Page 63 - Laboratory Manual in Physical Geology

North
5.4
Eurasian Plate American 1.8
▲ ▲ ▲ ▲ ▲ ▲ ▲ Plate Eurasian
Plate
▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ fault Caribbean 3.0
3.0 3.7 ▲ ▲
5.4 Juan de Fuca Plate San Andreas
▲ ▲ ▲
▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲
Arabian Plate 2.0 ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ Philippine 5.5 Galapagos Plate ▲ ▲ ▲ Mid-Atlantic Ridge 2.5
Rise
Plate
▲ ▲
10.8
▲ ▲ ▲ Pacific Plate Cocos ▲ ▲
2.7 10.5
African Plate ▲ ▲ ▲ ▲ ▲ Plate 7.0
13.4
4.4 ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ 3.5
▲ ▲ ▲ ▲ ▲ Nazca Plate South
7.3 Indo-Australian Pacific Rise 5.9 American
Plate
Plate ▲ ▲ 7.1 3.5
1.4 7.7 Peru-Chile
1.5 East 9.4 Trench
3.7 Chile
Rise
▲ ▲ ▲ ▲
7.2
5.7 Antarctic Plate
Divergent Convergent Transform fault 0 2000 4000 6000 8000 Miles
A. boundary B. ▲ ▲ ▲ ▲ boundary C. boundary
0 2000 4000 6000 8000 10,000 Kilometers

FIGURE 2.5 Earth’s lithospheric plates and their boundaries. Numerals indicate relative (in relation to each other) rates of plate

motion in centimeters per year (cm/yr); not the actual (absolute) rates of individual plate motion that you may have studied in Activity

2.1 . Divergent plate boundaries (red) occur where two adjacent plates form and move apart (diverge) from each other. Convergent plate

boundaries (hachured with triangular “teeth”) occur where two adjacent plates move together. Transform fault plate boundaries (dashed)

occur along faults where two adjacent plates slide past each other. Refer back to FIGURE 2.2 for another perspective of the three kinds of plate
boundaries.
is three orders of magnitude (1000 times) stronger than periodic reversals . During times of normal polarity ,
Earth’s magnetic field. Even so, you can use the tiny the north-seeking end of a compass needle (and tiny
magnetic needle in a compass to detect Earth’s magnetic iron-bearing mineral crystals in volcanic rock) points
field. Magnetic compass needles are not attracted to the in the direction of Earth’s present north magnetic pole.
geographic North Pole. Instead, they are attracted to But during times of reversed polarity , the north-
the magnetic north pole, which is located in the Arctic seeking end of a compass needle points in the opposite
Islands of Northern Canada, about 700 km (450 mi) direction (geographic south).
from the geographic North Pole.
Magnetic Anomalies and Paleomagnetic
Paleomagnetism
Stripes
Tiny crystals of iron-rich minerals, such as magnetite
(Fe O ), acquire and retain the directional signature Magnetic anomalies are deviations from the average
3 4
of Earth’s magnetic field when they form. This ancient strength of the magnetic field in a given area. Areas of
magnetism is called paleomagnetism . Magnetic mineral higher than average strength are positive anomalies, and
crystals lose this magnetism if heated above the Curie Point areas of less than average strength are negative anoma-
of about 580°C. Only when they cool below the Curie lies. In the 1950s the U.S. Coast and Geodetic Survey
Point do mineral crystals acquire and retain the signature scanned the ocean for magnetic anomalies and discov-
of Earth’s magnetic field for the time and place where they ered that rocks of the sea floor contained alternating
cooled. This happens when volcanic lava cools and crystal- striped patterns of high and low magnetic anomalies,

lizes below the Curie Point. called paleomagnetic stripes . They also discovered that
the pattern of paleomagnetic stripes was symmetrical
on opposite sides of mid-ocean ridges. In 1963, geolo-
Magnetic Reversals gists Fred Vine, Drummond Matthews, and Lawrence
When geologists first started detecting paleomagnetism Morley discovered that the symmetrical pattern of
in layers of cooled lava (volcanic rock) stacked one paleomagnetic stripes in seafloor rocks was the result of
atop the other, they discovered that Earth’s magnetic two processes: the formation of seafloor and reversals of
field has not always been the same. It has undergone Earth’s magnetic field. They proposed that as volcanoes

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