Sedimentary layers in Iceland can be grouped into three main categories on the basis of how they are formed:
Clastic sedimentary rocks are most common and are composed of detritus, which are fragments of rocks and minerals. Clastic rocks are classified on the basis of particle size, sediment transportation media (e.g., glacial till) or the sedimentary depositional environment (e.g., marine and fluvial sediment). Tephra is classified as a form of clastic sediment.
Chemical sedimentary rocksare not common in Iceland. Bog iron ore is a chemical sedimentary rock that forms in bogs due to high iron concentrations in the water. Mineral deposits in hot spring areas, called sinters, are also chemical sediments. The same is true of precipitates in geothermal areas, such as geyserite, hydrothermal clay, sulphur, and gypsum.
Organic sediment is formed from plant and animal material, which accumulates in thick layers. Examples of sedimentary organic matter and sedimentary rocks are petroleum, coal, lignite, and diatomite.
Soil, a mixture of clastic rock and organic matter, is a common sediment type in Iceland. Icelandic soils are classified as Andosols. Andosols are found only in those areas of the Earth where there is volcanic activity. Andosols are fertile but relatively rare in a global context.