# TOEFL IBT Reading Practice Test 40 from The Official Guide to the TOEFL

Passage 3: The Structure of the Earth’s Interior

P1: The area beneath the earth’s surface has always been a mystery for scientists. In the past, many speculated that the inner earth was hollow and inhabitable, much like the surface. Modem scientists are almost completely certain that this isn’t the case. Though they haven’t had the opportunity to directly observe the inner earth, scientists have studied many forces (such as earth’s gravity and earthquake waves) and employed complex mathematics to determine the earth’s weight, density, volume, and overall size. By applying these techniques to facts about the earth’s surface, researchers have concluded that there are three layers to the earth: the crust, the mantle, and the core, which is itself divided into an inner and outer core. Not only have they mapped out these layers, but they have also determined which kinetic forces are at work in each layer and how these affect the tectonic movements feit on the surface.

P2: The layer that is most accessible to scientists is the crust. This is a relatively thin outer layer of cool, flaky terrain that covers the planet, and all known living organisms exist on the surface of this crust, which includes the ocean floor as well as the continents. Scientists can easily study this layer through direct observation, drilling, earthquakes, and sonar waves. The crust is composed mostly of silicates and oxides, with some heavy metals scattered throughout the layer; these solids are much lighter than the materials that sink to the lower levels. Kinetic energy radiates through this stratum, and sections of the crust rub against one another as they float upon molten rock, which is part of the mantle.

P3: The mantle is the largest layer of the inner earth, being nearly two thousand miles thick. A In addition to the upper layer of the molten rock, the mantle consists of lower levels that surpass the magma’s temperature yet are completely solid, being a heavy mix of oxides and metals like aluminum, magnesium, and iron. B Scientists have determined this primarily by observing how sonar and earthquake waves move far beneath the surface. C The solid materials deep within the mantle exist within a viscous area, and they are slowly shifting due to the large amounts of heat that travel through the slimy layer; as a result, the convection expands to the upper layer of magma, moving the tectonic plates. D The heat that causes this movement emanates from the earth’s core.

P4: The core consists of an outer core and an inner core. The outer core is a mixture of iron and nickel that is even heavier and denser than the mantle. However, it is also hotter, so the outer core is entirely liquid. Scientists confirmed this when they noticed that earthquake waves completely reverberated off the core; the energy did this because it cannot travel through liquid. Scientists also believe that this liquid layer is slowly but constantly flowing and that such motion generates heat and causes the mantle to shift. This flowing of the liquid core produces another critical phenomenon that affects the earth: a planetary magnetic field . The constant movement somehow works with the earth’s rotation to create an electrical current, and, in turn, the current generates a magnetic field.

P5: At the center of the outer core and the very earth is the inner core. Naturally, the lowest level of the earth is the most difficult for scientists to study, so most of the details are technically still conjecture. The inner core is roughly the size of the moon, but its temperature is equal to that found on the surface of the sun. Despite being hotter than the outer core, the inner core is actually solid iron; this is because high levels of pressure compress the core’s molecules to such a degree as to prevent it from melting into liquid. Actually, scientists think that the center of the Earth is growing cooler and thus, paradoxically, freezing. They theorize that, as liquid from the outer core begins to cool and solidify, the inner core grows. It is the solidification and consolidation of liquid metal that releases massive amounts of heat and causes the outer core to flow, transferring the kinetic energy that causes tectonic motion.

Directions: Mark your answer by filling in the oval next to your choice.

27. The word they in the passage refers to

(A) researchers

(B) layers

(C) techniques

(D) facts

28. According to paragraph I, what was a common misconception about the earth’s interior?

(A) It was solid.

(B) It didn’t move.

(C) It could support life.

(D) It was empty.

29. According to paragraph 1, scientists determined the structure of the earth’s interior by

(A) using indirect observations and calculations

(B) obtaining a rock sample from a deep level

(C) traveling to the center of the pianet

(D) studying the temperature of magma

30. The word stratum in the passage is closest in meaning to

(A) steel

(B) layer

(C) wave

(D) method

31. According to paragraph 2, how does material in the crust compare to material found at lower levels?

(A) It is more metallic than the deeper elements.

(B) It is hotter than the solids in the deep earth.

(C) It is not as heavy or dense as lower materials.

(D) It is virtually identical to the lower materials.

32. The word viscous in the passage is closest In meaning to

(A) vicious

(B) oozy

(C) stable

(D)deep

33. The word reverberated in the passage is closest in meaning to

(A) ricocheted

(B) radiated

(C) incinerated

(D) washed

34. What creates a planetary magnetic field?

(A) the earth’s gravitational pull

(B) the flowing of liquid metals

(C) the intense heat from the core

(D) the shifting within the mantle

35.The word conjecture in the passage is closest in meaning to

(A) confusing

(B) evidence

(C) guesswork

(D) established

36. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

At the center of the outer core and the very earth is the inner core. Naturally, the lowest level of the earth is the most difficult for scientists to study, so most of the details are technically still conjecture. The inner core is roughly the size of the moon, but its temperature is equal to that found on the surface of the sun. Despite being hotter than the outer core, the inner core is actually solid iron; this is because high levels of pressure compress the core’s molecules to such a degree as to prevent it from melting into liquid. Actually, scientists think that the center of the Earth is growing cooler and thus, paradoxically, freezing. They theorize that, as liquid from the outer core begins to cool and solidify, the inner core grows. […]

(A) The inner core melts from its intense heat despite the pressure on its molecules.

(B) The immense pressure on the inner core counteracts intense heat, keeping it solid.

(C) The molecules of the solid iron inner core are under extreme pressure from above.

(DJ The inner core is hotter than the outer core, but it is actually a solid, not a liquid-

37. According to paragraph 5, all of the following about the earth’s inner core are true EXCEPT:

(A) It is the hottest part of the interior.

(B) It is actually freezing.

(C) It is the origin of all tectonic motion.

(D) It is continuously shrinking.

38. Based on the information in the passage, what can be inferred about the study of the earth’s interior?

(A) The internal heat has prevented a detailed examination.

(B) Scientists didn’t obtain all of the answers until recently.

(C) The deeper that a layer is, the more difficult it is to stuay-

(D) Earthquakes haven’t been very helpful in this research.

39. Look at the four squares c that indicate where the following sentence could be added to the passage.

“They must use these less direct methods to study the mantle because all previous drilling attempts have been futile.”
Where would the sentence best fit?

(A)

(B)

(C)

(D)

40 . Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

Scientists have used many techniques to observe the earth’s interior, its properties, and how it functions.

Answer Choices

(A) Studies reveal that the earth has multiple layers that form an interchanging pattern of solid, liquid, solid, etc.
(B) The inner core of the earth is actually the size of the moon and the temperature of the sun’s surface.
(C) Scientists aren’t absolutely certain about the earth’s interior, so theories persist that the earth is actually hollow.
(D) The layers of the earth get gradually cooler the deeper they are, and the core is actually freezing.
(E) Researchers believe that the core causes magnetism, mantle convection, and tectonic motion that results in earthquakes.
(F) Scientists read energy waves to discover how materials get progressively more metallic the deeper they are located.