Rocks are not motionless. They not only move but also break, and they can even change shape without breaking. When buried deep within the earth under conditions of great pressure, rocks can bend like soft metal or, like bread dough or modelling clay, be squeezed into new shapes. Most deformed rocks that we now see at the earth's surface underwent deformation when buried far below the surface. A surface along which rocks of any kind have broken and moved past one another is called a fault.  Faults have formed during the present century, and some of these have intersected the surface of the earth, where earth movements along them have left visible scars. Movements of the earth, which are often associated with faulting, are partially responsible for the origin of mountains, although buildup of volcanic igneous rocks has also contributed to the growth of most mountain chains. Movement along faults is sporadic. Sometimes after years of quiescence, rocks will suddenly move several metres along a fault. Uplift or subsidence (depression) of land, which may or may not be accompanied by faulting, can also be sporadic or gradual. Sudden uplift of local areas and subsidence of others accompanied the 1964 earthquake in Alaska.

Most of our knowledge about the structure of the earth's interior derives from the study of oscillatory movements called seismic waves, which travel through the earth as a consequence of natural or artificial disturbances. An earthquake is an example of a natural seismic disturbance that results from the sudden movement of one portion of the earth against another along a fault. Artificial explosions at the earth's surface also produce seismic waves.

Primary and secondary waves are two types of seismic waves that have provided particularly valuable insight into the nature of the earth's interior. Primary waves, which are also called P waves, propagate changes in volume as portions of the earth are alternately compressed and expanded. Secondary waves, or S waves, shake the material from side to side perpendicular to their direction of movement. Primary waves are so designated because they travel faster than secondary waves.

An earthquake always begins at a focus, which is a place within the earth where rocks move against other rocks along a fault, producing both P waves and S waves. Earthquake foci lie within the earth's mantle and crust, far from the centre of the earth—but the waves that foci emit often pass great distances through the earth to emerge at the surface, where they can be detected with machines called seismographs. Geophysicists can evaluate the nature of the earth's internal structure by recording at many locations the arrival times of P waves and S waves of an earthquake.

The study of seismic waves reveals that the materials that form the central part of the earth are much more dense than those near the earth's surface. The density gradient from the surface to the centre of the earth is not a gradual one, however; instead, the planet is divided into several discrete concentric layers. At the earth's centre is the core,whose solid, spherical inner portion and liquid outer portion are thought to consist primarily of iron. Forming a thick envelope around the outer core is the mantle, a complex body of less dense rocky material. Finally, capping the mantle is the crust, which consists of still less dense rocky material. The density gradient from the core to the crust developed early in the earth's history, when molten materials of low density came to float on materials of higher density.

There are several ways in which the study of seismic waves has revealed the aspects of the earth's interior just described and others as well. For example, when earth-quake waves reach a boundary between two concentric layers within the earth, they are usually both reflected from the boundary and transmitted through it, just as light striking the surface of a body of water is partially reflected and partially transmitted. S waves, however, do not penetrate the earth's outer core, and since it is known that liquids cannot transmit S waves, this strongly suggests that the outer core is made of liquid. It is also known that both P waves and S waves travel more rapidly through material of high density than through material of low density. Changes in wave velocity have thus revealed that the earth increases in density with depth and, further, that this increase is not gradual. The passage of seismic waves from the rocks of the crust to the denser rocks of the mantle, for example, is signalled by an abrupt increase in velocity known as the Mohorovicic discontinuity, or Moho, for short . Because continental crust is much thicker than the crust beneath the oceans, the Moho dips downward beneath the continents.

The rocks that form oceanic crust are the type known as mafic—a label whose first three letters indicate that these dark rocks are rich in magnesium (Mg) and iron (Fe). Mafic rocks are much less common in continental crust than are lighter coloured, less dense rocks that are described as felsic—an adjective derived from the first three letters of feldspar, which is the most common mineral of continental crust. In comparison to mafic rocks, felsic rocks are rich in silicon and aluminum and poor in the heavier elements magnesium and iron. Rocks of the mantle are even richer in magnesium and iron than is the oceanic crust—hence their great density—and they are known as ultra-mafic rocks.

Continental surfaces not only stand above the surface of the oceanic crust but also extend farther down into the mantle than oceanic crust. It also shows that continental crust beneath a mountain range extends even farther down into the mantle than that located elsewhere. Isostatic adjustment, or the upward or downward movement that keeps crust in gravitational equilibrium as it floats on the mantle, is responsible for this phenomenon. In effect, the root beneath a mountain acts to balance the mountain.

Although the crust and the upper mantle are separated by a difference in density, they are firmly attached to one another, forming a rigid layer known as the lithosphere. Below the lithosphere is the asthenosphere, which is also known as the "low-velocity zone" of the mantle because it has been found that seismic waves slow down as they pass through it. This property tells us that the asthenosphere is composed of partially molten rock—slushlike material consisting of solid particles with liquid occupying the spaces in between. Although the asthenosphere represents no more than 6 percent of the thickness of the mantle, the mobility of this layer allows the overlying lithosphere to move. The lithosphere does not move as a unit, however; instead it is divided into plates that move in relation to one another. Some plates carry continents with them as they move, while others carry only oceanic crust.

Some plates, such as the one that includes the continent of Asia, are enormous, while others, such as the one that forms the floor of the Caribbean Sea, constitute only a minute fraction of the skin of the earth. Plates move over the surface of the earth about as rapidly as your fingernails grow. Slow as this rate may seem, the progress of plates over millions of years has been considerable. Many have moved about 500 or 1000 kilometres (300 to 600 miles) in 10 million years. Since the early 1960s, it has been recognized that many earth movements can be attributed to the motions of plates. Movement of the edge of one plate over the edge of another is in fact a major source of mountain building.

Continental drift facilitated the development of separate, distinctive faunas and floras, not merely because of the physical separation of the new continents by ocean barriers, but in other ways also. The climates of land areas newly bordered by seas became milder and less variable. Where new mountain ranges lay across the path of the prevailing rain- bringing winds, new deserts grew in their lee. Finally, as the continents continued northward, their northern fringes reached such a high latitude that they became covered by permanent ice-sheets. This may have been the reason for the exaggeration and narrowing of the climatic zones; it may also have led in turn to the great Ice Ages of the Pleistocene, which wrought havoc upon the plant and animal life of the Northern Hemisphere. It is, perhaps, no mere coincidence that both the Permo-Carboniferous glaciation and the Pleistocene glaciations occurred at times when a considerable area of land lay near to one of the poles.

It is worth considering for a moment what patterns of distribution we might expect to find had the continents always had their present positions, so that the only changes would then have been the relatively minor climatic variations of the Northern Hemisphere Ice Ages, and changes in sea level making or breaking the intercontinental Bering and Panama land bridges. At times when the climate was warmer than it is today, the spread of animals and plants across the Bering region between Siberia and Alaska would have been possible. Similarly, in the absence of the deserts of the Middle East, there would also have been a single tropical fauna and flora stretching from West Africa to South- East Asia. It would not be surprising, however, if the later development of these deserts, dividing the tropical region into African and Asian sections, had allowed distinctive features to appear in the faunas and floras of each. Finally, it might have been expected that the complete isolation of Australia and the almost complete isolation of South America would have led to the development of unique faunas and floras on these continents.

Though these features are clearly visible in the accepted patterns of animal and plant distribution, other aspects of these patterns are less simple to explain. Today's floral realms are based upon the distribution of the flowering plants, or angiosperms. Perhaps the most fundamental feature of angiosperm distribution is the fact that, almost everywhere in the world, four families are among the six most numerous —the Compositae, Graminae, Leguminosae and Cyperaceae. Similarly, dicotyledonous angiosperms are almost everywhere more abundant and diverse than the monocotyledonous types. Even the floras of the isolated continents, Australia and South America, though they certainly display unusual features, are not basically unique, composed of major groups found nowhere else.

Furthermore, if the spread of angiosperms through the world had been through the pattern of continents we see today, one would expect each of the three southern temperate regions (Australia and the southern parts of South America and Africa) to have a flora derived from that of its own adjacent tropical region. This is not the case, as is shown most clearly by the contrast between the floras of Australia and of New Guinea. Twelve of the 28 most dominant families of angiosperm in each region are completely absent from the other region, and the Australian flora appears as an intrusive element in an otherwise uniform southern Pacific flora. In fact, rather than being related to the floras neighbouring them to the north, the floras of the three south temperate regions show considerable similarity to one another, despite their separation by wide expanses of ocean. For example, over 700 species of angiosperm are more or less entirely restricted to two or three of these regions, and six families (the Cunoniaceae, Escalloniaceae, Gunner-aceae, Philesiaceae, Proteaceae and Restionaceae) are found in all three. Even more well defined is the flora of the extreme cold temperate region south of 45°s, which is composed largely of groups of angiosperm which are scarcely, if at all, represented further north. This "Antarctic" flora is characteristic of the extreme southern end of South America, southeast Australia, Tasmania and New Zealand—it is absent from South Africa, which extends only to 35°S. This type of distribution has long puzzled biogeographers—see, for example, Darlington's discussion of the distribution of the southern beech, Nothofagus.

All these facts clearly suggest that the angiosperms spread through the world at a time when the continents had not yet split apart, so that a flora of fairly uniform composition (at family level) spread everywhere. Part of this uniform flora became adapted to the temperate climate of the southern part of Gondwanaland. When this supercontinent split up, the flora found itself on three separate continents and, though some genera have since become extinct, many are still to be found in more than one of these areas. This theory is compatible with what is known of the sequence of events.

The angiosperms originated in the Jurassic, were still a relatively insignificant part of the floras of the Lower Cretaceous, and became dominant in the Mid-Cretaceous. Gondwanaland is thought to have broken up during the Cretaceous and, though the precise timing is still uncertain, it is clearly possible that the angiosperms spread throughout that supercontinent before the break-up—some types still characteristic of the south temperate region today, such as the Proteaceae and the southern beech, Nothofagus, had already appeared in Australia in the Mid-Cretaceous. In Laurasia, two separate angiosperm floral realms had already appeared by the Upper Cretaceous. One covered eastern North America and the whole of Europe, the other extended throughout Asia and western North America. The two were separated by the shallow epicontinental Turgai Straits, which lay east of the Urals and ran across Asia from north to south, and this region still marks the separation between the European and the Asiatic sub-regions of the Boreal floral region today.

Though it is not yet possible to trace in detail the transition between the two, the basic characteristics of today's floral realms are already discernible in the Cretaceous floral distributions.