• Hot spot     —*- Direction of movement
^ FIGURE I2-I4 A map of the world showing the plates, their boundaries, direction of movement, and hot spots.
Ridge axis          Transform
* á á
Subduction zone
Zones of extension within continents
Uncertain plate boundary
^> FIGURE I2-22 This map shows the average rate of movement in centimeters per year and relative motion of the Earth's plates.
Areas of overlap Gaps
£> FIGURE 12-5 The best fit between continents occurs along the continental slope at a depth of 2,000 m.
Gíossopteris                                                                                                                                                    Lystrosaurus
^ FIGURE 12-9 Some of the animals and plants whose fossils are found today on the widely separated continents of South America, Africa, India, Australia, and Antarctica. These continents were joined together during the Late Paleozoic to form Gondwana, the southern landmass of Pangaea. Clossoptem and similar plants are found in Pennsylvanian- and Permian-aged deposits on all five continents. Mesosaums is a freshwater reptile whose fossils are found in Pemiian-aged rocks in Brazil and South Africa. Cyrtognathus and Lyströsciurus are land reptiles who lived during the Early Triassic Period. Fossils of Cynognathus are found in South America and Africa, while fossils of Lystrosaums have been recovered from Africa, India, and Antarctica.
(?
Käuai
3.8-5.6 million years
OahuVsO 2.3-3,3 miííion years
Direction of plate motion
Hawaii
0 7 million years
to present
Recent       \^ Indicates volcanism           active volcano
0
100
km
Maui 0-1.3 million years
Undersea volcano
(a)                                                                                                                (b)
^ FIGURE 4-26  (a) Generalized diagram showing the origin of the Hawaiian Islands. As the lithospheric plate moves over a hot spot, a succession of volcanoes forms. Present-day volcanism occurs only on Hawaii and beneath the sea just to the south, (b) Map showing the age of the islands in the Hawaiian chain.
Mantle  a
:                     -í SS                                                                                                                                                                                            Ä
>                                               Inner                                                /
core
Ě> FIGURE 1-13 The Earth's plates are thought to move as a result of underlying mantle convection cells in which warm material from deep within the Earth rises toward the surface, cools, and then, upon losing heat, descends back downward into the interior.
^* FIGURE I2-II  The sequence of magnetic anomalies preserved within the oceanic crust on both sides of an oceanic ridge is identical to the sequence of magnetic reversals already known from continental lava flows. Magnetic anomalies are formed when basaltic magma intrudes into Oceanic ridges; when the magma cools below the Curie point, it records the Earth's magnetic polarity at the time. Sea-floor spreading splits the previously formed crust in half, so that it moves laterally away from the oceanic ridge. Repeated intrusions record a symmetrical series of magnetic anomalies that reflect periods of normal and reversed polarity. The magnetic anomalies are recorded by a magnetometer, which measures the strength of the magnetic field.
^ FIGURE I2-I2 The age of the world's ocean basins established from magnetic anomalies demonstrates that the youngest oceanic crust is adjacent to the spreading ridges and that its age
increases away from the ridge axis.
Pleistocene to Recent (0-1.6 M.Y.A.
Pliocene (1.6-5 M.Y.A.) Miocene (5-24 M.Y.A.) Oligocene (24-37 M.YA) Eocene (37-58 M.Y.A.)
Paleocene (58-66 M.YA.) Late Cretaceous (66-88 M.Y.A.) Middle Cretaceous (88-118 M.Y.A.) Early Cretaceous (118-144 M.Y.A.) Late Jurassic (144-161 M.Y.A.)
£> FIGURE 1-15  An idealized cross section illustrating the relationship between the lithosphere and the underlying asthenosphere and the three principal types of plate boundaries: divergent, convergent, and transform.
Transform
plate
boundary
Upwelüng
Oceanic-oceanic convergent plate boundary
Lithosphere
^> FIGURE I2-I6 The East African rift valley is being formed by the separation of eastern Africa from the rest of the continent along a divergent plate boundary. The Red Sea represents a more advanced stage of rifting, in which two continental blocks are separated by a narrow sea.
^■Mi          _A__A__A_          _________
Oceanic          Zone of                Transform
ridge              subduction          fault
^> FIGURE I2-2I Transform plate boundary. The San Andreas fault is a transform fault separating the Pacific plate from the North American plate. Movement along this fault has caused numerous earthquakes. The photograph shows a segment of the San Andreas fault as it cuts through the Leona Valley, California. (Photo courtesy of Eleanora I. Robbins, U.S. Geological Survey.)