Sedimentary rocks are formed differently than those created by volcanic heat or intense pressure. They are often called the “storytellers” of the Earth because they are built up layer by layer, holding clues about the past. Each layer, or stratum, can reveal what the environment was like millions of years ago. Sedimentary rocks cover about 73% of the Earth’s land surface and are the only rock type that commonly holds evidence of ancient life.
Breaking Down the Big Stuff
The process of forming a sedimentary rock begins with weathering, which is the breaking apart of older rocks. Forces like wind, running water, ice, and plant roots slowly chip away at structures, crumbling them into smaller bits called sediment. This sediment can range from fine silt and clay to larger sand and pebbles. Water is also a powerful agent, causing chemical changes that dissolve minerals within the rock. Once the rock is broken into sediment, erosion takes over, moving the material away from its original spot. Flowing water and strong winds carry the sediment, preventing it from piling up where it fell.
The Journey to the Bottom
After erosion, the sediment begins transportation, carried by moving water, wind, or ice. Rivers move massive amounts of sediment downstream toward still bodies of water. Heavier, larger pieces, such as gravel and coarse sand, settle out first. Lighter particles, like clay and silt, are carried longer distances before falling out of the flowing water. This settling process, where the sediment comes to rest in a low-lying area, is called deposition. Deposition commonly occurs in lakes, river deltas, or the ocean floor. Over long periods, repeated deposition causes layers of sediment to pile up, forming thick beds of loose material.
Squeeze and Glue!
The final stage, lithification, transforms the loose sediment into a solid rock through two main steps: compaction and cementation. As layers of sediment pile up, the weight of the material above presses down on the layers below. This pressure causes the sediment grains to be tightly packed together, a process called compaction. Compaction reduces the empty spaces between grains and pushes out most of the trapped water. Next, cementation binds the pressed grains together. The remaining water contains dissolved minerals, such as silica or calcite, picked up during weathering. These dissolved minerals crystallize in the spaces between the sediment grains, acting as a natural cement. Common mineral cements include quartz and calcite, which permanently transform the loose sediment into a solid sedimentary rock.
What Sedimentary Rocks Tell Us
Because sedimentary rocks form in layers, they are valuable for understanding Earth’s history. The layers, called strata, appear as distinct bands of different colors or textures, reflecting changes in the ancient environment. For example, coarse sandstone might indicate an ancient beach, while fine shale suggests a deep, quiet sea. Sedimentary rocks are the only rock type where fossils—the preserved remains of ancient life—are commonly found. When an organism dies and is quickly buried by sediment, the coverage protects it from decay. As the sediment lithifies, the bone or shell material is often replaced by minerals, preserving it as a fossil within the rock. These rocks also contain features like ripple marks or mud cracks preserved on the bedding surface. Ripple marks are created by moving water or wind, and mud cracks form when wet sediment dries out. Studying the order and contents of these layers allows geologists to piece together past events.