Coal Glossary

Common Geological Terms
Relating to Coal

anthracite Hard coal that is mostly carbon and burns cleanly.

bituminous Soft coal that burns with a large flame and plenty of smoke.

charcoal A black substance of carbon and ash typically made by heating wood while depriving it of oxygen.

clinker Waste rock left behind when flammable minerals are burned at very high temperatures.

coal A hard mineral that is flammable, and gives off heat and light. It forms when decayed vegetable matter is compacted by Earth's crust. From soft to hard, giving less heat to more heat, the main types are lignite, subbituminous, bituminous, and anthracite.

coke a fuel with high carbon content typically made by baking coal.

formation In geology, a group of deposits, near each other, of the same type of rock.

lava What molten rock (magma) from inside Earth is called when it reaches the surface, spewed out by a volcano or leaking through fissures (cracks). At the moment it reaches the air, lava has a temperature of around 1500 to 2100 degrees Fahrenheit, and is literally red hot.

lignite The type of coal that first develops when peat is compacted. Lignite is high in water and low in carbon and sulfur, and so gives off less heat than do longer-compacted (called "harder") types of coal. Lignite is the type of coal Lewis and Clark saw.

member In geology, one of component deposits that make up a formation.

mudstone At the edges of ancient freshwater lakes, mudstone is formed when layers of rock and minerals are compacted under pressure.

peat When plants have partially decayed and been compacted in layers by pressure from above, the result is called peat. If left alone for eons, it turns into coal. All over the world, however, people have dug up peat todry and use for fuel. Peat also is used as a fertizler for plants, and even a packing material or bedding for cows, horses, and so on, if it dries light and fluffy.

pumice Actually volcanic glass, this stone was liquid, melted rock when it was expelled from a volcano.The air cooled it so quickly that it didn't form crystals again, but filled with tiny holes. In Lewis and Clark's time pumice was valued as a gentle abrasive, and it still is.

sandstone When sand is compacted hard and long by pressures from layers above it—which sometimes hold other minerals that act as glue—it turns to sandstone. The stone comes in many different colors, because of the minerals in the sand. It can also include organic matter, such as fossils of animals or leaves that were trapped in the sand before it was compacted into rock.

scoria True scoria is lava that has cooled slowly in the air, and formed rough rocks. As used in western North Dakota and eastern Montana, though, the term refers to slow-burned, exposed coal beds that have turned red or pink.

soft coal a synonym for bituminous coal.

Funded in part by a grant from the Montana Committee for the Humanities.