Steel and Coal

November 08, 2008


Photographer: Rob Sheridan
Summary Author: Rob Sheridan, Stu Witmer

About 500 million years ago (mya), what would become central Pennsylvania was coastal plain straddling the equator, covered by vast swamps that were periodically submerged and exposed as sea levels changed. Nearly 350 mya, the Iapetus Sea closed and the Pangaea land mass formed. This event, followed by other mountain building episodes, created the Appalachian range and buried nascent coal beds under variable amounts of distorted sedimentary and subsequently metamorphic rock. Approximately 200 mya, seismic plate movement split Pangaea apart, forming the Atlantic Ocean -- the Pennsylvania coal beds continued to mature deep underground.

Coal is a combustible rock made primarily of carbon from dead plant material that’s prevented from oxidizing and biodegrading because it quickly enters an anoxic (no oxygen) environment when buried in swamp mud. Over eons, this material is subject to variably high pressures and temperatures, explaining the variety of coal types. Coal matures from peat (coal precursor), to lignite (soft brown coal), to sub-bituminous coal (a hard lignite), to bituminous coal (most common), to anthracite (a hard glossy coal). Most coal is considered a sedimentary rock, although anthracite is considered metamorphic because of the high temperatures and pressures required to create it. The lower the grade, the easier the ignition, the lower temperature of burning, the greater the proportion of volatile materials, and the more abundant the emissions released. High-grade anthracite coal is hot-burning, clean-burning coal.

Steel is an alloy of iron with a reduced carbon content of less than two percent. Nineteenth century steel manufacture required very high temperatures, supplied by the hot-burning anthracite coal found in abundance in Pennsylvania hills. Pictured above are massive abandoned blast furnaces in Bethlehem, Pennsylvania, icons of the American steel industry. Steel came into prominence in the 1890s and started to die out about 100 years later. Although the reasons why this industry withered are debated, the stage was set for its emergence hundreds of millions of years earlier.