📚 Geology: Earth Structure, Rocks and Geological Processes | 地质学：地球结构、岩石与地质过程
Geology is the scientific study of the Earth: the materials it is made of, the way it is built inside, the landforms on its surface, and the immense history of change that produced all of them. Geologists read the rock record to explain how rocks and minerals form, and to trace the restless events and slow evolutionary processes that have shaped our planet over billions of years.
地质学是探索地球物质构成、内部构造、表层形态及漫长演变历史的学科。它深入解读地质现象，揭示岩石矿物的形成机理，并追溯地球动荡的地质事件与演化过程。
Because the Earth is roughly 4.54 billion years old, nobody can watch its past directly. Geologists therefore work like detectives: they examine structures, textures, fossils and chemical signatures preserved in rocks, and use the laws of physics and chemistry to reconstruct events that happened long before humans existed.
地球已有约45.4亿年历史，任何人都无法直接观察它的过去。地质学家因此像侦探一样工作：解读岩石中保存的构造、结构、化石与化学信号，并借助物理与化学定律，重建远在人类出现之前发生的重大事件。
1. What Geology Studies | 地质学的研究对象
Geology covers four broad questions: what the Earth is made of, how it is structured, what its surface looks like, and how it has changed through time. Answering these questions requires evidence from rocks, minerals, fossils, ice, sediments and even meteorites, because all of them record conditions that no longer exist today.
地质学围绕四个核心问题展开：地球由什么物质构成、内部如何分层、地表呈现何种形态、以及它在时间中如何变化。回答这些问题需要岩石、矿物、化石、冰层、沉积物甚至陨石提供的证据，因为它们都记录了今天已不复存在的环境条件。
Geology is also a practical science. It underpins the search for water, metals, oil and gas, guides the safe design of tunnels, dams and foundations, and supplies the evidence used to assess earthquake, volcanic and landslide hazard. In this sense it links pure scientific curiosity with decisions that affect everyday life.
地质学同样是一门实用科学。它为寻找水源、金属、石油与天然气提供基础，指导隧道、大坝与地基的安全设计，并为评估地震、火山与滑坡风险提供证据。就此而言，它把纯粹的科学好奇心与影响日常生活的决策连接起来。
2. Earth's Internal Structure | 地球的内部结构
The internal structure and composition of the Earth is a central topic in geology. The planet consists of four concentric layers: the inner core, the outer core, the mantle and the crust. The inner core lies at the very centre, is composed mainly of iron and nickel, and endures extreme temperature and pressure; the outer core is a liquid metallic shell surrounding it; the mantle is a thick layer of rocky material; and the crust is the surface beneath our feet.
地球的内部结构与组成是地质学的核心议题。地球由内核、外核、地幔和地壳四重同心层构成。内核深居地球最深处，主要由铁和镍构成，承受着极端的温度与压力；外核是一圈包围内核的液态金属；地幔则是岩质物质构成的厚层；地壳便是我们脚下所踏的地表。
| Layer 圈层 | Approximate thickness 大致厚度 | State 状态 | Main composition 主要成分 |
|---|---|---|---|
| Inner core 内核 | radius about 1,220 km 半径约1,220公里 | solid 固态 | Fe and Ni 铁与镍 |
| Outer core 外核 | about 2,260 km 约2,260公里 | liquid 液态 | Fe, Ni and lighter elements 铁、镍及轻元素 |
| Mantle 地幔 | about 2,900 km 约2,900公里 | solid rock that flows very slowly 固态但可缓慢流动 | Mg- and Fe-rich silicates 富镁铁硅酸盐 |
| Crust 地壳 | about 5 to 70 km 约5至70公里 | solid 固态 | silicate rocks 硅酸盐岩石 |
These boundaries are not guesses. They were detected with seismic waves: P-waves pass through both solids and liquids, whereas S-waves cannot travel through liquids, and the resulting S-wave shadow zone is the classic evidence that the outer core is molten. The flow of liquid iron in the outer core also generates the Earth's magnetic field.
这些圈层界面并非猜测，而是通过地震波探测出来的：P波可穿过固体与液体，S波却无法穿过液体，由此形成的S波影区正是外核为熔融液态的经典证据。外核中液态铁的流动还产生了地球磁场。
3. The Crust: Continental and Oceanic | 地壳：陆壳与洋壳
By analysing the crust, geologists can understand surface features and the course of geological events. The crust is the Earth's outermost rocky shell, and it includes both continental crust and oceanic crust. Its thickness varies from place to place, from only a few kilometres at the thinnest points to more than 70 km beneath major mountain ranges.
通过解析地壳，地质学家得以洞察地表特征和地质事件的演进脉络。地壳是地球最外层的岩石外壳，囊括了陆壳与洋壳。地壳厚度因地而异，最薄处仅有数公里，最厚处可超过70公里。
The two types of crust differ in composition, density and age. Oceanic crust is thin, dense and made mainly of basalt and gabbro; it is geologically young because it is continuously created at mid-ocean ridges and destroyed at subduction zones. Continental crust is thicker, less dense and dominated by granite-like rocks, and parts of it are billions of years old.
两类地壳在成分、密度与年龄上都不相同。洋壳薄而致密，主要由玄武岩与辉长岩构成，且地质年龄较轻，因为它在洋中脊不断新生、在俯冲带不断消亡。陆壳更厚、密度更小，以花岗质岩石为主，其中部分岩石已有数十亿年历史。
The boundary between crust and mantle is the Mohorovičić discontinuity, usually called the Moho, where seismic wave velocities increase sharply. Because the crust floats on the denser mantle, loading by ice sheets or mountain belts causes it to sink, and unloading lets it rise again, a slow adjustment known as isostasy.
地壳与地幔的分界是莫霍洛维契奇不连续面，通常简称莫霍面，此处地震波速度会突然增大。由于地壳"漂浮"在更致密的地幔之上，冰盖或山带的加载会使其下沉，卸载后又会缓慢回升，这种缓慢调整称为均衡补偿。
4. Plate Tectonics and Crustal Movement | 板块构造与地壳运动
Plate tectonics is the unifying theory that explains crustal movement. The rigid outer shell, called the lithosphere, is broken into a number of plates that move over the hotter, weaker asthenosphere below. Their relative motion pushes and pulls against neighbouring plates, and the resulting stresses are stored and released in the crust.
板块构造是解释地壳运动的统一理论。由刚性外壳构成的岩石圈破裂为若干板块，在下方更热、更软的软流圈之上运动。板块之间的相对运动形成推挤与拉扯，由此产生的应力在地壳中积累并释放。
Three types of plate boundary are recognised. At divergent boundaries plates move apart and new oceanic crust is created; at convergent boundaries one plate descends beneath another, or two continents collide and thicken the crust into mountain belts; at transform boundaries plates slide horizontally past one another.
板块边界分为三类。离散边界上板块相互分离，并生成新的洋壳；汇聚边界上板块俯冲消亡，或两个大陆碰撞、把地壳加厚成山带；转换边界上板块沿水平方向相互错动。
Mountain building, ocean basins, earthquakes and volcanoes are all consequences of plate interaction. Crustal movement is therefore the relative pushing of plates, which raises mountains and makes earthquakes frequent along plate margins.
山脉隆起、洋盆形成、地震与火山活动，都是板块相互作用的结果。地壳运动正是板块的相对推挤，它催生了山脉的隆起，也使板块边缘的地震频繁发生。
5. Rocks and the Rock Cycle | 岩石与岩石循环
Rocks and minerals are important objects of geological study. Rocks make up the surface of the Earth and are aggregates of one or more minerals; minerals are naturally occurring substances with a fixed chemical composition and a definite crystal form. Investigating them helps geologists trace the formation and evolution of the Earth.
岩石与矿物是地质学重要的研究对象。岩石组成地表，由多种矿物聚集而成；矿物则是拥有固定化学组成和晶体形态的天然物质。一番探究，能帮助地质学家回溯地球的形成与演化轨迹。
Rocks fall into three great classes. Igneous rocks form when magma or lava cools and crystallises, and are coarse-grained if they cool slowly underground and fine-grained if they cool quickly at the surface. Sedimentary rocks form when loose grains, shells or dissolved material are compacted and cemented, usually in layers, and they may contain fossils. Metamorphic rocks form when existing rocks are altered by heat and pressure without melting.
岩石分为三大类。火成岩由岩浆或熔岩冷却结晶形成，在地下缓慢冷却时颗粒较粗，在地表快速冷却时颗粒较细。沉积岩由松散颗粒、生物壳体或溶解物质经压实胶结而成，通常呈层状，并可能含有化石。变质岩则由已有岩石在高温高压下、但未熔融的条件下改造而成。
These classes are linked in the rock cycle, which has no fixed starting point because any rock can be transformed into any other given the right conditions:
三类岩石在岩石循环中彼此相连。这一循环没有固定起点，只要条件合适，任何一类岩石都可以转变为另一类：
岩浆 magma → 火成岩 igneous rock → 风化剥蚀 weathering and erosion → 沉积物 sediment → 沉积岩 sedimentary rock → 变质作用 metamorphism → 变质岩 metamorphic rock → 熔融 melting → 岩浆 magma
6. Minerals and Their Properties | 矿物及其性质
Minerals are identified by their physical and chemical properties. These include colour, streak, lustre, cleavage, fracture, crystal habit and hardness, which is measured on Mohs' scale from 1 for talc to 10 for diamond. A mineral is defined by a specific chemical composition and an ordered internal atomic arrangement, so its properties are predictable.
矿物凭物理与化学性质鉴定，包括颜色、条痕、光泽、解理、断口、晶习以及硬度。硬度用莫氏硬度计衡量，从1度的滑石到10度的金刚石。矿物的定义取决于确定的化学成分与有序的内部原子排列，因此其性质具有可预测性。
Most rock-forming minerals are silicates, built from a basic unit called the silica tetrahedron, which contains one silicon atom surrounded by four oxygen atoms and is written as SiO₄. The way these units join, either as isolated tetrahedra or as chains, sheets and frameworks, controls how the mineral behaves when it is heated, squeezed or weathered.
最主要的造岩矿物是硅酸盐，其基本单元称为硅氧四面体：一个硅原子被四个氧原子包围，写作 SiO₄。这些单元以孤立四面体、链状、层状或架状方式连接，决定了矿物在受热、受压或风化时的表现。
Common examples include quartz (SiO₂), feldspar, mica, olivine ((Mg,Fe)₂SiO₄), calcite (CaCO₃) and halite (NaCl). Calcite is a useful illustration of chemical weathering, because weak carbonic acid dissolves it in the reaction:
常见例子包括石英 SiO₂、长石、云母、橄榄石 (Mg,Fe)₂SiO₄、方解石 CaCO₃ 与岩盐 NaCl。方解石是化学风化的典型例证：弱碳酸会把它溶解，反应式为：
CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂
7. Geological Events: Earthquakes | 地质事件：地震
Geology is equally concerned with geological events and processes such as earthquakes, volcanic eruptions and crustal movement, because these events profoundly shape the relief and landforms of the surface. An earthquake is the shaking of the ground caused by crustal movement.
地质学同样关注地震、火山喷发、地壳运动等地质事件与过程，因为这些事件深刻塑造了地表的地形与地貌。地震源自地壳运动引发的大地颤动。
Stress accumulates along a fault until the rock suddenly breaks or slips. The stored elastic energy is released as seismic waves: fast P-waves, which are longitudinal and travel through solids and liquids; slower S-waves, which are transverse and pass only through solids; and surface waves, which usually cause the greatest damage. The point of origin underground is the focus, and the point on the surface directly above it is the epicentre.
应力沿断层不断积累，直至岩石突然破裂或滑动。储存的弹性应变能以地震波形式释放：速度快、属纵波且能穿过固体与液体的P波；速度较慢、属横波且只能通过固体的S波；以及通常造成最大破坏的面波。地下发源处称为震源，震源正上方的地表点称为震中。
Earthquake size is now normally expressed by the moment magnitude scale, which is based on the seismic moment:
如今地震规模通常用矩震级表示，其依据是地震矩：
M₀ = μ × A × D M𝓌 = (2/3) log₁₀ M₀ − 6.07
Here μ is the shear modulus of the rock, A is the fault area and D is the average slip. Consequences of large earthquakes include strong ground shaking, soil liquefaction, landslides and, in the ocean, tsunamis generated by sudden sea-floor displacement.
其中 μ 为岩石的剪切模量，A 为断层面积，D 为平均滑动量。大地震的后果包括强烈地面震动、土壤液化、滑坡，以及在海域中由海底突然位移引发的海啸。
8. Geological Events: Volcanoes | 地质事件：火山
A volcanic eruption is the scene of magma from below breaking through the surface. Mag
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