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TOEFL Directory > TOEFL writing > TOEFL Reading Class Unit 1_Passage 22_Question 232-242
TOEFL Reading Class Unit 1_Passage 22_Question 232-242
You have about 15 minutes to finish this passage.
First,use about 3-4 minutes to read the passage, try to understand the main idea of this passage.
Don't read it so slowly or try to remember all details.You need to do "fast reading",and "scan" the passage.
Second, read questions 1-11, and with questions you go back the passage again and look for correct answers.
Questions 232-242
Passage 22
In essence, the formation of a limestone cave is not particularly complicated, but it may take millions of years to first produce the basic cavity and then to decorate it with cavern formations. Caves can be said to have 3 growth phases: excavation, decoration, and dormancy.
Beds, or strata, of limestone or marble (metamorphosed limestone) are commonly faulted, cracked, and fractured by movements of the Earth's surface and certain parts of a strata may be much softer than others. It is along these planes of weakness that caverns develop. Water excavates or hollows out the caverns.
Rain, snowmelt, and other groundwaters containing carbon dioxide seep or flow downward along cracks, joints, fault planes, and fissures. This water actually constitutes a weak form of carbonic acid and slowly dissolves the limestone. Gradually this dissolving action enlarges the cracks, joints, and crevices into passageways, rooms, or huge halls. These barren, dissolved areas are called solution channels.
The rate of erosion for a cave is relative, depending on the amount of water involved, the length of time, the rate of flow, and the resistance of the particular limestone. ■Some caves have apparently been created by an underground stream or river flowing along the line of least resistance deep in the Earth. ■After solution channels have been excavated, rooms may fill up with very slow moving water. ■This water acts on the walls of the room, gradually expanding the chamber by dissolving the walls and causing the ceiling to collapse. ■The ceiling rubble is then attacked by the water, which in turn dissolves it and carries it away. ■This is somewhat of an oversimplification, but gigantic rooms, such as those in Carlsbad Caverns in the western United States, have been created by this process. ■
Eventually the prime dissolving waters leave the caverns. This may be due to geological uplift of the region, causing the water to drain away, or perhaps the original source is diminished, terminated, or diverted. Then air fills the chambers---most accessible caverns have an abundance of fresh air in them. Air and water are essential to the second phase of cavern development, decoration.
232. Which of the following is the main topic of the passage?
a) How limestone caves develop
b) Dating limestone caves
c) Dangers associated with limestone caves
d) Effects of climate on limestone caves
233. The phrase In essence in the passage is closest in meaning to
b) in the meantime
c) basically
d) significantly
e) over time
234. The word constitutes in the passage is closest in meaning to
a) finds out
b) consists of
c) causes
d) prevents
235. The word Gradually in the passage is closest in meaning to
a) powerfully
b) as a result
c) little by little
d) predictably
236. All of the following are mentioned as factors in the rate of erosion in a cave EXCEPT the
a) age of the limestone
b) strength of the limestone
c) amount of water present
d) speed at which groundwater moves
237. Omitted.
238. Which of the following is true of the caves found at Carlsbad Caverns?
a) They are now filled with water.
b) They were only recently discovered.
c) They contain unusually large chambers.
d) They were formed more quickly than the average limestone cave.
239. The word diminished in the passage is closest in meaning to
a) polluted
b) replaced
c) reduced
d) endangered
240. Which of the following is primarily responsible for cave excavation?
a) Limestone
b) Water
c) Heat
d) Air
241. The passage probably continues with a discussion of
a) erosion in caves
b) the decoration phase of cave development
c) the geology of regions where caves are found
d) Carlsbad Caverns
242. The following sentence can be added to paragraph 4.
Others may have been formed just below the water table where the ground is saturated with water.
Where would it best fit in the passage? Click on the [■] to add the sentence to paragraph 4.
Click here to get the detail explanations and correct answers.
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