocke. Similar to a compass needlef. When the lava cools - earth’s magnetic field is locked in

3. Geomagnetic time scalea. Is a pattern of magnetic reversals over time

COS 7: Theory of Plate Tectonics Notes

4. Magnetic reversal a. A change in the earth’s magnetic field

5. Magnetic symmetrya. The magnetic pattern of one side of the ridge is a

mirror image of the pattern on the other side of the ridge

b. By matching patters on the seafloor with known land reversal, the age of the ocean floor was discovered

c. Isochron map is a line on a map that connect points that have the same age

Seafloor spreadingd. Harry Hess proposed this theorye. New ocean crust is being formed at ocean ridges

and destroyed in deep sea trenches

COS 7: Theory of Plate Tectonics Notes

Quiz: 5 Question Quiz