本篇文章共分 6 段，針對地球核心進行分析，從地震波對於地核研究的效用、地核化學物質的組成、到地球磁場的成因，對地球核心的物理性質做有系統的整理與說明。

本篇考題英文原文與對應之中文翻譯整理如下。練習作答解題時若有對語意不清楚之處，請仔細查閱對照，以提升閱讀理解能力。

Earth's Core 地球的核心

1. 地震波的分類

   Knowledge of Earth’s deep interior is derived from the study of the waves produced by earthquakes, called seismic waves. Among the various kinds of seismic waves are primary waves (P-waves) and secondary waves (S-waves). Primary and secondary waves pass deep within Earth and therefore are the most instructive. Study of abrupt changes in the characteristics of seismic waves at different depths provides the basis for a threefold division of Earth into a central core; a thick, overlying mantle; and a thin, enveloping crust. Sudden changes in seismic wave velocities and angles of transmission are termed discontinuities.

   對地球內部深處的瞭解來自於對地震產生的波的研究，稱為地震波。在各種地震波中，有一級波 (P波) 和二級波 (S波)。一級波和二級波在地球深處通過，因此最具有指導意義。對不同深度的地震波特徵的突然變化的研究，為將地球分為中央核心、厚重的上覆地幔和薄薄的包覆地殼三部分提供了依據。地震波速度和傳播角度的突然變化被稱為不連續現象。

2. 地震波的傳遞特性

   One of the discontinuities is the Gutenberg discontinuity, which is located nearly halfway to the center of Earth at a depth of 2,900 kilometers and marks the outer boundary of Earth's core. At that depth, the S-waves cannot propagate, while at the same time P-wave velocity is drastically reduced. S-waves are unable to travel through fluids. Thus, if S-waves were to encounter a fluidlike region of Earth’s interior, they would be absorbed there and would not be able to continue. Geophysicists believe this is what happens to S-waves as they enter the outer core. As a result, the S-waves generated on one side of Earth fail to appear at seismograph stations on the opposite side of Earth, and this observation is the principal evidence of an outer core that behaves as a fluid. Unlike S-waves, P-waves are able to pass through liquids. They are, however, abruptly slowed and sharply refracted (bent) as they enter a fluid medium. Therefore, as P-waves encounter the molten outer core of Earth, their velocity is reduced and they are refracted downward

   其中一個不連續點是古登堡不連續點，它幾乎位於地球中心的一半，深度為 2,900 公里，是地球核心的外部邊界。在這個深度，S 波無法傳播，同時 P 波速度急劇下降。S 波也無法穿過流體。因此，如果 S 波遇到地球內部的流體狀區域，它們將在那裡被吸收，無法繼續下去。地球物理學家認為這就是 S 波進入外核時發生的情況。因此，在地球一側產生的 S 波未能出現在地球另一側的地震儀站，而這一觀察是外核表現為流體的主要證據。與 S 波不同，P 波能夠穿過液體。然而，當它們進入液體介質時，它們會突然變慢並發生急劇的折射（彎曲）。因此，當 P 波遇到地球的熔融外核時，它們的速度被降低，並被向下折射。

3. 固體內核

   The radius of the core is about 3,500 kilometers. The inner core is solid and has a radius of about 1,220 kilometers, which makes this inner core slightly larger than the Moon. Most geologists believe that the inner core has the same composition as the outer core and that it can only exist as a solid because of the enormous pressure at the center of Earth. Evidence of the existence of a solid inner core is derived from the study of hundreds of records of seismic waves produced over several years. These studies showed that the inner core behaves seismically as if it were a solid.

   地球核心的半徑約為 3,500 公里。內核是固體，半徑約為 1,220 公里，這使得這個內核比月球略大。大多數地質學家認為，內核的成分與外核相同，由於地球中心的巨大壓力，它只能作為固體存在。固體內核存在的證據來自於對幾年來產生的數百條地震波記錄的研究。這些研究表明，內核的地震行為就像它是一個固體一樣。

4. 地核之化學組成

   Earth has an overall density of 5.5 grams per cubic centimeter, yet the average density of rocks at the surface is less than 3.0 grams per cubic centimeter. This difference indicates that materials of high density must exist in the deep interior of the planet to achieve the 5.5 grams per cubic centimeter overall density. Under the extreme pressure conditions that exist in the region of the core, iron mixed with nickel would very likely have the required high density. Laboratory experiments, however, suggest that a highly pressurized iron-nickel alloy might be too dense and that minor amounts of such elements such as silicon, sulfur, carbon, or oxygen may also be present to lighten the core material.

   地球的總體密度為每立方厘米 5.5 克，然而表面岩石的平均密度低於每立方厘米 3.0 克。這一差異表明，高密度的材料必須存在於地球的內部深處，才能達到每立方厘米 5.5 克的總密度。在核心區域存在的極端壓力條件下，鐵與鎳的混合將非常可能具有所需的高密度。然而，實驗室的實驗表明，高度加壓的鐵鎳合金可能密度太大，而且還可能存在少量的矽、硫、碳或氧等元素，以減輕核心材料的重量。

5. 隕石提供之證據

   Support for the theory that the core is composed of iron (85 percent) with lesser amounts of nickel has come from the study of meteorites. Many of these samples of solar system materials are iron meteorites that consist of metallic iron alloyed with a small percentage of nickel. Some geologists suspect that iron meteorites may be fragments from the core of a shattered planet. The presence of iron meteorites in our solar system suggests that the existence of an iron-nickel core for Earth is plausible.

   對核心由鐵 (85%) 和較少數量的鎳組成的理論的支持來自對隕石的研究。這些太陽系物質的樣本中有許多是鐵隕石，它們由金屬鐵與少量鎳的合金組成。一些地質學家懷疑，鐵隕石可能是來自破碎的行星核心的碎片。太陽系中鐵隕石的存在表明，地球存在一個鐵鎳核心是合理的。

6. 地球磁場與地核

   There is further evidence that Earth may have a metallic core. Anyone who understands the functioning of a compass is aware that Earth has a magnetic field. The planet itself behaves as if there was a great bar magnet embedded within it. A magnetic field is developed by the flow of electric charges and requires good electrical conductors. Silicate rocks, such as those in the mantle and crust, do not conduct electricity very well, whereas metals such as iron and nickel are good conductors. Heat-driven movements in the outer core, coupled with movements induced by Earth’s spin, are thought to provide the necessary flow of electrons (very small particles that carry a negative charge) around the inner core that produces the magnetic field. Without a metallic core, Earth’s magnetic field would not be possible.

   有進一步的證據顯示地球可能有一個金屬核心。任何瞭解指南針功能的人都知道，地球有一個磁場。這個星球本身的行為就像有一個巨大的條形磁鐵嵌入其中。磁場是由電荷的流動形成的，需要良好的電導體。矽酸鹽岩石，如地幔和地殼中的岩石，無法充分導電，而鐵和鎳等金屬是良好的導體。外核的熱驅動運動，加上地球自旋引起的運動，被認為提供了圍繞內核的必要的電子流（攜帶負電荷的非常小的粒子），產生了磁場。如果沒有一個金屬核心，地球的磁場就不可能存在。