381
Testing the Plate Tectonics Model
dant in lava flows of basaltic
composition.* Basaltic lavas
erupt at the surface at
temperatures greater than
1000 °C, exceeding a threshold temperature for magnetism known as the Curie point
(about 585 °C). Consequently, the magnetite grains
in molten lava are nonmagnetic. However, as the lava
cools these iron-rich grains become magnetized and align themselves in
the direction of the existing magnetic lines of force. Once the minerals
solidify, the magnetism they possess will usually remain “frozen” in this
position. Thus, they act like a compass needle because they “point”
toward the position of the magnetic poles at the time of their formation.
Rocks that formed thousands or millions of years ago and contain a
The JOIDES Resolution, one of the drilling ships
of the Integrated Ocean Drilling Program. (Ocean
Drilling Program) Inset photo shows the newly
launched Japanese vessel Chikyu, the world’s
most advanced drilling ship. (AP/Wide World)
Geographic
north
Magnetic
north
FIGURE 15.20 Earth’s magnetic field consists of
lines of force much like those a giant bar magnet
would produce if placed at the center of Earth.
D I D Y O U K N O W ?
When all of the continents were joined
to form Pangaea, the rest of Earth’s
surface was covered with a huge ocean
called Panthalassa (
,
). Today all that remains
of Panthalassa is the Pacific Ocean,
which has been decreasing in size since
the break-up of Pangaea.
thalassa = sea
pan = all
*Some sediments and sedimentary rocks contain enough iron-bearing mineral grains
to acquire a measurable amount of magnetization.
Testing the Plate Tectonics Model
dant in lava flows of basaltic
composition.* Basaltic lavas
erupt at the surface at
temperatures greater than
1000 °C, exceeding a threshold temperature for magnetism known as the Curie point
(about 585 °C). Consequently, the magnetite grains
in molten lava are nonmagnetic. However, as the lava
cools these iron-rich grains become magnetized and align themselves in
the direction of the existing magnetic lines of force. Once the minerals
solidify, the magnetism they possess will usually remain “frozen” in this
position. Thus, they act like a compass needle because they “point”
toward the position of the magnetic poles at the time of their formation.
Rocks that formed thousands or millions of years ago and contain a
The JOIDES Resolution, one of the drilling ships
of the Integrated Ocean Drilling Program. (Ocean
Drilling Program) Inset photo shows the newly
launched Japanese vessel Chikyu, the world’s
most advanced drilling ship. (AP/Wide World)
Geographic
north
Magnetic
north
FIGURE 15.20 Earth’s magnetic field consists of
lines of force much like those a giant bar magnet
would produce if placed at the center of Earth.
D I D Y O U K N O W ?
When all of the continents were joined
to form Pangaea, the rest of Earth’s
surface was covered with a huge ocean
called Panthalassa (
,
). Today all that remains
of Panthalassa is the Pacific Ocean,
which has been decreasing in size since
the break-up of Pangaea.
thalassa = sea
pan = all
*Some sediments and sedimentary rocks contain enough iron-bearing mineral grains
to acquire a measurable amount of magnetization.