IELTS Reading 練習 火山如何形成（含題目+詳解）

How Volcanoes Form

A. A volcano is, in the simplest sense, an opening through which molten rock from inside the Earth reaches the surface. The processes that drive these openings are extraordinarily varied, however, and modern geologists distinguish at least three principal mechanisms. Each is linked to the slow movement of the rigid plates that make up the outermost shell of the planet.

B. The most common setting is the subduction zone, where one tectonic plate slides beneath another. As the descending plate sinks into the hotter mantle, water locked inside its rocks is released, lowering the melting point of the surrounding material. The resulting magma is comparatively rich in dissolved gas and silica, which makes it sticky. When this magma rises and reaches the surface, the trapped gas often expands violently, producing the explosive eruptions characteristic of volcanoes such as Mount St Helens or Mount Pinatubo.

C. A second setting occurs at mid-ocean ridges, where two plates pull apart. As the plates separate, hot rock from below flows upward to fill the gap and partially melts as the pressure on it drops. The lava produced here is generally low in silica, runs easily and erupts gently. Because most mid-ocean ridges lie thousands of metres below sea level, these eruptions usually go unseen, but they account for the majority of volcanic activity on Earth by volume.

D. A third, more puzzling setting is the so-called hot spot. Here, plumes of unusually hot rock rise from deep within the mantle, possibly from as far as the core-mantle boundary. They burn through the overlying plate to produce isolated volcanoes far from any plate boundary. Hawaii is the classic example: as the Pacific plate drifts north-westward over a stationary plume, a chain of progressively older, extinct islands trails behind the active volcanoes.

E. Forecasting eruptions remains difficult. Although seismic instruments now track the movement of magma in unprecedented detail, the precise moment when a volcano will erupt — and how violently — is still beyond reliable prediction.

The passage has five paragraphs, A-E. Choose the correct heading for paragraphs B, C, and D from the list below.

• i. Why prediction is still imprecise
• ii. Three settings, one underlying cause
• iii. Eruptions hidden beneath the sea
• iv. Sticky magma and explosive eruptions
• v. When two plates collide head-on
• vi. Volcanoes far from plate edges

1. Paragraph B
2. Paragraph C
3. Paragraph D

1. The presence of water in subducting rocks helps magma form by lowering the melting point.
2. Mid-ocean ridge eruptions produce more lava overall than any other type of volcano.
3. Hot-spot plumes have been confirmed to originate exactly at the core-mantle boundary.

Complete the sentences below using NO MORE THAN TWO WORDS from the passage.

1. Magma at subduction zones is rich in dissolved gas and ______, which makes it sticky.
2. The lava produced at mid-ocean ridges is generally ______ in silica.
3. The Hawaiian island chain becomes progressively ______ as you move away from the active volcanoes.

1. iv — Sticky magma and explosive eruptions Paragraph B explains: "The resulting magma is comparatively rich in dissolved gas and silica, which makes it sticky" and links sticky magma to "the explosive eruptions characteristic of volcanoes such as Mount St Helens". Heading v is a trap — plates here slide under each other, not collide head-on.

2. iii — Eruptions hidden beneath the sea Paragraph C ends: "Because most mid-ocean ridges lie thousands of metres below sea level, these eruptions usually go unseen". "Hidden beneath the sea" paraphrases "below sea level... usually go unseen".

3. vi — Volcanoes far from plate edges Paragraph D opens with hot spots that "burn through the overlying plate to produce isolated volcanoes far from any plate boundary". Direct paraphrase of "far from plate edges".

4. TRUE Supporting sentence: "water locked inside its rocks is released, lowering the melting point of the surrounding material". The statement is a direct paraphrase.

5. TRUE Supporting sentence: "they account for the majority of volcanic activity on Earth by volume". "Majority by volume" matches "more lava overall than any other type".

6. FALSE Supporting sentence: "plumes of unusually hot rock rise from deep within the mantle, possibly from as far as the core-mantle boundary". The word "possibly" means this has not been confirmed. The statement contradicts the hedged language.

7. silica Supporting sentence: "The resulting magma is comparatively rich in dissolved gas and silica, which makes it sticky". Single-word answer; the question already supplies "gas".

8. low Supporting sentence: "The lava produced here is generally low in silica". One-word answer matching exactly.

9. older Supporting sentence: "a chain of progressively older, extinct islands trails behind the active volcanoes". One-word answer; "older" describes movement away from active volcanoes.