EARTH SCIENCES - Notable Research and Discoveries Part 2

earth ScienceS Theuppermost layer of Earth, called the crust, contains the moun-tains, plains, and deserts of the continents and the seafloor. Most of the rocks of the crust are composed of silicates—compounds contain- ing the elements silicon (Si) and oxygen (O), such as silica (SiO2), a molecule which consists of one silicon atom and two oxygen atoms. Sand and quartz are common examples. Another common silicate known as olivine contains iron and magnesium along with silicon and oxygen. In terms of chemical elements, the weight of Earth’s crust is about 46 percent oxygen, 28 percent silicon, 8 percent aluminum, 6 percent iron, 4 percent magnesium, and a small percentage of other elements. Major features such as mountains do not seem to change much in a human lifetime, yet Earth is a dynamic place. The top of Mount Everest, which soars more than 29,030 feet (8,850 m) above sea level, is rich in Despite seemingly permanent features, such as Mount Rushmore in South Dakota, Earth is constantly, albeit slowly, changing. (William Walsh/ iStockphoto) eploring earth’s Depths limestone—a sedimentary rock—that contains marine fossils and was once under water! In 1912 the German researcher Alfred Wegener (1880–1930) noticed that the coasts of continents such as Africa and South America seemed to fit together and displayed remarkable simi-larities in the kind of fossils they contained, as if these now-separated continents were once adjoined. He proposed the notion of continental drift and hypothesized that continents had once been joined. Wegener had a dificult time convincing people that something as massive as a continent moves, and he was wrong, as it turned out, in some of his ideas—Wegener was unable to propose a viable mechanism by which continents move, and he incorrectly believed continents float across oceans. But anyone who has ever lived through an earthquake knows the ground can certainly move. SEISMIC WaVES Wiechert, Wegener, and other researchers encouraged their colleagues to reexamine assumptions about the dynamics and structure of Earth’s interior. But ideas alone are not suficiently convincing. Scientific evi-dence that supports a hypothesis or a particular point of view is essential before the scientific community is willing to accept an idea. Although obtaining evidence on the nature of Earth’s depths or on any other loca-tion where it is not yet possible to venture is extremely dificult, geolo-gists of the early 20th century began using seismic waves as their eyes into the planet’s interior. These waves continue to be the most impor-tant tool for these studies today. Waves are important in many branches of science, especially the study of sound and light, both of which behave (at least under certain conditions) as waves. A wave is a vibration or disturbance that prop-agates across space or in a material such as water or air. To make a wave, something has to fluctuate—electromagnetic fields in the case of light, air pressure in the case of sound, or water in the case of sea or lake waves—and it is this fluctuation that propagates. For instance, a stone dropped in a pond will create ripples spreading out from the point at which the stone fell. The fall of the stone created a disturbance that moved the water in the small region surrounding the impact zone, and these water molecules pushed against their neighbors, and so on, propagating the disturbance throughout the pond. earth ScienceS Disturbances can propagate in several different ways. A transverse wave propagates in a direction perpendicular (at a 90 degree angle) to the vibrations or oscillations, as illustrated in the bottom of the figure on page 7. Light waves are examples of transverse waves. Inside solid materials, the side-to-side oscillation (with respect to the direction of travel) is associated with a kind of force known as shear stress, so these waves are sometimes called shear waves, a term geologists often use be-cause many of the waves they study travel through solids. The top of the figure illustrates another kind of wave, called a longitudinal wave, which propagates in the same direction as the vibrations. Sound waves are longitudinal waves, since a sound wave consists of a compression propagating through air, water, or some other material, caused by mol-ecules moving toward (and then away) from each other in the same direction that the wave propagates. The compression gives these waves an alternative name—compression waves. Wave behavior is critical in optics (the study and use of light) and acoustics (the study and use of sound). Camera lenses form images on film or digital sensors by bending and focusing light, and eyeglasses and contact lenses perform a similar service for people whose vision would otherwise be blurry. The focusing is due to refraction—the bending of the wave when passing from one substance to another. For instance, when a light wave passes from air into the transparent glass of a lens, light changes speed, which causes its path to bend, or refract. Another property of waves that occurs at a boundary between two different sub-stances is reflection—some of the motion is sent back. For example, the glass of a window transmits a lot of light but also reflects some of it, so an observer looking through a window can see outside but may also notice his or her reflection in the glass. The speed of waves is also crucial. Waves travel at a specific speed in the material, or medium, through which the disturbance is propagat-ing. In general, compression waves travel faster in a medium that resists compression. For example, sound waves travel faster in the denser air at (opposite page) Compression waves consist of contractions and expan-sions in the same direction (longitudinally) as the propagation of the wave. Shear or transverse waves consist of up-and-down motions perpen-dicular to the wave’s propagation. eploring earth’s Depths Earth’s surface than the thinner air high in the atmosphere. Chuck Yea-ger, who in 1947 made the first documented flight exceeding the speed of sound, flew at an altitude of about 45,000 feet (13.7 km), where the earth ScienceS Seismic recording equipment, part of the Earthquake Arrival Recording Seismic System (EARSS) in New Zealand (New Zealand © GNS Science/SSPL/ The Image) speed of sound is 660 miles per hour (MPH) (1,056 km/hr), compared to 760 MPH (1,216 km/hr) at the surface. (Temperature also affects the speed of sound.) In water, sound waves travel about five times faster than in air. In diamond, one of the hardest substances, sound travels about 40,000 MPH (64,000 km/hr)! Compression waves generally trav-el faster than shear waves in solids, since solids tend to be more dificult to compress than to bend or twist (which is what shear forces will do). Shear waves do not propagate in water because water does not resist shear forces. ... - tailieumienphi.vn