The Tiny Tunnel Inside a Stalactite

A thin stone tube hangs from a cave ceiling. A drop of water shines at its tip, then falls. The tube looks like a drinking straw, and it really does have a tiny tunnel inside. This kind of stalactite is called a soda straw. Despite its delicate appearance, it is made of stone. Its construction begins above the cave.

As rainwater passes through soil, it picks up carbon dioxide gas released by roots and other living things. The gas mixes with the water to make a weak acid. If the water reaches limestone, the acid dissolves a small amount of the rock. The dissolved material travels with the water through narrow cracks. It is not a load of visible pebbles, even though it came from rock.

Eventually, the water reaches the ceiling of an open cave. The cave air often contains less carbon dioxide than the water does, so some gas escapes from the water into the air. This change allows some of the dissolved material to become solid again. It forms a mineral called calcite. A tiny ring of calcite can remain around the edge of a drop before the drop falls.

More drops arrive at the same spot. Each may add a little calcite to the ring's rim, or outer edge. Slowly, the rim extends downward into a tube. Water keeps moving through the center, so the growing tube stays hollow. The stone is added mainly at its lower end. The result is a narrow soda straw, built by many small additions rather than one large deposit.

Sometimes, a mineral plug blocks the tunnel. The water then needs another route. If it can flow around the blockage, it runs down the tube's outside instead. Calcite can build up along that outer surface, making the formation thicker. Over time, it may become a tapered stalactite, wider near the ceiling and narrower at the tip. Not every stalactite begins as a soda straw, however.

These formations do not grow at one dependable speed. Growth depends on the water supply, the amount of dissolved material, and conditions in the cave air. A dry period can interrupt the supply of drops and pause growth. The main process does not require the water to dry up: escaping carbon dioxide helps turn dissolved material back into solid stone.

A diagram under the heading "A Drop's Journey" can show arrows leading from soil, through limestone, and into a cave. Its caption might read, "Water carries dissolved rock to the ceiling; escaping gas helps leave stone behind." The arrows explain a surprising connection: a hanging stone tube can begin with rain soaking into the ground.