fig. 15-1 (a) A normal fault is an inclined surface along which a rock mass has slipped downward; it is the result of forces that tend to stretch the earth's crust. (b) A thrust fault is an inclined surface along which a rock mass has moved upward to override the neighboring mass; it is the result of forces that tend to compress the crust. (c) A strike-slip fault is a surface along which one rock has moved horizontally with respect to the other; it is the result of oppositely directed forces in the crust that do not act along the same line. Erosion modifies the fault scarps left by normal and thrust faults.

fig. 15-2 Cross section showing effect of folding in horizontal strata. Folds always shorted the crust and hence are produced by compressional forces. An anticline is an arch (a fold convex upward), and a syncline is a trough (a fold convex downward). In regions of intense folding, anticlines and synclines follow one another in long series.

fig. 15-3 Drowned valleys on the Atlantic Coast of the United States.

fig. 15-4 Successive stages in the evolution of the Appalachian Mountains. (a) Sediments accumulating in the Appalachian basin; (b) folding and thrust faulting of rocks in the basin; (c) original mountains word down to a nearly level plain by stream erosion; (d) renewed erosion of the folded strata following vertical uplift, producing the parallel ridges and valleys of the present landscape.

fig. 15-5 How some of the continents fit together. The boundary of each continent is taken at a depth of 900 m on its continental slope; the tan regions represent land above sea level at present, and the light orange regions represent submerged land on the continental shelf and slope. Overlaps are shown in dark orange and gaps in blue.

fig. 15-6 The landmasses of the earth as they may have appeared in the past and as they are today. The breakup of Pangaea into Laurasia and Gondwana began about 200 million years ago. There is evidence for even earlier continental drift.

fig. 15-7 Large blocks of wood float higher and extend farther downward than smaller blocks of wood. This is why the thicker continental crust extends lower into the mantle than the oceanic crust, as shown in Fig. 14-12.

fig. 15-8 The principle of echo sounding. (a) A pulse of high-frequency sound waves is sent out by a suitable device on a ship. (b) The time at which the pulse returns to the ship is a measure of the sea depth.

fig. 15-9 The worldwide system of oceanic ridges, rises, and trenches. The ridges and rises are offset by transverse fracture zones.

fig. 15-10 Pattern of magnetization along the Mid-Atlantic Ridge southwest of Iceland. Sea-floor rocks whose directions of magnetization are the same as that of today's geomagnetic field are shown in dark blue; the intervening spaces represent rocks whose magnetization is in the opposite direction.

fig. 15-11 Ocean-floor spreading. A midocean ridge forms where molten rock rises from the asthenosphere along a crack in the ocean floor and pushes apart the lithosphere on both sides.

fig. 15-12 The chief lithospheric plates whose motion results in continental drift. The plates are bounded by ridges, rises, trenches, and faults. The arrows show the directions of plate motions. Dashed lines indicate uncertain boundaries. The African plate is thought to be stationary. Because Japan lies on or near the intersections of four plates, one-tenth of the world's earthquakes rock its islands.

fig. 15-13 (a) Origin of the Hawaiian Islands. (b) The islands today.

fig. 15-14 Three situations can occur when lithospheric plates come together. (a) Oceanic-continental plate collision. The Andes Mountains of South America are the result of such a collision. (b) Oceanic-oceanic plate collision. The islands of the West Indies originated in this way. (c) Continental-continental plate collision. A collision of this kind thrust up the Himalaya Mountains between India and the rest of Asia.

fig. 15-15 The sea floor spreads apart at midocean ridges where molten rock rises to the surface of the lithosphere. At a trench, one lithosphere plate is forced under another into the asthenosphere, where it melts. Mountain ranges, volcanoes, and island arcs are found where plates collide. The vertical scale is greatly exaggerated.

fig. 15-16 A transform fault separates two plates that are sliding past each other. An example is the San Andreas Fault in California.

fig. 15-17 How the earth may appear 30 million years from now if present plate motions continue.

fig. 15-18 Schematic cross section showing folded sedimentary rocks that were displaced along a fault and then intruded by granite.

fig. 15-19 An unconformity is an irregular eroded surface that separates one set of rock layers from an earlier set. Shown is an unconformity above tilted lower layers; the layers above and below an unconformity can also be parallel.

fig. 15-20 The principle of radiocarbon dating. The radioactive ¹⁴C content of a sample of dead animal or plant tissue decreases steadily, while its ¹²C content remains constant. Hence the ratio of ¹⁴C to ¹²C contents indicates the time that has elapsed since the death of an organism. The half-life of ¹⁴C is 5700 years.

fig. 15-21 How the number of families living things has varied throughout history. A family is a group of related species, and the greater the number of families, the greater the diversity of life. Each dip in the curve represents a biological extinction; there have been 20 major ones in all. The most severe extinction marks the end of the Paleozoic Era; the dinosaurs disappeared in the extinction at the end of the Mesozoic Era. Life is at its most diverse today, although human activity has begun to turn the graph downward.

fig. 15-22 Two common types of structural traps in which petroleum accumulates: (a) a trap formed by an anticline. (b) a trap formed by a fault. In both cases petroleum in a porous reservoir rock (such as sandstone) is prevented from migrating upward by an impermeable cap rock (such as shale). A well drilled at A would strike petroleum, one drilled at B would strike gas, and one at C only water. About 80 percent of known petroleum deposits are found in anticline traps.

fig. 15-23 A gigantic crater whose traces were found in Mexico's Yucatan Peninsula may have been formed by the impact of the comet or asteroid that led to the extinction of the dinosaurs and many other forms of life at the end of the Mesozoic Era.

fig. 15-24 The maximum extent of Pleistocene glaciers in North America. There were four major ice advances that covered up to 30 percent of the earth's land surface. Today about 10 percent lies under ice.

fig. 15-25 Evolutionary tree of humans and apes, based on DNA evidence. Monkeys split off earlier. Chimpanzees, bonobos, and humans have 98 percent of their genes (and every bone in their bodies) in common.

fig. 15-26 World population according to United Nations figures. Estimates for future population vary widely; shown is the midrange projection.