DIRECTIONS: In the passages that follow, some words and phrases are underlined and numbered. In the answer column, you will find alternatives for the words and phrases that are underlined. Choose the alternative that you think is best, and fill in the corresponding bubble on your answer sheet. If you think that the original version is best, choose “NO CHANGE,” which will always be either answer choice A or F. You will also find questions about a particular section of the passage, or about the entire passage. These questions will be identified either by an underlined portion or by a number in a box. Look for the answer that clearly expresses the idea, is consistent with the style and tone of the passage, and makes the correct use of standard written English. Read the passage through once before answering the questions. For some questions, you should read beyond the indicated portion before you answer.
How Volcanoes Work
What causes the formation of volcanoes? Before humans understood that the center of the Earth was made of molten iron, scientific hypotheses pointed to chemical reactions in superficial layers of magma to explain the phenomenon. Through modern geology, humans have a clearer—though hardly complete—understanding of the mechanism of magma flow, and can Q1 analyze the vibrations of the earth to warn of recent Q2 eruptions.
Most volcanoes were being Q3 the result of magma flowing out of the surface of the earth and hardening, usually near a subduction zone. As two tectonic plates collide, one of them is Q4 forced under the other, and the seabed rock melts to form new, low-density magma.
This magma is red-hot Q5 and eventually penetrates unstable pockets of the Earth’s surface. Some magma will succeed in reaching the surface either to form a new volcano or adding Q6 more mass to an existing one. Not all volcanoes are formed at continental boundaries, however.
Hotspot volcanoes form Q7 by a different mechanism. One theory, proposed in the 1960s, seeks to explain volcanoes such as those that formed the Hawaiian Islands, which are not at a plate boundary. The probable explanation is a hotspot, which is a fixed point beneath the Earth’s crust where a narrow plume of magma rises into the crust and appears at the surface as a continental volcano or a volcanic island. Chains, of volcanic islands called archipelagos, Q8 provide evidence that the hotspot stays in place as the tectonic plate passes over it.
Geologic processes are slow, while Q9 research must include the study of ancient human accounts of eruptions and layers of rock millions of years old. In general, eruptions Q10 seem to occur every several hundred or even thousand years, and many volcanoes seem to be completely dormant, that is unlikely to erupt soon. Q11 No one can say for sure that these volcanoes will not erupt again in the future, so scientists take serious Q12 the study of every volcano situated where a future eruption could bring significant human loss and environmental damage. Predicting volcanic eruptions is not an exact science, and only within the briefest geological moment can people warn Q13 to evacuate. Through studying the earth’s vibrations, physical deformation, and gas emissions, geologists in recent decades have made several excellent predictions of volcanic eruption, as if Q14 at the Philippines’ Mount Pinatubo in 1991 and Popocatépetl outside Mexico City in 2000.
Understanding the mechanisms of the earth’s interior and continuing to study volcanoes will advance the development of reliable early warning systems for dangerous eruptions. Volcanoes are both feared and revered for their beauty and awesome destructive power, but Q15 they show that humans have much more to learn about the planet Earth.