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Chapter 2
HOW WE STUDY EARTH
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
Artist’s depiction of the space shuttle Endeavour during the Shuttle Radar Topography
Mission. The SRTM mapped 80 percent of Earth’s land surface using a dual-antenna
radar.
[NASA.]
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
FIGURE 2.1 Research is the process of discovery and confirmation through observation
of the natural world. These geologists are researching soil samples near a lake in
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
Minnesota. [Courtesy of USCS.]
2.3 Marine scientists Craig Marquette and Will Ostrom, from the Woods Hole
Oceanographic Institution, deploy a mooring to measure temperatures from the
Research Vessel Oceanus during a gale off Cape Hatteras. [Chris Linder, Woods
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
Hole Oceanographic Institution.]
Don Swanson and Christina Heliker of the USGS survey
benchmarks on the Lava Dome from the crater floor of
Mount St. Helens, circa 1981. (Lyn Topinka photo)
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
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The geologic record preserves
evidence of Earth’s long history.
These multicolored layers of sand
at Colorado National Monument
were deposited more than 200
million years ago when this part of
the western United States was a
vast Sahara-like desert. They were
subsequently overlain by other
rocks, welded by pressure into
sandstone, uplifted by mountainbuilding events, and eroded by
wind and water into today’s stunning
landforms.
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
Sensitive instruments like this mass spectrometer can determine
the age of a rock by measuring minute amounts of radioactive
elements
and how that radioactivity has decayed since the rock
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
first formed. [Rosenfeld Images Ltd./Photo Researchers.]
Jordan, The phenomena
Essential Earth 1e © 2008
by W.
H. Freeman
and Company
Geologic
can
stretch
over
thousands of centuries or can occur with
dazzling speed.
How Eratosthenes measured Earth’s circumference. At noon on the summer solstice
(June 21), a vertical well in the southern Egyptian town of Syene (near modern Aswan)
was filled with light, indicating that the Sun was directly overhead, whereas a vertical pole
in his hometown of Alexandria cast a shadow. From the length of the shadow, he found
that Syene and Alexandria were separated by about 1/50 of Earth’s circumference, about
Jordan,of
The360°.
EssentialSince
Earth 1e ©the
2008 by
W. H. Freeman and
Company between the two towns was approximately
7° out
measured
distance
800 km, he calculated a circumference close to its modern value of 40,000 km.
Earth’s topography is measured with respect to sea level.
Elevations vary from almost 9 km above sea level to about 11 km
Jordan, Thesea
Essentiallevel.
Earth 1e © 2008
W. H. Freeman
and Company
below
Inbythis
picture,
the elevation scale is greatly
exaggerated
FIGURE 2.10 Earth’s major layers, showing their volume
and mass as a percentage of Earth’s total volume and
mass.
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
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FIGURE 2.11 (a) This
stony meteorite, which
is similar in
composition to Earth’s
silicate mantle, has a
density of about 3
g/cm3.
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
FIGURE 2.11 (b) This ironnickel meteorite, which is similar in
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
composition
to Earth’s core, has a density of about 8 g/cm3. [John
FIGURE
2.13 Jumps in density between Earth’s major layers are
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
caused by differences in their chemical composition.
FIGURE 2.14 The Earth system is an open system that
exchanges energy and mass with its surroundings.
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
Grotzinger/Ramón Rivera-Moret/Harvard Mineralogical Museum.]
FIGURE 2.12 Crustal rocks are denser than mantle
rocks, allowing the crust to float on the mantle.
Continental crust is thicker and has a lower density than
oceanic crust, which causes it to ride higher, explaining
the elevation difference between continents and the
deep sea floor.
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
FIGURE 2.15 The Earth
system encompasses all parts
of our planet and their
interactions. This diagram
illustrates the basic
components of the Earth
system, how they are
energized, and which among
them interact to form the
climate system, the plate
tectonic system, and the
geodynamo system.
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FIGURE 2.16 Convection carries heat upward through
the motion of matter.geodynomo system.
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
FIGURE 2.17 (left) The magnetic field of a bar magnet is revealed by the alignment of
iron filings on paper. (right) Earth’s magnetic field is much like the field
that would be produced if a giant bar magnet were placed at
Earth’s center and slightly inclined (11°) from the axis of rotation. The lines of force
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
produced
by Earth’s magnetic field are shown. At any place on Earth, a compass
needle will point to the north magnetic pole because it orients to the local line of force.
Jordan, The Essential Earth 1e © 2008 by W. H. Freeman and Company
FIGURE 2.18 This ribbon of
geologic time shows some
of the major events
observed in the geologic
record, beginning with the
formation of the planets.
(Ma, million years ago.)
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