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Internal Structure of Earth
and Plate Tectonics
2
Learning Objectives
After reading and studying this chapter, students should
 Understand the basic internal structure and processes of the Earth
 Learn the basic ideas behind and evidence for the theory of plate tectonics
 Understand the mechanisms of plate tectonics
 Understand the relationship of plate tectonics to environmental geology
Chapter Summary
Internal Structure of Earth and Plate Tectonics describes the internal structure of the Earth and the
movement of lithospheric plates on the Earth’s surface, as well as the evidence for both. The chapter
opens with a discussion of the major layers of Earth structure and explains how seismic waves have
been used to discern those layers. The subsequent sections focus on the broad-scale patterns of plate
movement (including Wegener’s early idea of continental drift), the evidence for plate motion
(volcanoes, earthquakes, etc.), the specific types of plate boundaries, the rates of plate motion, and the
use of magnetic polarity stripes and hot spot patterns to discern past plate motions. The chapter closes
with a discussion of the supercontinent Pangaea, the driving forces of plate tectonics, and the relevance
of plate tectonics to environmental geology.
Chapter Outline
I. Internal structure of the Earth
A. Earth is complex and dynamic
1. Internal processes
a. Affect continents, oceans, climate, weather, and all life on Earth
B. The Earth is layered and dynamic
1. Inner core
a. solid
b. mostly metallic iron (90%)
c. 1300 km thickness
2. Outer core
a. fluid (similar to water)
b. similar composition to inner core
c. 2000 km thick
3. Mantle
a. mostly solid
b. rocks are primarily iron and magnesium silicates
c. 3000 km thick
d. Mohorovicic discontinuity marks boundary between mantle and crust
4. Crust
a. Variable thickness
b. Outer rock layer
C. Continents and ocean basins have significantly different properties and history
1. Lithosphere
a. cool, strong, outermost layer
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Internal Structure of Earth and Plate Tectonics
b. average thickness is 100 km (few km near mid-ocean ridges up to 400 km thick beneath
continents)
c. crust is the uppermost portion of the lithosphere; crust is less dense than mantle rocks
beneath
d. oceanic crust thinner than continental crust
e. oceanic and continental crusts have very different geologic histories
2. Asthenosphere
a. hot, slowly flowing layer of relatively weak rock
b. lies beneath lithosphere
c. circulation occurs by convection
3. Continental and oceanic crust have contrasting geologic histories
II. How we know about the internal structure of Earth
A. What we have learned about Earth from earthquakes
1. Seismology: study of earthquakes and the passage of seismic waves through the Earth
B. Wave behavior
1. wave refraction and reflection reveal discontinuities between layers of the Earth’s interior
a. wave reflection: waves may reflect off discontinuities such as the core-mantle boundary
b. wave refraction: waves may change direction as they enter a material with different
properties
2. more sophisticated seismological techniques have revealed finer details of Earth structure
III. Plate tectonics
A. Movement of the lithospheric plates
1.What is plate tectonics?
a. Lithosphere is broken into plates, which move relative to one another
b. Plate tectonics describes the processes associated with the creation, movement, and
destruction of lithospheric plates
2. Locations of earthquakes and volcanoes define plate boundaries
a. lithospheric plates may include a continent and part of an ocean basin or just an oceanic
region
b. Most volcanoes and earthquakes are associated with plate boundaries
3. Seafloor spreading is the mechanism for plate tectonics
a. Continental drift
1) proposed by Alfred Wegener, 1915
2) fit of continents and distribution of fossils on southern continents
3) not taken seriously—no known mechanism
b. sea floor spreading: new crust is added to edge of lithospheric plates at mid-ocean ridges
c. subduction zones
4. Sinking plates generate earthquakes
a. as oceanic crust sinks in subduction zones, earthquakes are created
b. earthquakes evidenced by Wadati-Benioff earthquake zone
5. Plate tectonics is a unifying theory
B. Types of plate boundaries
1. Divergent boundaries
a. new lithosphere is produced
b. plates move apart
c. marked by mid-ocean ridges
d. molten rock erupts along ridge and cools in rift valley at center of ridge
2. Convergent boundaries
a. plates converge
b. higher density plate subducts
© 2012 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they
currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.
8
Chapter 2 – Instructor’s Manual
c. compression shortens crust, causing folding and thrust faulting
d. produces major mountain chains and volcanoes (e.g., Andes, Cascades; see A Closer Look:
The Wonder of Mountains)
e. island arc marks ocean-ocean subduction zone
f. subduction produces submarine trenches
g. continental collision produces high mountain ranges
h. A Closer Look: The wonder of mountains
3. Transform boundaries
a. two plates slide past one another
b. most are in oceans, but some are in continental crust (e.g., San Andreas Fault)
c. triple junctions: locations where three plates border one another
C. Rates of plate motion
1. plate motion is a fast geologic process
2. few cm/yr (about as fast as fingernail or hair growth)
a. plate interiors move along steadily
b. plate boundaries can stick, causing earthquakes
IV. A detailed look at seafloor spreading
A. Introduction
1. Wegener’s idea of continental drift lacked a solid mechanism for continental movement
a. seafloor spreading provided a mechanism for moving continents
B. Paleomagnetism
1. Definition
a. The study of the magnetism of rocks at the time their magnetic signature formed
2. Magnetic field
a. Dipole magnetic field
b. Earth has had a magnetic field for at least 3 billion years
c. Convection in outer core creates magnetic field
3. Cooling volcanic rocks record prevailing magnetic polarity
4. Earth’s magnetic field periodically reverses, every few hundred thousand years on average
a. rocks in some areas were found to have reversed magnetic polarity
5. What produces magnetic stripes?
a. stripes of magnetic polarity on the Atlantic Ocean floor near Iceland were found to match
on either side of the mid-ocean ridge
b. symmetrical magnetic patterns explained by sea-floor spreading
6. Why is the seafloor no older than 200 million years?
a. age of ocean floors can be determined by magnetic patterns: youngest near ridges, older
away from ridges to maximum oceanic crust age of 200 million years
b. continental crust is more stable than are rocks of ocean basins
C. Hot spots
1. volcanic centers from hot materials produced deep in mantle
2. as plate moves over hot spot, a chain of volcanoes is formed
a. Yellowstone volcanic region lies over a continental hotspot
a. best example is Hawaiian-Emperor chain in Pacific Ocean
b. patterns of volcanoes reveal plate motions
V. Pangaea and present continents
A. Plate tectonics shapes continents and dictates the location of mountain ranges
1. The supercontinent of Pangaea
a. supercontinent composed of all continents
b. composed of Laurasia (north) and Gondwana (south)
© 2012 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they
currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.
9
Internal Structure of Earth and Plate Tectonics
c. split 180 million years ago as modern continents began to move apart
2. Understanding plate tectonics solves long-standing geologic problems
a. occurrence of similar fossil plants and animals on different continents
b. evidence of ancient glaciation on several continents; inferred ice flow directions make sense
only if continents were together
VI. How plate tectonics works: Putting it together
A. Driving mechanisms that move plates
1. Ridge push
a. Driven by high elevation of mid-ocean ridges
b. Ridge push involves gravitational push of rock away from ridge toward subduction zone
c. Of relatively little importance compared to slab pull
2. Slab pull
a. Driven by weight of descending slab
b. Weight of slab descending into mantle at subduction zone pulls rest of plate
c. Probably of greater importance than ridge push
VII. Plate tectonics and environmental geology
A. Plate tectonics affects us all
1. Plate movement produces zones of oil and mineral resources as well as earthquakes and
volcanoes
2. Tectonic processes determine types and properties of rocks, which determine characteristics of
land, resources, and soil
3. Plate motions modify oceanic and atmospheric circulation patterns, influencing global climate
a. Climatic changes affect productivity of the land and its desirability
Answers to Review Questions and Critical Thinking Questions
Review Questions
1. The major difference between inner core and outer core is that the outer core is liquid, while the
inner core is solid. Their composition is similar.
2. The lithosphere differs from the asthenosphere in two main ways. First, the lithosphere includes the
crust, which is of lower density than the mantle. Second, the lithosphere is solid, while the
asthenosphere consists of hot, slowly moving, weak rock.
3. The three major types of plate boundaries are divergent (plates moving apart), convergent (plate
moving together), and transform (plates sliding past one another).
4. The Earth’s magnetic field is produced by convection of the liquid outer core around the solid inner
core.
5. Paleomagnetism and magnetic reversals have revealed long-term patterns of plate motion, and led to
the concept of sea-floor spreading.
6. Hot spots are volcanic centers fed by hot materials produced deep in the mantle. As plates move
over hot spots, chains of volcanoes are formed.
7. Ridge push and slab pull differ in that ridge push results from gravitational movement of plates from
the high-elevation mid-ocean ridge, while slab pull depends on forces generated as the subducting
plate sinks into the mantle. Slab pull is generally thought to be the dominant driving force of plate
motion.
Critical Thinking Questions
1. An answer to this question should focus on the importance of plate tectonics to the distribution of
continents and landscapes we see today. The ocean basins, which play a large part in determining
the patterns of oceanic circulation, would be very different, as would the Earth’s climate. North
America and other continents would look much different today if Pangaea had never broken up, as
© 2012 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they
currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.
10
Chapter 2 – Instructor’s Manual
much of western North America and substantial portions of other continents have been modified
significantly by plate motions of the past 180 million years. Environmental conditions would also
be much different, as extreme continental climates would dominate land areas, with moist climates
dominating a relatively small land area near the supercontinent edges. There would be other
marked differences, as well.
© 2012 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they
currently exist. No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher.
11
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