Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". ... The protolith may be a sedimentary, igneous, or existing metamorphic rock.Metamorphic rocks make up a large part of the Earth's crust and form 12% of the Earth's land surface.

**Sedimentary Rocks: The Result of Gradual Deposition**

**Explanation:**

**1. Introduction to Sedimentary Rocks:**
Sedimentary rocks are one of the three main types of rocks found on Earth, alongside igneous rocks and metamorphic rocks. They are formed through a process called sedimentation, in which sediments (particles of rock, minerals, or organic matter) are gradually deposited and compacted over time.

**2. Formation Process:**
The formation of sedimentary rocks involves several distinct steps:

- **Weathering and Erosion**: The process begins with the weathering and erosion of existing rocks on the Earth's surface. This can occur due to various factors such as wind, water, ice, and even biological activity. As the rocks are exposed to these forces, they break down into smaller particles.

- **Transportation**: The eroded particles or sediments are then transported by wind, water, or ice to a new location. This can occur over long distances, with sediments being carried downstream by rivers, deposited in lakes or oceans, or transported by glaciers.

- **Deposition**: Once the sediments reach a new location, they settle and accumulate on the Earth's surface. This deposition occurs when the transporting agent (such as water or wind) loses its energy and can no longer carry the sediments. The sediments then settle out of the transporting agent and accumulate as layers.

- **Compaction and Cementation**: Over time, the weight of the overlying sediments, as well as the pressure from the Earth's crust, compacts the deposited sediments. This compaction squeezes out any remaining water or air between the particles, causing them to become tightly packed. Additionally, minerals dissolved in groundwater can precipitate and fill the spaces between the particles, cementing them together.

- **Lithification**: The final step in the formation of sedimentary rocks is lithification, which involves the transformation of loose sediment into solid rock. This occurs through the processes of compaction and cementation, which bind the sediments together and create a cohesive rock mass.

**3. Characteristics of Sedimentary Rocks:**
Sedimentary rocks have several distinguishing characteristics:

- **Layering**: Sedimentary rocks often exhibit distinct layers or strata, which are formed by the successive deposition of different sediments over time. These layers can provide valuable information about past environments and geological history.

- **Fossils**: Sedimentary rocks are known for their abundance of fossils. Fossils are the preserved remains or traces of ancient plants and animals, and they can provide insights into past life forms and ecosystems.

- **Softness**: Compared to igneous or metamorphic rocks, sedimentary rocks are generally softer and more easily eroded. This is due to their composition, which is often made up of loosely packed particles or organic matter.

- **Variety**: Sedimentary rocks can be further classified into various types based on their composition and origin. Common examples include sandstone, limestone, shale, and conglomerate.

**Conclusion:**
Sedimentary rocks are formed by the gradual deposition of sediments, which are then compacted and cemented over time. They are characterized by their layering, abundance of fossils, softness, and variety. Understanding the formation and characteristics of sedimentary rocks is essential for various fields of study, including geology, civil engineering, and environmental science.