📚 The Structure of the Earth | 地球的结构
The Earth may look solid beneath our feet, but it is actually made of several distinct layers. Each layer has its own composition, temperature, and state of matter. Understanding these layers helps chemists and geologists explain volcanoes, earthquakes, and even the magnetic field that protects us.
我们脚下的大地看似坚固，但实际上地球由几个不同的圈层组成。每一层都有各自的成分、温度和物质状态。了解这些圈层有助于化学家和地质学家解释火山、地震，甚至保护我们的地球磁场。
1. What is inside the Earth? | 地球内部有什么？
If we could cut the Earth in half, we would see a layered structure like an onion. From the surface to the centre, the main layers are the crust, the mantle, the outer core, and the inner core. These layers differ in thickness, temperature, pressure, and the materials they contain.
如果我们能把地球切成两半，就会看到像洋葱一样的层状结构。从地表到中心，主要圈层依次是地壳、地幔、外核和内核。这些圈层在厚度、温度、压力以及所含物质方面各不相同。
The crust is the thin outer skin, the mantle is the thick rocky middle, and the core is divided into a liquid outer part and a solid inner part. The deeper you go, the hotter and denser the material becomes.
地壳是薄薄的外层，地幔是厚厚的岩石中间层，地核则分为液态的外核和固态的内核。越往深处，物质就越热、越致密。
2. The crust | 地壳
The crust is the outermost and thinnest layer of the Earth. It is made of solid rock and is divided into two types: continental crust and oceanic crust. Continental crust is thicker, about 30 to 70 km, and rich in silicon and aluminium, so it is sometimes called ‘sial’. Oceanic crust is thinner, about 5 to 10 km, and richer in silicon and magnesium, so it is often called ‘sima’.
地壳是地球最外面、最薄的一层。它由坚硬的岩石组成，分为大陆地壳和大洋地壳两种。大陆地壳较厚，约30至70千米，富含硅和铝，因此有时被称为硅铝层。大洋地壳较薄，约5至10千米，富含硅和镁，因此常被称为硅镁层。
From a chemical point of view, the most common elements in the crust are oxygen, silicon, aluminium, iron, calcium, sodium, potassium, and magnesium. Oxygen alone makes up nearly half of the crust’s mass because it combines with other elements to form silicate minerals.
从化学角度看，地壳中最常见的元素是氧、硅、铝、铁、钙、钠、钾和镁。仅氧就占地壳质量的近一半，因为它与其他元素结合形成硅酸盐矿物。
3. The mantle | 地幔
Below the crust lies the mantle, which is about 2,900 km thick and makes up roughly 84% of the Earth’s volume. The mantle is mostly solid rock, but over very long timescales it can flow slowly. Its main elements are oxygen, silicon, magnesium, and iron, forming minerals such as olivine and pyroxene.
地壳之下是地幔，厚约2900千米，约占地球体积的84%。地幔大部分是固态岩石，但在非常长的时间尺度上可以缓慢流动。地幔的主要元素是氧、硅、镁和铁，形成橄榄石和辉石等矿物。
The upper part of the mantle contains a weak, partially molten zone called the asthenosphere. Heat from the core causes convection currents in the mantle, where hot rock rises, cools, and sinks again. These currents drive plate tectonics and cause earthquakes and volcanoes.
地幔上部包含一个软弱、部分熔融的区域，称为软流层。来自地核的热量在地幔中形成对流，热岩石上升、冷却后再次下沉。这些对流驱动板块运动，并引发地震和火山。
4. The outer core | 外核
The outer core lies beneath the mantle, from about 2,900 km to 5,150 km below the surface. It is made mainly of liquid iron and nickel, with smaller amounts of lighter elements such as sulfur and oxygen. The temperature here ranges from about 4,000 °C to 5,000 °C.
外核位于地幔之下，大约在地表以下2900千米到5150千米之间。外核主要由液态铁和镍组成，还含有少量的硫和氧等较轻元素。这里的温度约为4000°C到5000°C。
Because the outer core is liquid, it can flow. The movement of this electrically conducting liquid iron generates electric currents, which produce the Earth’s magnetic field. This process is called the geodynamo.
由于外核是液态的，它可以流动。这种导电液态铁的运动会形成电流，从而产生地球磁场。这一过程被称为地球发电机效应。
5. The inner core | 内核
At the very centre of the Earth is the inner core, a ball with a radius of about 1,220 km. Despite temperatures of around 5,400 °C, the inner core is solid. This is because the pressure is extremely high, over 3 million times atmospheric pressure, which forces iron and nickel atoms into a solid state.
地球的最中心是内核，半径约为1220千米。尽管温度高达约5400°C，内核仍然是固态的。这是因为那里的压力极高，超过大气压的300万倍，迫使铁和镍原子保持固态。
The inner core is mainly iron with some nickel. It grows very slowly as the Earth cools, because liquid iron at the boundary with the outer core freezes onto the inner core.
内核主要由铁和少量镍组成。随着地球冷却，内核非常缓慢地增长，因为外核与内核边界处的液态铁会冻结到内核上。
6. How do we know? Seismic waves | 我们如何知道？地震波
We cannot drill to the centre of the Earth, so scientists study earthquakes. When an earthquake happens, it sends out seismic waves that travel through the Earth. Two important types are P-waves and S-waves. P-waves can travel through solids and liquids, but S-waves can only travel through solids.
我们无法钻探到地球中心，因此科学家通过研究地震来了解地球内部。地震发生时，会发出穿过地球的地震波。两种重要的波是P波和S波。P波可以通过固体和液体，但S波只能通过固体。
S-waves disappear when they reach the outer core, which tells us that the outer core is liquid. P-waves slow down and bend as they pass through different layers, giving clues about density and composition. This is how the layered model of the Earth was built.
S波在到达外核时消失，这告诉我们外核是液态的。P波在穿过不同圈层时会减速并发生弯曲，为密度和成分提供线索。地球层状模型就是这样建立起来的。
7. Composition and elements | 组成与元素
Each layer has a distinct chemical composition. The crust is rich in silicate minerals, the mantle is rich in magnesium and iron silicates, and the core is dominated by iron and nickel. This difference in composition affects density: the core is much denser than the crust.
每一圈层都有独特的化学成分。地壳富含硅酸盐矿物，地幔富含镁和铁的硅酸盐，地核则以铁和镍为主。这种成分差异影响密度：地核的密度比地壳大得多。
We can summarise the overall composition of the whole Earth by mass as roughly 32% iron, 30% oxygen, 15% silicon, 14% magnesium, and smaller amounts of nickel, sulfur, calcium, and aluminium. These elements are not evenly distributed; heavy iron sank to the centre during Earth’s early molten stage.
按质量计算，整个地球的总体成分大致为：铁约32%、氧约30%、硅约15%、镁约14%，以及少量的镍、硫、钙和铝。这些元素并不是均匀分布的；在地球早期熔融阶段，重的铁下沉到了中心。
8. Plate tectonics and the lithosphere | 板块构造与岩石圈
The crust and the uppermost solid part of the mantle together form the lithosphere. The lithosphere is broken into large pieces called tectonic plates. These plates float on the softer asthenosphere below and move a few centimetres each year.
地壳和地幔最上部的固态部分共同构成岩石圈。岩石圈被分割成称为板块的大块。这些板块漂浮在下方较软的软流层上，每年移动几厘米。
Where plates move apart, magma from the mantle rises to form new crust. Where plates collide, one plate can sink into the mantle and melt. These processes recycle elements between the crust and mantle, linking Earth’s structure to the rock cycle.
在板块分离的地方，地幔中的岩浆上升形成新的地壳。在板块碰撞的地方，一个板块会俯冲到地幔中并熔化。这些过程使元素在地壳和地幔之间循环，将地球结构与岩石循环联系起来。
9. The rock cycle connection | 岩石循环的联系
Earth’s internal heat drives the rock cycle. Igneous rocks form when magma or lava cools. Sedimentary rocks form from compressed sediments at the surface. Metamorphic rocks form when existing rocks are changed by heat and pressure, often near plate boundaries.
地球内部的热量驱动着岩石循环。岩浆或熔岩冷却形成火成岩。沉积物在地表被压实形成沉积岩。现有岩石在热量和压力作用下发生变化，形成变质岩，这通常发生在板块边界附近。
The mantle is the source of magma for igneous rocks, and the movement of plates buries rocks deep enough for metamorphism. This shows that the Earth’s layered structure is not static; materials are constantly being moved and transformed.
地幔是火成岩岩浆的来源，而板块的运动将岩石埋藏到足够深处发生变质。这表明地球的层状结构并不是静止的；物质在不断地移动和转化。
10. Earth’s magnetic field | 地球磁场
The liquid outer core is essential for life on Earth because it produces the magnetic field. This magnetic field acts like a shield, deflecting harmful charged particles from the Sun and outer space. Without it, the solar wind could strip away our atmosphere.
液态外核对地球生命至关重要，因为它产生了磁场。这个磁场就像一面盾牌，偏转来自太阳和外太空的有害带电粒子。如果没有它，太阳风可能会剥离我们的大气层。
The magnetic field also causes compasses to point north. The magnetic poles are not exactly the same as the geographic poles, and they slowly move over time because the flow in the outer core changes.
磁场还使指南针指向北方。磁极与地理极点并不完全重合，并且随着外核流动的变化而缓慢移动。
11. Comparing the layers | 层与层的比较
The table below compares the four main layers of the Earth by state, thickness, temperature, and main elements.
下表从状态、厚度、温度和主要元素四个方面比较了地球的四个主要圈层。
| Layer | State | Thickness | Temperature | Main elements |
|---|---|---|---|---|
| Crust | Solid | 5–70 km | Up to about 1,000 °C | O
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