Lab #4

Plate Tectonics: Global Patterns

Name : ________________________ Date: _____________Period: _____ Earth Science

IntroductionA revolution in twentieth-century Earth science is the plate tectonics theory. This is a time of

profound breakthroughs in our understanding of how the continents and oceans evolved, why earthquakes and volcanoes occur where they do, and the reasons for the present arrangement and movement of landmasses. One task of physical geography is to explain the spatial implications of all this new knowledge and its effect on Earth's surface and society.

Many billions of years ago, as the Earth solidified, heavier elements (materials) slowly gravitated toward the center, and lighter elements (materials) slowly welled upward to the surface, concentrating in the crust. Earth's interior is highly structured, with uneven heating generated by the radioactive decay of unstable elements. The results of this heating and instability are irregular patterns of moving, warping, and breaking of the crust.

In this lab exercise we work with plate tectonics and the evolution of the present configuration of continents and ocean basins. A reality of plate tectonics is that pieces of Earth's crust are on the move, actively migrating and interacting. Plate interactions are characterized and portrayed in this lab. We examine the correlation among types of plate boundaries and how the continents arrived at the positions that they are today.

Earth's Internal Structure and the CrustAs Earth solidified, gravity sorted materials by density. Heavier substances such as iron gravitated

slowly to its center, and lighter elements such as silica slowly welled upward to the surface and became concentrated in the crust. Consequently, Earth's interior is sorted into roughly concentric layers, each one distinct in either chemical composition or temperature.

Heat energy migrates outward from the center by conduction and by physical convection (movement) in the more fluid or plastic layers in the mantle and nearer the surface.

Our knowledge of Earth's internal differentiation into these layers is acquired entirely through indirect evidence, because we are unable to drill more than a few kilometers into Earth's crust. There are several physical properties of Earth materials that enable us to approximate the nature of the interior. For example, when an earthquake or underground nuclear test sends shock waves through the planet, the cooler areas, which generally are more rigid, transmit these seismic waves at a higher velocity than do the hotter areas, where seismic waves are slowed to lower velocity. This is the science of seismic tomography, as if Earth is subjected to a kind of CAT scan with every earthquake. This difference in speed allows us to calculate the different layers of the Earth based on each layer's density.

Completion and analysis items concerning Earth's internal structure.1. Use the space in Figure 1 (next page) and sketch Earth's interior from the core to the crust. Consult page 10 of your Reference Booklet. Color and fill in the following labels (name, density, depth) at the appropriate locations. Crust - Green, Asthenosphere - Yellow, Mantle - Orange, Outer Core - Red, Inner Core - Blue.

2. What 2 layers does the lithosphere contain? ________________________________________________

3. How have scientists figured out the layers of the Earth's interior since no one has ever sampled the interior below the crust directly? Explain. _________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________Plate Tectonics

Have you ever looked at a world map and noticed that a few of the continental land-masses appear to match shape, like pieces of a jigsaw puzzle—particularly South America and Africa? The incredible reality is that the continental pieces indeed once were fitted together!

Continental landmasses not only migrated to their present locations but continue to move at annual speeds up to 6 cm (2.4 in.) per year.

In 1912, German geophysicist and meteorologist Alfred Wegener publicly presented in a lecture his idea that Earth's landmasses migrate. His book, Origin of the Continents and Oceans, appeared in 1915. Wegener today is regarded as the father of this concept, called continental drift. Wegener postulated that all landmasses were united in one supercontinent approximately225 million years ago, during the Triassic period. This one landmass he called Pangaea, meaning “all Earth.” Wegener's proposal launched a half-century debate leading up to the adoption of plate tectonics as an all-encompassing theory of continental drift, sea-floor spreading, and subduction (one plate sliding over

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another). Aided by an avalanche of discoveries, the theory today is nearly universally accepted as an accurate model of the way Earth's surface evolves, and virtually all Earth scientists accept the fact that continental masses move about.

The Breakup of Pangaea Figure 2 gives you an opportunity to work with an updated version of Wegener's Pangaea, 225 to 200 million years ago (Triassic–Jurassic periods). Areas of North American, North Africa, the Middle East, and Eurasia were near the equator and therefore were covered by plentiful vegetation. The landmasses focused near the South Pole and attached to the sides of Antarctica were covered by ice. We also see that Africa shared a common connection with both North and South America. The Appalachian Mountains in the eastern United States and the Atlas Mountains of northwestern Africa reflect this common ancestry; they are, in fact, portions of the same mountain range. The Atlantic Ocean did not exist. The only oceans were Panthalassa (“all seas”), which would become the Pacific, and the Tethys Sea (partly enclosed by the African and the Eurasian plates), which formed the Mediterranean Sea, with trapped portions becoming the present-day Caspian Sea.

Activity and questions about the development of the plate tectonic theory

1.The landmasses outlined on the last page of this lab represent the continental crustal pieces as they were 225 million years ago. Cut out these landmass shapes using scissors. Then, using the Mollweide's (elliptical) map projection provided, arrange the pieces to construct Pangaea. Pay particular attention to latitudinal relationships. After you have decided on an appropriate arrangement paste the continental pieces to the map projection. Also, label Panthalassa and Tethys seas. Color each continent: North America - Yellow, South America - Green, Africa - Red, Antarctica - Blue, India - Orange, Australia - Purple, and Eurasia - Brown

2. What physical clues and evidence prove that these landmasses were together 225 million years ago as Pangaea and why do scientists think the continents are drifting (continent and sea-floor movements)? _______________________________________________________________________________________________________________________________________________________________________________________________________________________________

3. If you were a petroleum geologist searching for oil deposits (derived from ancient animals and plants), why would you want to know the positioning of these landmasses 225 million years ago? Explain. ____________________________________________________________________________________________________________________________________________________________________________________________________________________________________

4. In your opinion, what factors produced the long delay between the time of Wegener's lectures and book until the time of acceptance of his theory? ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

5. Why did Wegener coin the name “Pangaea”? Explain its meaning. ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Principal Motions of Plates and Plate BoundariesTo visualize the mechanisms that drive Earth's lithospheric plates refer to Figure 4. Demonstrated in cross-section are: sea-floor spreading, upwelling currents, subduction, and plate movements.

1. At the time of Pangaea, South America and Africa were attached forming a continuous landmass. Using Figure 4 (above), describe what transpired in these past 225 million years to position the two continents where they are today? ________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

2. With respect to the movement of crustal plates in the vicinity of the Andes Mountains, why is the oceanic plate diving beneath South America? Use the word density in your explanation. ______________________________________________________________________________________________________________________________________________________________________________________________________________________________________________

Plates and Plate BoundariesFigure 5 shows tectonic features in modern times (the late Cenozoic Era). A spreading center

extends down the center of the Atlantic and into the southern Indian ocean. Subduction trenches operate along the west coasts of Central and South America and throughout the western Pacific Ocean basin. The northern reaches of the India plate have under thrust the southern mass of Asia through subduction, forming the Himalayas in the upheaval created by the collision.

Of all the major plates, India traveled the farthest. From this time until the present, according to Robert Dietz and John Holden, more than half of the ocean floor was renewed. The three inset blocks demonstrate specific types of plate boundaries: convergent (subduction), divergent (sea-floor spreading, mid-ocean ridges), and transform (lateral motion between crests of spreading ridges produces transform faults).

The continents today and types of plate boundary interactions.Each arrow represents 20 million years of plate movement. The longer arrows indicate that the Pacific and Nazca plates are moving more rapidly than the Atlantic plates.

Questions and completion items about Earth's plates.1. Using page 5 of your Reference Booklet, identify types of plate boundary interactions of the following.a) On the ocean floor south of Alaska: ____________________________________b) Indian Ocean between the Antarctic and Indo-Australian plates: _______________________________c) Beneath the Red Sea: ___________________________________d) Along the west coast of Central America: _________________________________e) Through Iceland: _______________________________________f) Along the northeast coast of the Persian Gulf: ____________________________

2. Which three plates converge on Japan? ______________________________________________________________

3. Mark your location on the map above. On which plate do you live? Which direction is your plate moving? ______________________________________________________________________________________________________________Color , cut and paste onto the map on page 4 of this lab. Use the color scheme described in that part of the lab.

North America - Yellow, South America - Green, Africa - Red, Antarctica - Blue, India - Orange, Australia - Purple, and Eurasia - Brown