MASS WASTING PROCESSES
1The downslope movement of rock, mud, or other material under the influence of gravity is known as mass wasting. While the angle of the slope is a major factor in the potential for mass wasting, the slope is not the sole determiner of mass wasting events. Water plays a significant role, especially where it is plentiful during the rainy season.Earthquakes may cause rockslides, mudflows, and other mass movements. Factors such as the presence or absence of vegetation and human activities can also influence the potential for mass wasting.
2One way to classify mass wasting processes is on the basis of the material involved, such as rock, debris, earth, or muD、The manner in which the material moves is also important and is generally described as a fall, a slide, or a flow.A、fall occurs when weathering loosens boulders from cliffs or rock faces, causing the boulders to break away and fall.A、slide takes place whenever material remains fairly coherent and moves along a well-defined surface.A、flow involves the movement of debris containing a large amount of water.
3Many mass wasting processes are described as slides. Rockslides occur when a coherent mass of rock breaks loose and slides down a slope as a unit. If the material involved is mostly separate pieces, it is called a debris slide. Slides are among the fastest and most destructive mass movements. Usually rockslides occur in a geologic setting where the rock layers are inclined, or where there are joints and fractures in the rock that are parallel to the slope. When such a rock unit is undercut at the base of the slope, it loses support and the rock eventually collapses. Rain or snowmelt can trigger a rockslide by wetting the underlying surface to the point that friction can no longer hold the rock in place. The fastest type of slide is a rock avalanche, in which a mass of rock literally floats on air as it moves downslope. The high speed of a rock avalanche is the result of air becoming trapped and compressed beneath the falling mass of debris, allowing it to move down the slope as a buoyant sheet.
4Mudflows are relatively rapid mass wasting events that involve soil and a large amount of water.Because of their fluid properties, mudflows follow canyons and stream channels. Mudflows often take place in semiarid mountainous regions and on the slopes of some volcanoes.Although rainstorms in semiarid regions are infrequent, they are typically heavy when they occur. When a rainstorm or rapidly melting snow creates a sudden flood, large quantities of soil and loose rock are washed into nearby stream channels because there is usually little or no vegetation to anchor the surface material. The result is a flowing mass of well-mixed mud, soil, rock, and water. The consistency of the mudflow may be similar to that of wet concrete, or it may be a soupy mixture not much thicker than muddy water. The water content influences the rate of flow across the surface. When a mudflow is dense, it moves more slowly, but it can easily carry or push large boulders, trees, and even houses along with it.
5In dry mountainous areas such as southernCalifornia, mudflows are a serious hazard to development on and near canyon hillsides. The removal of native vegetation by brush fires has increased the probability of these destructive events. Past mudflows have contributed to the buildup of fan-shaped deposits at canyon mouths. Such fans are relatively easy to build on and often have scenic views, so many have become desirable sites for residential development. However, because mudflows occur infrequently, homeowners are often unaware of the potential danger of building on the site of a previous mudflow.
6Highly fluid, fast-flowing mudflows incorporate fine-grained sediment and are common after volcanic eruptions that produce large volumes of volcanic ash. Mudflows containi

THE、FIVE—SEVEN SHIFT
1 All major theories of child psychology state that children undergo a major change between the ages of five and seven. In classical learning theory, this is a time when the simplest forms of learning give way to learning that involves more complex mental processes.According to psychologist Jean Piaget, the period from five to seven years old is a transition to operational thought, when children are able to move beyond using only their senses toward using a new set of rational-thinking skills.Because several cognitive changes occur in children between ages five and seven, this period is called the five-seven shift. The shift is biological in nature and involves fundamental growth in the brain and stabilization of brain-wave rhythms into a basically adult pattern. The five-seven shift involves many physical changes, such as the loss of the "baby teeth" and an increase in the rates of height acquired and weight gaineD、
2 By the time they are five years old, children can understand and use symbols. They have developed the ability to use words, gestures, and pictures to stand for "real life" objects, and they are skilled in deploying various symbol systems, such as language or drawing. However, a five-year-old child is able to focus attention on only one quality of an object at a time, such as the object’s size or shape. The use of symbolization continues to evolve, reaching a peak around the age of seven or eight, when children become capable of concrete operations. When this happens, they can solve problems by using rational thought to make generalizations from their own experience.
3 By the age of seven or eight, a new set of abilities allows children to reason systematically about the world of objects, quantity, time, space, and causality.According to Piaget, this is because an "extra card" is added to the child’s mental "computer" during the five-seven shift. The development of operational thought enables the child to appreciate the relations among a series of actions upon objects. For example, the child understands that a scene can be viewed from a different perspective and still contain the same elements. The child also understands that objects can be rearranged and still have the same quantity and that a substance can be changed in shape without its mass or volume being affecteD、
4Piaget discovered the most widely known hallmark of the five-seven shift, an understanding of conservation, the idea that some properties stay the same despite changes in appearance. In one of Piaget’s classic experiments on the conservation of quantity, the experimenter shows children of different ages two straight rows of coins, each with six coins pressed close together, beside each other on a table. The experimenter asks each child subject whether both rows have the same number of coins or whether one row has more. Then the experimenter spreads out the coins of one row to make the line look longer. The child must now say whether one row has more coins. Children younger than five years old cannot understand conservation, so they invariably say that the spread-out row has more coins than the other row.
5Like most age-related tasks for children, there are other ways to set up the task. In a similar experiment, water is poured into two identical glasses until the child subject agrees that each contains an equal amount. Then the experimenter pours water from one of these glasses into a tall, thin glass.At that point, the child is asked whether one glass has more water than the other. Five-year-old children will say that there is more water in the tall, thin glass. When asked why they think that, many will confidently say, "Because it’s taller." Older children, however, are likely to reply, "It looks like there’s more water in this one because it’s taller, but they’re really the same." Such exp

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