Sedimentary Rock
Sedimentary rock is formed from the accumulation and compaction of sediments — particles of minerals, organic matter, or chemical precipitates — deposited by water, wind, ice, or gravity over time. This process, called lithification, involves compaction (pressure from overlying sediment) and cementation (mineral-rich groundwater binding particles together). Sedimentary rocks cover about 75% of Earth's land surface, host nearly all fossil records, and contain major reserves of coal, oil, natural gas, and economically important minerals.
Classification of Sedimentary Rocks
| Category | Formation Process | Example Rock | Key Feature |
|---|---|---|---|
| Clastic | Fragments of other rocks cemented together | Sandstone, Shale, Conglomerate | Grain size varies |
| Chemical | Precipitation from mineral-rich water | Limestone, Rock Salt, Chert | Crystal or layered texture |
| Organic (Biogenic) | Accumulation of organic remains | Coal, Chalk, Fossiliferous Limestone | Contains fossils |
| Evaporite | Evaporation of saline water | Gypsum, Halite | Soluble minerals |
| Pyroclastic | Volcanic ash and fragments compacted | Tuff, Breccia | Mixed volcanic material |
Interactive Tools
Khan Academy — Sedimentary Rocks
Detailed lessons on how sedimentary rocks form and their classification
Open ToolUSGS Sedimentary Rocks
USGS overview of sedimentary processes, rock types, and geological significance
Open ToolMindat.org
Searchable database of sedimentary rock types with specimen photos and descriptions
Open Tool
Wikimedia Commons, CC BY-SA
Related Terms
Rock Cycle
The rock cycle is a continuous geological process by which rocks are transformed from one type to another through processes such as melting, cooling, erosion, deposition, compaction, cementation, heat, and pressure over geological timescales. The three main rock types — igneous, sedimentary, and metamorphic — are interconnected in this cycle, each capable of being converted into another type under the right conditions. Understanding the rock cycle helps geologists interpret Earth's history, locate mineral resources, and predict geological hazards.
Igneous Rock
Igneous rock is formed by the cooling and solidification of magma (underground molten rock) or lava (magma that reaches the surface), making it one of the three fundamental rock types in Earth's crust. Intrusive igneous rocks such as granite solidify slowly deep underground, forming coarse-grained crystals, while extrusive igneous rocks such as basalt cool rapidly at the surface, forming fine-grained or glassy textures. Igneous rocks make up about 90% of Earth's crust by volume and are essential raw materials for construction, ceramics, and as indicators of volcanic history.
Continental Drift
Continental drift is the hypothesis that Earth's continents were once joined together in a single supercontinent called Pangaea and have since moved apart over geological time to their current positions. Proposed by Alfred Wegener in 1912, the hypothesis was supported by the jigsaw-fit of continents, matching fossil records across oceans, and similar rock formations on different continents. Continental drift later became a cornerstone of plate tectonic theory, which provided the mechanism of seafloor spreading to explain how continents move.
From Latin "sedimentum" (settling, a deposit), derived from "sedere" (to sit, to settle). The word reflects the process by which particles settle and accumulate. The systematic study of sedimentary rock layers (stratigraphy) was pioneered by William Smith in England around 1815, who showed that rock strata could be identified by the fossils they contained.