Speleothems, sometimes referred to as formations or decorations, are cave features formed by the deposition of minerals. The word speleothem is derived from the Greek words spelaion meaning "cave" and thema meaning "deposit".
Speleothems form from the movement of water through the ground and cave. As surface water from streams and precipitation seeps through cracks in the rock, it passes through organic material and picks up carbon dioxide gas, creating carbonic acid. This weak acid passes through joints and cracks in the soluble bedrock. The mineral calcite is dissolved from that soluble bedrock like limestone or marble, in the case of Oregon Caves. When this water that now holds the dissolved rock is exposed to the air in the cave, it releases the carbon dioxide gas, much like when a bottle of soda is opened. As the carbon dioxide is released, calcite is precipitated (redeposited) on cave walls, ceilings and floors. This calcite builds up over time to form speleothems like stalagmites and stalactites.
Speleothems form at varying rates as calcite crystals build up. Several factors can determine the rate of growth. Two important factors are the temperature outside (which affects the rate of decay of plants and animals, hence the amount of carbon dioxide in the soil), and the amount of rainfall. The shapes of speleothems are determined by how the acidic water enters the cave (by dripping, seeping, or splashing) and how the water stands or flows after entering the cave.
Most scientists believe that the color of speleothems are determined by the mineral content. Pure calcite is white and almost colorless. Iron and other minerals, as well as acids from surface vegetation, combine with calcite crystals to add shades of red, orange and black to the color of speleothems. Others believe that humic and fulvic acids in the soil may also contribute to speleothem coloration.