Scattered around the globe are more than one hundred regions of volcanic activity known as hot spots (hot spot: a place in the upper mantle of the earth at which hot magma from the lower mantle upwells to melt through the crust usually in the interior of a tectonic plate to form a volcanic feature; also: a place in the crust overlying a hot spot). Unlike most volcanoes, hot spots are rarely found along the boundaries of the continental and oceanic plates that comprise the Earth’s crust; most hot spots lie deep in the interior of plates and are anchored deep in the layers of the Earth’s surface. Hot spots are also distinguished from other volcanoes by their lavas, which contain greater amounts of alkali metals than do those from volcanoes at plate margins.
In some cases, plates moving past hot spots have left trails of extinct volcanoes in much the same way that wind passing over a chimney carries off puffs of smoke. It appears that the Hawaiian Islands were created in such a manner by a single source of lava, welling up from a hot spot, over which the Pacific Ocean plate passed on a course roughly from the east toward the northwest, carrying off a line of volcanoes of increasing age. Two other Pacific island chains—the Austral Ridge and the Tuamotu Ridge—parallel the configuration of the Hawaiian chain; they are also aligned from the east toward the northwest, with the most recent volcanic activity near their eastern terminuses.
That the Pacific plate and the other plates are moving is now beyond dispute; the relative motion of the plates has been reconstructed in detail. However, the relative motion of the plates with respect to the Earth’s interior cannot be determined easily. Hot spots provide the measuring instruments for resolving the question of whether two continental plates are moving in opposite directions or whether one is stationary and the other is drifting away from it. The most compelling evidence that a continental plate is stationary is that, at some hot spots, lavas of several ages are superposed instead of being spread out in chronological sequence. Of course, reconstruction of plate motion from the tracks of hot-spot volcanoes assumes that hot spots are immobile, or nearly so. Several studies support such an assumption, including one that has shown that prominent hot spots throughout the world seem not to have moved during the past ten million years.
Beyond acting as frames of reference, hot spots apparently influence the geophysical processes that propel the plates across the globe. When a continental plate comes to rest over a hot spot, material welling up from deeper layers forms a broad dome that, as it grows, develops deep fissures. In some instances, the continental plate may rupture entirely along some of the fissures so that the hot spot initiates the formation of a new ocean. Thus, just as earlier theories have explained the mobility of the continental plates, so hot-spot activity may suggest a theory to explain their mutability.
20. The primary purpose of the passage is to
(A) describe the way in which hot spots influence the extinction of volcanoes
(B) describe and explain the formation of the oceans and continents
(C) explain how to estimate the age of lava flows from extinct volcanoes
(D) describe hot spots and explain how they appear to influence and record the motion of plates（D）
(E) describe the formation and orientation of island chains in the Pacific Ocean
21. According to the passage, hot spots differ from most volcanoes in that hot spots
(A) can only be found near islands
(B) are active whereas all other volcanoes are extinct
(C) are situated closer to the earth’s surface
(D) can be found along the edges of the plates（E）
(E) have greater amounts of alkali metals in their lavas
22. It can be inferred from the passage that evidence for the apparent course of the Pacific plate has been provided by the
(A) contours of the continents
(B) dimensions of ocean hot spots
(C) concurrent movement of two hot spots
(D) pattern of fissures in the ocean floor（E）
(E) configurations of several mid-ocean island chains
23. It can be inferred from the passage that the spreading out of lavas of different ages at hot spots indicates that a
(A) hot spot is active
(B) continental plate has moved
(C) continental rupture is imminent
(D) hot spot had been moving very rapidly（B）
(E) volcano contains large concentrations of alkali metals
24. The passage suggests which of the following about the Hawaiian Islands, the Austral Ridge, and the Tuamotu Ridge?
(A) The three chains of islands are moving eastward.
(B) All the islands in the three chains have stopped moving.
(C) The three island chains are a result of the same plate movement.
(D) The Hawaiian Islands are receding from the other two island chains at a relatively rapid rate.（C）
(E) The Austral Ridge and the Tuamotu Ridge chains have moved closer together whereas the Hawaiian Islands have remained stationary.
25. Which of the following, if true, would best support the author’s statement that hot-spot activity may explain the mutability of continental plates?
(A) Hot spots move more rapidly than the continental and oceanic plates.
(B) Hot spots are reliable indicators of the age of continental plates.
(C) Hot spots are regions of volcanic activity found only in the interiors of the continental plates.
(D) The alignment of hot spots in the Pacific Ocean parallels the alignment of Pacific Ocean islands.（E）
(E) The coastlines of Africa and South America suggest that they may once have constituted a single continent that ruptured along a line of hot spots.
26. The author’s argument that hot spots can be used to reconstruct the movement of continental plates is weakened by the fact that
(A) hot spots are never found at the boundaries of plates
(B) only extinct volcanoes remain after a plate moves over a hot spot
(C) lava flow patterns for all hot spots have not been shown to be the same
(D) the immobility or near immobility of hot spots has not been conclusively proven（D）
(E) the changing configurations of islands make pinpointing the locations of hot spots difficult
27. The author’s style can best be described as
Climatic conditions are delicately
Climatic conditions are delicately adjusted to the composition of the Earth’s atmosphere. If there were a change in the atmosphere—for example, in the relative proportions of atmospheric gases—the climate would probably change also. A slight increase in water vapor, for instance, would increase the heat-retaining capacity of the atmosphere and would lead to a rise in global temperatures. In contrast, a large increase in water vapor would increase the thickness and extent of the cloud layer, reducing the amount of solar energy reaching the Earth’s surface.
The level of carbon dioxide, CO2, in the atmosphere has an important effect on climatic change. Most of the Earth’s incoming energy is short-wavelength radiation, which tends to pass through atmospheric CO2 easily. The Earth, however, reradiates much of the received energy as long-wavelength radiation, which CO2 absorbs and then remits toward the Earth. This phenomenon, known as the greenhouse effect, can result in an increase in the surface temperature of a planet. An extreme example of the effect is shown by Venus, a planet covered by heavy clouds composed mostly of CO2, whose surface temperatures have been measured at 430℃. If the CO2 content of the atmosphere is reduced, the temperature falls. According to one respectable theory, if the atmospheric CO2 concentration were halved, the Earth would become completely covered with ice. Another equally respectable theory, however, states that a halving of the CO2 concentration would lead only to a reduction in global temperatures of 3℃.
If, because of an increase in forest fires or volcanic activity, the CO2 content of the atmosphere increased, a warmer climate would be produced. Plant growth, which relies on both the warmth and the availability of CO2 would probably increase. As a consequence, plants would use more and more CO2. Eventually CO2 levels would diminish and the climate, in turn, would become cooler. With reduced temperatures many plants would die; CO2 would thereby be returned to the atmosphere and gradually the temperature would rise again. Thus, if this process occurred, there might be a long-term oscillation in the amount of CO2 present in the atmosphere, with regular temperature increases and decreases of a set magnitude.
Some climatologists argue that the burning of fossil fuels has raised the level of CO2 in the atmosphere and has caused a global temperature increase of at least 1℃. But a supposed global temperature rise of 1℃ may in reality be only several regional temperature increases, restricted to areas where there are many meteorological stations and caused simply by shifts in the pattern of atmospheric circulation. Other areas, for example the Southern Hemisphere oceanic zone, may be experiencing an equivalent temperature decrease that is unrecognized because of the shortage of meteorological recording stations.
21. The passage supplies information for answering which of the following questions?
(A) Why are projections of the effects of changes in water vapor levels on the climate so inaccurate?
(B) What are the steps in the process that takes place as CO2 absorbs long-wavelength radiation?
(C) How might our understanding of the greenhouse effect be improved if the burning of fossil fuels were decreased?
(D) What might cause a series of regular increases and decreases in the amount of CO2 in the atmosphere?（D）
(E) Why are there fewer meteorological recording stations in the Southern Hemisphere oceanic zone than elsewhere?
22. The author is primarily concerned with
(A) explaining the effects that the burning of fossil fuels might have on climate
(B) illustrating the effects of CO2 on atmospheric radiation
(C) discussing effects that changes in the CO2 level in the atmosphere might have on climate
(D) challenging hypotheses about the effects of water vapor and CO2 on climate（C）
(E) refuting hypotheses by climatologists about the causes of global temperature fluctuations
23. The passage suggests that a large decrease in the amount of CO2 in the atmosphere would result in
(A) at least a slight decrease in global temperatures
(B) at the most a slight increase in short-wavelength radiation reaching the Earth
(C) a slight long-term increase in global temperatures
(D) a large long-term increase in the amount of volcanic activity（A）
(E) a slight short-term increase in atmosphere water vapor content
24. The author refers to Venus primarily in order to
(A) show the inherent weakness of the greenhouse effect theory
(B) show that the greenhouse effect works on other planets but not on Earth
(C) show the extent to which Earth’s atmosphere differs from that of Venus
(D) support the contention that as water vapor increase, the amount of CO2 increases（E）
(E) support the argument that the CO2 level in the atmosphere has a significant effect on climate
25. The passage suggests that if there were a slight global warming at the present time, it would be
(A) easy to measure the exact increase in temperature because of the abundance of temperature recording stations throughout the world
(B) difficult to measure the increase of CO2 in the atmosphere because of local variations in amounts
(C) easy to demonstrate the effects of the warming on the water vapor in the atmosphere
(D) difficult to prove that the warming was caused by the burning of fossil fuels（D）
(E) easy to prove that the warming was caused by an increase of cloud cover
26. The discussion of climate in the passage suggests which of the following conclusion?
I. Climate is not perfectly stable, and slight regional temperature variations can be considered a normal feature of the environment.
II. We are unable at present to measure global temperature changes precisely.
III. The most important cause of regional climatic fluctuations is the change in CO2 levels in the atmosphere.
(A) I only
(B) III only
(C) I and II only
(D) II and III only（C）
(E) I, II, and III
27. All of the following can be found in the author’s discussion of climate EXCEPT
(A) a statement about the effects of increased volcanic activity on the Earth’s temperatures
(B) an indication of the effect of an increase in water vapor in the atmosphere
(C) a contrast between two theories about the effects of a lowering of CO2 levels in the atmosphere
(D) a generalization about the efficiency of meteorological recording stations（D）
(E) a hypothesis about the relationship between atmospheric gases and changes in climate
The primary method previously used
The primary method previously used by paleontologists to estimate climatic changes that occurred during Pleistocene glacial cycles was the determination of 18O/16O ratios in calcareous fossils. However, because this ratio is influenced by a number of factors, the absolute magnitude of the temperature difference between Pleistocene glacial and interglacial cycles could not be unequivocally ascertained. For example, both temperature fluctuations and isotopic changes in seawater affect the 18O/16O ratio. And, since both factors influence the ratio in the same direction, the contribution of each to the 18O/16O cannot be determined.
Fortunately, recent studies indicate that the racemization reaction of amino acids can be used to determine more accurately temperatures that occurred during Pleistocene glacial cycles. Only L-amino acids are usually found in the proteins of living organisms, but over long periods of geological time these acids undergo racemization, producing D-amino acids, which are not found in proteins. This reaction depends on both time and temperature; thus, if one variable is known, the reaction can be used to calculate the other.
24. It can be inferred from the passage that determination of the temperatures mentioned in line 17 through 18O/16O ratios and determination through racemization reactions both require which of the following?
(A) Calcium deposits known to be from Pleistocene seas
(B) Proteins containing both L-amino acids and D-amino acids
(C) Glacial debris from both before and after the Pleistocene period
(D) Fossil material from organisms living during the Pleistocene period（D）
(E) Proteins containing both amino acids and 18O
25. The passage suggests that the 18O/16O ratio could be used more successfully as a means of measurement if scientists were able to
(A) determine the 18O/16O ratio in living animals as well as in fossil remains
(B) locate a greater number of calcareous fossils from the Pleistocene glacial and interglacial cycles
(C) locate the factors other than temperature fluctuations and isotopic changes in seawater that affect the 18O/16O ratio
(D) arrive at more exact determinations of which amino acids are found in the proteins of living organisms（E）
(E) isolate the relative effects of temperature fluctuations and isotopic changes in seawater on 18O/16O ratios
26. The information in the passage can be used to answer which of the following questions?
I. Do temperature variations and isotopic changes in seawater cause the 18O/16O ratio to shift in the same direction?
II. What are the methods used to determine the 18O/16O ratio?
III. Is the study of racemization reactions useful in estimating climatic changes that occurred during Pleistocene glacial cycles if only one of the two important variables is known?
(A) I only
(B) I and II only
(C) I and III only
(D) II and III only（C）
(E) I, II, and III
27. According to the passage, before the recent experiments described in the passage were completed, scientists could
(A) determine temperatures only for Pleistocene seas
(B) determine temperatures that occurred during Pleistocene glacial cycles only by examining fossil remains
(C) measure changes in temperatures that occurred during Pleistocene glacial cycles with only questionable accuracy
(D) only partially identify factors tending to lower Pleistocene temperatures（C）
(E) accurately determine temperatures only for land masses affected by glaciation
The stratospheric ozone layer is
The stratospheric ozone layer is not a completely uniform stratum, nor does it occur at the same altitude around the globe. It lies closest to the Earth over the poles and rises to maximum altitude over the equator. In the stratosphere, ozone is continuously being made and destroyed by natural processes. During the day the Sun breaks down some of the oxygen molecules to single oxygen atoms, and these reacting with the oxygen molecules that have not been dissociated, form ozone. However, the sunlight also breaks down ozone by converting some of it back to normal oxygen. In addition naturally occurring nitrogen oxides enter into the cycle and speed the breakdown reactions. The amount of ozone present at any one time is the balance between the processes that create it and those that destroy it.
Since the splitting of the oxygen molecules depends directly upon the intensity of solar radiation, the greatest rate of ozone production occurs over the tropics. However ozone is also destroyed most rapidly there, and wind circulation patterns carry the ozone-enriched upper layers of the atmosphere away from the equator. It turns out that the largest total ozone amounts are found at high latitudes. On a typical day the amount of ozone over Minnesota, for example, is 30 percent greater than the amount over Texas, 900 miles farther south. The density and altitude of the ozone layer also change with the seasons, the weather, and the amount of solar activity. Nevertheless, at any one place above the Earth’s surface, the long-term averages maintained by natural processes are believed to be reasonably constant.
The amount of ozone near the Earth is only a small percent of the amount in the stratosphere, and exchange of molecules between the ozone layer and the air at ground level is thought to be relatively small. Furthermore, the ozone molecule is so unstable that only a tiny fraction of ground-level ozone could survive the long trip to the stratosphere, so the ozone layer will not be replenished to any significant degree by the increasing concentrations of ozone that have been detected in recent years near the earth’s surface. The long-term averages of ozone both near ground level (ground level: n.[地质]地水准平面) and in the stratosphere are regulated by continuous processes that are constantly destroying and creating it in each of these places. This is why scientists are so concerned about human beings injection into the stratosphere of chemicals like nitrogen oxides, which are catalysts that facilitate the breakdown of ozone. If the ozone layer is depleted significantly, more ultraviolet radiation would penetrate to the Earth’s surface and damage many living organisms.
17. The passage suggests that factors contributing to the variation in the amount of ozone above different areas of the Earth’s surface include which of the following?
I. Some of the ozone found at higher latitudes was produced elsewhere.
II. There is usually a smaller amount of naturally occurring nitrogen oxide over high latitudes.
III. The rate of ozone production over the poles is less than that over the tropics.
(A) II only
(B) III only
(C) I and II only
(D) I and III only（D）
(E) I, II, and III
18. Which of the following best states the central idea of the passage?
(A) Naturally occurring nitrogen oxides, as well as those introduced by humans, threaten to deplete the layer of ozone in the stratosphere.
(B) A delicate but reasonably constant balance exists between the natural processes that produce and those that destroy ozone in the stratosphere.
(C) There is little hope that the increased concentrations of ground-level ozone observed in recent years can offset any future depletion of stratospheric ozone.
(D) Meteorologically induced changes in the concentration of ozone in the stratosphere tend to cancel themselves out over a period of time.（B）
(E) Solar radiation not only produces and destroys zone but also poses a hazard to human life.
19. The processes that determine the amount of ozone in a given portion of the stratosphere most resemble which of the following?
(A) Automobile emissions and seasonal fog that create a layer of smog over a city
(B) Planting and harvesting activities that produce a crop whose size is always about the same
(C) Withdrawals and deposits made in a bank account whose average balance remains about the same
(D) Assets and liabilities that determine the net worth of a corporation（C）
(E) High grades and low grades made by a student whose average remains about the same from term to term
20. According to the passage, which of the following has the LEAST effect on the amount of ozone at a given location in the upper atmosphere?
(D) Ground-level ozone（D）
(E) Solar activity
21. The author provides information that answers which of the following questions?
I. What is the average thickness of the stratospheric ozone layer?
II. Why does increased exposure to ultraviolet radiation damage many living organisms?
III. What is the role of oxygen in the production of stratospheric zone?
(A) I only
(B) II only
(C) III only
(D) I and II（C）
(E) II and III
22. In explaining what determines the amount of ozone in the stratosphere, the author describes natural processes that form
(A) an interactive relationship
(B) a reductive system
(C) a linear progression
(D) a set of randomly occurring phenomena（A）
(E) a set of sporadically recurring events