(For class on Tuesday, Oct. 4)
Introduction. The questions under "Reading
Questions" below are based on the intermediate version of Tarbuck and
Lutgens, "The Theory of Plate Tectonics" (version 2.01), 2003, Tasa
Graphic Arts. Inc., Chapters 1-6 (on CD). (Part II of this assignment will
focus on the remaining chapters.) These questions are designed to help you
focus on what we think are some of the most important parts of these chapters.
Although you don't need to hand in your answers to these questions, answering
them should help you prepare for an in-class exercise that you will
have to turn in for credit, as well as Exam #2.
As a prelude to this reading assignment, we first summarize
features of the global patterns of earthquakes, volcanoes, and topographic
features that we discovered during the in-class activities referred to in
the "Objectives" section above. Then you'll read through the first
six chapters of the intermediate version of Tarbuck and Lutgens, "The
Theory of Plate Tectonics" (version 2.01, on CD), looking for answers
to the reading questions while watching for ways in which the theory of plate
tectonics might help explain the patterns of earthquake, volcano, and topographic
feature distributions that we identified earlier.
Review and Recap. In the in-class activities on global distributions
of earthquakes and volcanoes, we discovered that there are systematic global
patterns of earthquake locations and depths, active volcano locations, and topographic
features such as mountains and deep-sea trenches. Moreover, there are strong
associations between most of these patterns. In particular, we discovered that:
- Earthquakes are organized in narrow, interconnecting bands or belts that
create a kind of webbed pattern around the globe. Roughly speaking, most of
the most prominent of these bands are located:
- around most of the Pacific Ocean where it borders with continents, including:
- from New Zealand around the northern side of Australia (along islands
north of Australia)
- splitting, with one branch extending to China and then Japan, and
the other through the Philippines to Japan
- along the Aleutian Islands of Alaska
- along the western side of North America
- the length of Central America
- along the western side of South America
- across the Caribbean Sea;
- across the northeastern corner of the Indian Ocean parallel to Indonesia;
- from western China (the Tibetan Plateau) westward through the Middle
East through western Europe;
- along the Red Sea and cutting across East Africa; and
- through the the Atlantic, Indian, and Antarctic Oceans, and the southeastern
corner of the Pacific.
- Earthquakes originate anywhere from immediately at the earth's surface down
to depths of 800 km (500 miles) below the surface (about 1/8 of the distance
to the center of the earth). However, in terms of depth patterns, the earthquake
bands fall into two broad categories:
- Bands that consist almost exclusively of relatively shallow earthquakes.
These include bands D through F above.
- Bands that consist of earthquakes occurring across a range of depths,
from the shallowest to deepest, starting with the shallowest on the side
adjoining an ocean and becoming progressively deeper toward the side facing
the nearest continent (roughly speaking). These include bands A through
- Earthquake bands are accompanied closely by mountain ranges, and sometimes
by deep ocean trenches. In particular:
- Earthquake bands consisting of earthquakes that are shallow on one side
of the band and deep on the other (bands A, B and C above) have (1) deep
ocean trenches next to the shallow-earthquake side of the band, and (2)
mountains (sometimes mostly underwater but protruding as chains or arcs
of islands above the sea surface) on the deep-earthquake side. [For example,
the Aleutian Islands of Alaska; Japan; the Philippines; Indonesia; a number
of islands in the western and southwestern Pacific; islands in the Caribbean;
and the Andes Mountains in South America are all associated with these
types of bands, for example.]
- Earthquake bands down the middle of most of the world's oceans (bands
F above) are associated with undersea mountain chains called mid-ocean
- The belt across southern Europe, the Middle East, and south Asia, are
associated with the Pyrenees Mts., the Alps, the Caucasus Mts., and the
Himalaya Mts., to name a few.
- Most of the world's active volcanoes are associated with earthquake bands.
(This is especially true around the edges of the Pacific Ocean, the so-called
"Ring of Fire".)
- Based on earthquake magnitude data from several recent years:
- The strongest earthquakes tend to occur along bands A and C (and even
B?), and along the Himalaya Mts. in south-central Asia.
- The mid-ocean ridges (bands F) tend to have relatively weak earthquakes.
Based on the first six chapters of the intermediate version of Tarbuck and
Lutgens, "The Theory of Plate Tectonics" (version 2.01), 2003, on
- answer the questions posed below; and
- look for explanations for the global patterns summarized above, in terms
of the theory of plate tectonics
- What is plate tectonics?
- Continental Drift: An Idea Before Its Time
- What was Pangaea? When did it exist? What happened
- In the early part of the 20th century, what were four types of evidence
that Alfred Wegener cited in support of his theory of continental
drift? Give a specific example of each.
- How can we explain similar fossils in rocks around the world that are
older than 200 million years (my), and fossil diversity in rocks younger
than 200 my?
- How can we explain (1) evidence for tropical forests in ancient rocks
found today in cold northern regions; and (2) evidence of glaciation in
ancient rocks found today in tropical regions?
- What was one of the reasons why Wegener's ideas were not widely accepted
at the time he proposed them?
- Exploring Continental Drift
(This is a review chapter. If you like,
try your hand at reconstructing Pangaea; test your knowledge of world geography;
practice using longitude as a measure of position east and west on the earth;
and become more familiar with the positions of continents and creation of
ocean basins since Pangaea began breaking up.)
- Earth's Interior
- Geologists divide the earth's interior into four layers, like layers
of an onion. What are these four layers, and what distinguishes each of
them from the others?
- The outermost layer of the earth (which is also by far the thinnest)
is the crust. It consists of continental crust and oceanic
crust. In continental crust, what is one of the most common types of rock
(among many)? What is the most common type of rock found in oceanic crust?
(One of these rock types is volcanic igneous and the
other plutonic igneous—which is which?)
- Which is thicker (that is, deeper from top to bottom)—continental
crust or oceanic crust? Which is denser (and hence tends to lie
lower on the earth's surface than the other)?
- Geologists also divide the upper portion of the earth's interior into
two layers, the asthenosphere and the lithosphere,
which differ from the four layers that you identified in (a) above because
different criteria are used to define them. The lithosphere contains the
crust and part of the upper mantle, while the asthenosphere,
which lies beneath the lithosphere, contains some more of the upper mantle.
What characteristics distinguish the lithosphere from the asthenosphere?
- New Evidence: Mapping the Ocean Floor
- In the 1950s and 1960s, what topographic features of the ocean floor
did mapping done using echo sounding technology reveal?
- What ocean bottom feature tends to be associated with active volcanoes?
- Plate Tectonics: A Scientific Revolution Unfolds
- When did the theory of plate tectonics finally emerge from the earlier
theory of continental drift? (One of your instructors remember when this
was taking place, though he was just a kid—it was a major and very
exciting development in the world of science at the time, and it occurred
at the same time as the U.S. was preparing to try to land a person on
- What allows the seven major lithospheric plates (and
several other smaller plates) to move around? (Lithospheric plates are
also called tectonic plates, or just plates.)
- Do these lithospheric plates consist either exclusively of continental
land mass or exclusively of ocean floor? If not, can you cite an example?
- With respect to lithospheric plates, where do most earthquakes, volcanoes,
and mountain building occur?
- What are the three types of plate boundaries? At each
type of boundary, what are the motions of the plates relative to each
- Are all three types of boundaries ever found around the periphery of
any single plate?
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