When peats are buried, the weight of the overlying sediments squeezes out much of the water from the peat and reduces its volume (called compaction). Continued burial deeper into the earth also exposes the material to higher temperatures. On average, heat in the earth rises one degree Farenheit per 70 to 100 ft of depth. Heating causes hydrocarbon compounds (compounds composed of hydrogen, carbon, and oxygen) in the peat to break down and alter in a variety of ways resulting in coal. In general, moisture and gases (e.g., methane, carbon dioxide) are systematically expelled from the peat and resulting coal with increasing burial and heat. This results in increasingly carbon-rich coals. The stages of this trend proceed from peat through different ranks of coal (lignite, sub-bituminous coal, bituminous coal, anthracite coal).