The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. How does coal form, and what class of rock is it? (B)

Coal is classified as an organic sedimentary rock, a type of rock that is characteristically rich in organic carbon and formed from the accumulation and lithification of organic material. This material primarily consists of plant remains that have been preserved due to the lack of oxygen, which inhibits the decomposition process. Over time, these remains are compacted and buried under layers of sediments.

Sedimentary rocks are broadly categorized into three groups: clastic, chemical, and organic. Clastic sedimentary rocks are made up of particles that have been weathered and eroded from other rocks, chemical sedimentary rocks are formed from precipitated minerals, and organic sedimentary rocks, like coal, are the result of organic matter accumulation. During the transformation process, plant remains undergo physical and chemical changes, eventually becoming solid rocks. The degree of compaction and the amounts of heat and pressure applied play a crucial role in determining the rock's properties.

The Carboniferous period, approximately 360 to 300 million years ago, was a critical time in Earth's history for coal formation. This period received its name due to the large deposits of coal that were formed globally. Earth's climate during the Carboniferous was warm and humid, creating extensive swampy forests where plant life thrived.

During this era, there was a significant evolution of vegetation, with the dominance of giant ferns, horsetails, and other plants, which eventually gave rise to the immense coal deposits we find today. The environment provided the perfect conditions for the accumulation of plant debris, which, due to waterlogged conditions, did not fully decompose. As these thick layers of plant material were buried by sediment, the initial steps for coal formation were set in motion. The significance of the Carboniferous period is such that it has marked a distinct point in geologic and biological timelines, where the precursors to modern coal reserves were deposited.

Coal formation is a process that begins with peat and, given enough time and geological pressures, ends with the formation of anthracite. This transformation occurs in distinct stages, each representing a step towards greater carbon content and structural complexity.

From Peat to Lignite

Initially, dead plant material accumulates in wetland areas, forming peat – a soggy precursor to coal. When peat gets buried under sediment, the incremental increase in temperature and pressure from the overlying material compresses the peat, driving out moisture and impurities and forming lignite, or brown coal.

Progressing to Bituminous Coal

Further burial and the ongoing Earth's geological processes increase the heat and pressure, which expels more water and increases carbon content, transforming lignite into bituminous coal. Bituminous coal is valued for its higher calorific content and is widely used in the energy sector.

The Transition to Anthracite

The final stage in this transformative journey is the formation of anthracite, the highest grade of coal. Under even greater pressures and temperatures, bituminous coal metamorphoses into anthracite, which boasts the highest carbon content, the greatest energy density, and a lustrous sheen. The transformation from peat to anthracite is not only characterized by an incremental increase in carbon content but also by a decrease in the volatile compounds and water content, resulting in a denser, harder, and cleaner-burning fuel.