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In the 1930s a broad artistic trend called “the documentary impulse” emerged in the United States. Its attempt to combine social relevance with distinctively American themes came to characterize American artistic expression during that decade. The documentary impulse focused on the communication of real life, the presentation of facts and events in a way that aroused the interest and emotions of its audience. It emphasized observation and narration, but without relying on overly elaborate language for impact. A photograph or some other visual image often supplied the emotional force that words could not convey. This treatment of subject material elevated the ordinary, finding beauty and emotion in topics not usually considered within the range of art. It tried to make the audience experience the subject as though it were actually on the scene.

The documentary impulse influenced every facet of American creative expression in the 1930s. It can be seen in novels, such as John Steinbeck’s The Grapes of Wrath (1939), and in Erskine Caldwell’s Tobacco Road (1932). John Dos Passos’ U.S.A. trilogy used actual newspaper clippings, news reports, and headlines throughout its imaginary story. Filmmaker Pare Lorentz commissioned the composer Virgil Thomson to create music that set the mood for several documentary movies. The new photojournalism magazines, Life and Look, founded in 1936 and 1937, respectively, also reflected a documentary approach. The March of Time newsreels, which movie audiences saw before the feature film, presented images of the world in this period before television. The radio news broadcasts with their standard opening, “We now take you to...” did the same thing.

The camera was the prime instrument of the documentary impulse. The use of cameras to document social conditions and awaken social consciousness dated back to the probing photography of social reformers at the turn of the twentieth century. When the nation entered another period of intense social and economic questioning in the 1930s, photographers revived and updated this technique. Photographers such as Dorothea Lange and Walker Evans shaped the image of the economic depression of the decade, and documentary photography became widely popular in magazines and books that combined photography with narration.

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While there is a popular assumption that volcanic activity is always the same, eruptions come in all sorts and sizes, ranging from hissing gas to gushing molten or “melted” lava and huge explosions of ash and dust; and the side effects of eruptions can produce great sea waves, mudflows, brilliant sunsets, and fires. These many kinds of eruptions occur when hot materials, similar to molten rock and containing gases and steam, rise up from the nearly molten layer that lies under great pressure below the cool and solid crust of Earth. These materials reach the surface via chimneylike vents or along fissures that crack the crust. Specialists have spent their whole lives arguing about exactly how this happens. Basically, as the mass approaches the surface, the pressures acting upon it are reduced and the gases and steam can separate out. When the materials are fluid and hot, the gases and steam can hiss out fairly easily to the surface. Most of the molten material runs across the land as lava flows, although mild gas explosions can shatter some of it into ash and cinders that pile up as little volcanic cones usually rising less than 250 meters above the chimney. This kind of activity, with small variations, is described in the accounts of such famous eruptions as Stromboli, Etna, Lanzarote, Laki, and Paricutin.

On the other hand, the rising materials can often be much more viscous-glue-like rather than fluid-and much cooler. In this case, the steam and gas cannot easily escape as the pressure is reduced. They form bubbles that honeycomb and weaken the molten rock until it becomes a stiff, frothy mass. Suddenly the multitude of bubbles explodes with enormous force. The molten froth shatters into an infinity of small pieces that are thrust into the air, where they cool to dust, ash, or pumice (a spongy form of volcanic glass). Sometimes, these fragments form huge, incandescent clouds that can race down a volcano at 500 kilometers an hour. Even more frequently, eruptions hurl colossal, billowing, gray columns of gas and fine fragments into the stratosphere.

Water has always been one of the principal obstacles to the improvement of transportation by land. In early times travelers were limited to wading through shallow streams or stepping across a series of large rocks in the streambed. In some cases a fallen tree would conveniently span a stream, creating a beam bridge so that people could cross from one side to the other. Occasionally, large rocks could fall across the water in such a way that a natural arch was formed. In warmer climates intertwined vines formed natural suspension chains on which one could swing across a river or gorge. Thus beam, arch, and chain bridges all probably developed from natural bridges. Each of these forms of construction has been traced back several thousand years, though which type developed first is a matter of speculation. Arch bridges have been recorded as far back as 4000 B.C., and it is likely that crude forms of beam bridges were developed even earlier in which logs or slabs of stone were used to connect raised areas on either side of the water. A historic example of a form of beam bridge is that constructed on floating supports, or pontoons, by Xerxes, the king of Persia, across the Hellespont in 480 B.C. The first organized building of bridges on a large scale took place in association with military campaigns. During the expansion of their empire, the Romans built a large number of aqueducts, which were bridges of stone and timber designed to support roads and to carry water. The Romans were excellent builders.

They were able to direct large labor forces supervised by technical experts. They developed an extensive network of roads in Europe; their bridges were about 4.5 meters wide to allow two vehicles or two units of armed men to pass each other. There remain several fine examples of Roman bridge building on the European continent, perhaps the most famous of which is the Porit du Gard aqueduct in the south of France. Towering about 47 meters above the river, it has an overall length of 275 meters.

Through most of the nineteenth century, United States art had been strongly influenced by the established artistic styles of Europe. By 1900, however, a number of American artists had broken from the old European traditions and experimented with new styles. Winslow Homer, for example, created vigorously American scenes in his paintings of New England maritime life and other native subjects. James McNeill Whistler was one of the first Western artists to appreciate the beauty of Japanese color prints and to introduce Asian concepts into American art.

By the first years of the new century, some American artists were turning decisively away from the traditional, conventionally accepted style of painting. Instead, many younger painters were exploring the same grim aspects of modern life that were becoming the subject of American literature. Members of the so-called Ashcan School produced paintings that were startling in their naturalism and stark in their portrayal of the social realities of the era. John Sloan portrayed the dreariness of American urban slums; Edward Hopper explored the starkness and loneliness of the modern city. The Ashcan artists were also among the first Americans to appreciate the new Expressionist and Abstract Art movements, and showed their interest in new art forms in 1913 when they helped stage the famous "Armory Show" in New York City, which displayed controversial works of contemporary artists.

The work of these and other artists marked the beginning in the United States of an artistic movement known as Modernism, a movement that had counterparts in many other areas of cultural and intellectual life as well. Rejecting the heavy reliance on established forms that had characterized the genteel tradition of the nineteenth-century art world, Modernists rejected the past and embraced new subjects and new forms. Where the genteel tradition had emphasized the "dignified" and "elevated" aspects of civilization, Modernism gloried in the ordinary, even the coarse. Where the genteel tradition had placed great importance on respect for the past and the maintenance of "standards," Modernism looked to the future and exulted in the new. Eventually, Modernism developed strict orthodoxies of its own. However, in the early stages, it seemed to promise an escape from rigid, formal traditions and an unleashing of individual creativity.