Rocks in Soil

Rock Sizes in Soil

• As different rock types weather, the particles sizes become smaller and may degrade to form new minerals in the soil. Sand particles are among the largest soil particles, and the grains are generally easy to see with the naked eye. Common minerals that form sand particles are quartz, feldspar, magnetite, olivine, feldspar and hornblende. Silt particles are smaller than sand, and you need a microscope to see the individual grains; however, you may be able to feel the grains between your fingertips. Quartz, feldspars and micas commonly form silt particles. Feldspars and micas can further degrade to clay particles, which are the smallest particles in soil and are not easy to see or feel.

Igneous Rocks in Soil

• Igneous rocks form from volcanoes both below and above ground. Common igneous rocks include basalt, granite, syenite, monzonite and scoria, among others. The minerals in these various igneous rocks will break down over time and form soil particles through the weathering process. Minerals such as quartz, feldspar and micas from a variety of igneous rocks form sand, silt and clay particles in sandy, silty and clayey soils and in mixtures known as loams.

Sedimentary Rocks in Soil

• Sedimentary rocks form in rivers, lakes, oceans and deserts. Limestone, sandstone and shale are common sedimentary rocks that weather over time to form soil particles. The quartz in sandstone will form sand and silt particles that are present in sandy, silty and loamy soils. Limestone forms from the mineral calcite, and particles that weather from a larger rock can incorporate as part of the soil and alter the pH of the area. Shale is largely clay minerals that are common in soils but also contain calcite and quartz that form larger soil particles.

Metamorphic Rocks in Soil

• A metamorphic rock is one that was formerly an igneous or sedimentary rock but changed due to heat and pressure at depth. These metamorphic rocks also contain minerals that form soil particles such as quartz, feldspars and micas, among others. Common metamorphic rocks that degrade to soil constituents are gneiss, marble, quartzite and slate.