Sedimentary Rocks: Formation, Types and Features | 沉积岩：形成、类型与特征

📚 Sedimentary Rocks: Formation, Types and Features | 沉积岩：形成、类型与特征

Sedimentary rocks are one of the three main rock groups in the rock cycle. They form from loose particles called sediments, which are laid down in layers, compacted and cemented over millions of years. In KS3 science, you need to know how they form, how to recognise them, and why they are important for fossils and useful materials.

沉积岩是岩石循环中三大岩类之一。它们由松散的颗粒（沉积物）经过层层堆积、压实和胶结，在数百万年间形成。在 KS3 科学课程中，你需要掌握沉积岩的形成过程、识别特征，以及它们为何对化石和有用资源如此重要。

1. What Are Sedimentary Rocks? | 什么是沉积岩？

Sedimentary rocks are rocks made from sediments: small pieces of older rocks, minerals, shells or organic matter. When these sediments are transported by water, wind or ice, deposited in layers and then compacted and cemented, they become solid rock.

沉积岩是由沉积物形成的岩石：这些沉积物包括较老岩石的碎屑、矿物、贝壳或有机物质。当它们被水、风或冰搬运、分层沉积，再经过压实和胶结后，就变成坚硬的岩石。

Sedimentary rocks cover about 75% of the Earth’s land surface. Common examples include sandstone, limestone, shale, chalk and conglomerate. Although they are widespread at the surface, they are only a thin layer compared with the whole crust.

沉积岩覆盖了地球陆地表面约 75%。常见例子包括砂岩、石灰岩、页岩、白垩和砾岩。虽然它们在地表分布很广，但与整个地壳相比只是很薄的一层。

2. From Weathering to Sediment | 从风化到沉积物

All sedimentary rocks begin with weathering, the breakdown of existing rock. Physical weathering breaks rock into smaller pieces without changing its composition, for example by freeze-thaw action. Chemical weathering changes minerals into new substances, such as when rainwater dissolves limestone. Biological weathering is caused by plants and animals, for example roots growing into cracks.

所有沉积岩都始于风化作用，即原有岩石的破坏。物理风化在不改变成分的情况下把岩石破碎成小块，例如冻融作用。化学风化把矿物变成新物质，例如雨水溶解石灰岩。生物风化由动植物引起，例如树根长入裂缝。

After weathering, erosion moves the loose particles away. The main agents of erosion are gravity, running water, wind and glaciers. During transport, particles are rounded, sorted and broken down further.

风化之后，侵蚀作用将松散颗粒搬运走。侵蚀的主要营力包括重力、流水、风和冰川。在搬运过程中，颗粒会被磨圆、分选并进一步破碎。

3. Deposition and Layering | 沉积与分层

Deposition happens when the transporting agent loses energy. For example, a river drops heavier gravel first, then sand, then silt and clay. Wind deposits sand in deserts to form dunes, and the sea deposits shells and mud on the seabed.

当搬运介质能量降低时就会发生沉积。例如，河流先沉积较重的砾石，再沉积砂，然后是粉砂和黏土。风在沙漠中沉积沙子形成沙丘，海洋则在海底沉积贝壳和泥。

Over time, sediments build up in layers called beds or strata. The oldest layers are normally at the bottom, and younger layers are laid down on top. This layering is one of the most obvious features of sedimentary rocks and is used by geologists to read Earth history.

随着时间推移，沉积物堆积成层，称为地层。最老的岩层通常在底部，较新的岩层覆盖在上面。这种层理是沉积岩最明显的特征之一，地质学家用它来解读地球历史。

4. Compaction and Cementation | 压实与胶结

Loose sediment becomes solid rock through two processes: compaction and cementation. As more layers build up, the weight squeezes out water and air from the spaces between grains. This is compaction. The grains are pressed closer together and the sediment becomes denser.

松散的沉积物通过两个过程变成坚硬岩石：压实和胶结。随着更多岩层堆积，重力挤压出颗粒之间的水和空气，这就是压实作用。颗粒被压得更紧密，沉积物变得更致密。

Cementation occurs when dissolved minerals crystallise in the spaces between grains and bind them together. Common natural cements include silica, calcite and iron oxides. Calcite cement in sandstone is often formed from dissolved shells or limestone.

当溶解的矿物在颗粒之间的空隙中结晶并把颗粒粘结在一起时，就发生胶结作用。常见的天然胶结物包括二氧化硅、方解石和氧化铁。砂岩中的方解石胶结物通常来自溶解的贝壳或石灰岩。

Loose sediment → compaction → cementation → solid sedimentary rock

松散沉积物 → 压实 → 胶结 → 坚硬沉积岩

5. Types: Clastic Sedimentary Rocks | 类型：碎屑沉积岩

Clastic sedimentary rocks are made from fragments of other rocks. They are classified by grain size. Conglomerate has rounded pebbles and gravel, sandstone has sand-sized grains, siltstone has fine silt particles, and shale has very fine clay particles.

碎屑沉积岩由其他岩石的碎块组成，并按颗粒大小分类。砾岩含有磨圆的卵石和砾石，砂岩含有砂粒大小的颗粒，粉砂岩含有细小的粉砂颗粒，页岩则含有极细的黏土颗粒。