三、閱讀測驗：每題 2 分，本大題共 10 分
 
(E)About 760,000 years ago a cataclysmic volcanic eruption in the Long Valley area of easternCalifornia blew out 150 cubic miles—600 cubic kilometers of magma (molten rock) form a depth ofabout 4 miles beneath the earth’s surface. Rapid flows of glowing hot ash (pyroclastic flows) coveredmuch of east-central California, and airborne ash fell as far east as Nebraska. The earth’s surface sankmore than 1 mile into the space vacated by the erupted magma, forming a large volcanic depressionthat geologists call a caldera.
        Long Valley Caldera is part of a large volcanic system in eastern California that also includes theMono-Inyo Craters chain. This chain extends from Mammoth Mountain at the southwest rim of thecaldera northward 25 miles to Mono Lake. Eruptions along this chain began 400,000 years ago, andMammoth Mountain was formed by a series of eruptions ending 58,000 years ago. (F) The volcanicsystem is still active—eruptions occurred in both the Inyo Craters and Mono Craters parts of thevolcanic chain as recently as 600 years ago, and small eruptions occurred in Mono Lake sometimebetween the mid-1700s and mid-1800s.
 (G)Although no volcanic eruptions are known to have occurred in eastern California since thosein Mono Lake, earthquakes occurr frequently. These are caused by movement along faults and by the2pressure of magma rising beneath the surface, two closely related geologic processes. In 1872, amagnitude 7.4 earthquake centered 80 miles south of Long Valley was felt throughout most ofCalifornia, and moderate(magnitude 5 to 6)earthquakes have shaken the Long Valley area since 1978.
        In 1978, a magnitude 5.4 earthquake struck 6 miles southeast of the caldera, heralding a periodof geologic unrest in the Long Valley area that is still ongoing. (H) That temblor ended two decadesof low quake activity in eastern California. The area has since experienced numerous swarms ofearthquakes, especially in the southern part of the caldera and the adjacent Sierra Nevada.
        The most intense of these swarms began in May 1980 and included four strong magnitude 6shocks, three on the same day. Following these shocks, scientists from the U.S. Geological Survey(USGS) began a reexamination of the Long Valley area, and they soon detected other evidence ofunrest—a dome-like uplift within the caldera. Measurements showed that the center of the calderahad risen almost a foot since the summer of 1979—after decades of stability. This swelling, which by2014 totaled more than 2.5 feet and affected more than 100 square miles, is caused by new magmarising beneath the caldera.
        In response to this increased unrest, USGS intensified its monitoring in the Long Valley region.(I) Today, a state-of-the-art network of seismometers and geodetic equipment closely monitorsearthquake activity and the swelling in the caldera. Data from these instruments help scientist toassess the volcanic hazard in the Long Valley are and to recognize early signs of possible eruptions.
        During the early 1990s, trees began dying at several places on Mammoth Mountain on thesouthwest edge of Long Valley Caldera. Studies conducted by USGS and U.S. Forest Servicescientists showed that the trees are being killed by large amounts of carbon dioxide gas seeping upthrough the soil from magma deep beneath Mammoth Mountain. (J) Such emissions of volcanic gas,as well as earthquake swarms and ground swelling, commonly precede volcanic eruptions. When theyprecede an eruption of a “central vent” volcano, such as Mount St. Helens, Washington, they normallylast only a few weeks or months. (K)However, symptoms of volcanic unrest may persist for decadesor centuries at large calderas, such as Long Valley Caldera. Studies indicate that only about one in sixsuch episodes of unrest at large calderas worldwide actually culminates in an eruption.
        Over the past 4,000 years, small to moderate eruptions have occurred somewhere along theMono-Inyo volcanic chain every few hundred years, and the possibility remains that the geologicunrest in the Long Valley area could take only weeks to escalate to an eruption. Nonetheless,geologists think that the chances of an eruption in the area in any given year are quite small.
【題組】21. What is the reason geologists have increased their monitoring of the Long Valley Caldera?
(A) It has been more than 150 years since the last eruption.
(B) Eruptions happen frequently in volcanic chains of such size.
(C) The area is experiencing geologic activity indicative of an impending eruption.
(D) The swelling of the caldera may damage the sensitive geodetic equipment.