Coal Formation in Illinois
There are numerous Illinois State Geological Survey publications on coal formation in Illinois, including our latest book, Geology of Illinois, chapter 14.
Coal formation is an involved, but interesting story. Approximately 300 million years ago during the Pennsylvanian age, Illinois was near the equator. As it is today, the long-term global climate of the Pennsylvanian was controlled by glacial cycles, with ice building up and melting away in a repeating pattern. Where Illinois was at the equator, climate oscillated from being very wet during glacial "polar ice" intervals, to more dry during interglacial "no polar ice" intervals. During the very wet glacial intervals, Illinois saw luxuriant plant growth develop on broad, flat, poorly drained landscapes, and the creation of vast peat swamps.
The plants of the time looked very different from our familiar, modern, species. Giant, 100 foot tall, stalk-like ancestors of present-day club mosses called Lycopods dominated the forests (and today comprise 85 to 90% of our coal seams). Also present were 40 foot tall ancestors of horsetails called Calamites, large tree ferns, seed ferns, and a variety of ground cover plants. Living in the forests were insects of impressively large size. Ancient dragonflies with wingspans 1 foot across flew through the lycopod trees, while millipede ancestors 5 feet long, and the largest land invertebrate of all time (called Arthropleura) scuttled in the leaf litter and debris eating the downed vegetation.
When the wet, peat forming, glacial periods came to an end, drying climate, combined with rising sea level, brought eroded sediment flooding on top of the peat swamps, covering the peat and beginning the coal forming process. Over time, additional sediment was deposited, and the peat was buried deeper and deeper. Heat and pressure from the overlying sediment (which eventually reached a mile in thickness) expelled water, and cooked out volatile gases, gradually concentrating and reconstituting the carbon bearing plant remains into first lignite, then subbituminous, and finally the bituminous coal we see today.
Historically, the first written record of coal in the New World is from Illinois, along the Illinois River near what is now the town of Ottawa. Coal-bearing rocks underlie 37,000 square miles, or 68% of the state, and more than 211 billion tons of coal are estimated to remain in place in Illinois. (Source: Geology of Illinois).