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Web Extras for Chapter 17: Plate Tectonics
4.
Web Extras: The age of the seafloor. The Marine Geology and Geophysics section of the National
Geophysical Data Center features spectacular images showing the topography and age of the seafloor:
www.ngdc.noaa.gov/mgg/image/crustalimages.html. Use the Crustal Age Image with Plates to answer the
following questions:
a.
Which ocean hosts the oldest oceanic crust on Earth?
b.
Where are the oldest parts of the Atlantic Ocean?
How old is the oldest crust, and to which time period does this correspond (see Fig. 13.7 for a time scale)?
c.
Where are the oldest parts of the Pacific Ocean?
How old is this crust, and to which time period does this correspond (see Fig. 13.7 for a time scale)?
d.
Describe where we find most of the youngest oceanic crust. What is the age of the youngest oceanic crust?
e.
Explain the relationship between the youngest oceanic crust and the plate boundaries running through the
Atlantic, Indian, and eastern Pacific oceans.
f.
Compare the ages of the Atlantic oceanic crust along the coasts of Greenland and northern Europe (north
Atlantic), the United States and Africa (central Atlantic), and Argentina and southern Africa (south
Atlantic). Which part of the Atlantic Ocean opened up first, which second, and which part last?
g.
The age of the seafloor east of South America parallels the east coast of South America as well as the plate
boundary in the middle of the Atlantic. In contrast, seafloor of a wide range of ages occurs off the west
coast of South America. Explain why there is this contrast. (Hint: A map showing the tectonic boundaries
and their senses of relative motion appears inside the front cover of the lab manual.)
If you pay attention to detail, you may have noticed that the western Pacific is rather complicated and
features several small patches of young oceanic crust. You can compare the age of the seafloor map with
Web Extras for the Laboratory Manual for Physical Geology by Jones and Jones (McGraw-Hill).
this detailed tectonic plates map (http://earthquake.usgs.gov/learn/topics/plate_tectonics/plates.php) to
further investigate the mysteries of the Earth’s ocean bottom.
5.
Plate tectonics and sediment accumulation on the seafloor. Visit
www.ngdc.noaa.gov/mgg/sedthick/sedthick.html and click on the map to see the total thickness of sediment on
the ocean floors. Use this map to answer the following:
a.
Where is sediment the thickest, near the continents or in the deep ocean? Why?
b.
If you compare the trend of the ocean ridges (see the previous problem) with the sediment thickness map,
you will see that the ocean ridges have the thinnest sediment cover of anywhere in the oceans. Explain
whether this observation is consistent with the theory of plate tectonics.
c.
If you consult the tectonic map inside the front cover of the lab manual, you will see that a slab of oceanic
lithosphere—the Nazca Plate—is subducting under the western edge of South America (as in Fig. 17.5B).
New lithosphere for the Nazca Plate is produced along the East Pacific Rise, which is also on the map
inside the front cover. The Andes Mountains run along the western edge of South America, just east of the
subduction zone. Meanwhile, the east side of South America is a trailing continental margin, which was
once a divergent-plate boundary but now lies well within the South American plate.
Using what you know about plate tectonics, propose an explanation (hypothesis) for why so little sediment
is present on the subducting Nazca Plate whereas it is so thick on the Atlantic side.
d. Note the thickness of the sediment in the Bay of Bengal (east of India). This area is also close to a
convergent plate boundary, but it is of a type like that in Figure 17.5C.
i. How does the sediment thickness in the Bay of Bengal compare with that on the west side of South
America?
ii. Formulate a hypothesis, based on plate-tectonic theory, to explain why this is so.
The PaleoMap Project features a host of maps and animations showing how the Earth’s tectonic plates have moved
over the past 1100 million years. Click on the link to go straight to the animations:
www.scotese.com/newpage13.htm.
Web Extras for the Laboratory Manual for Physical Geology by Jones and Jones (McGraw-Hill).