Glowing in the Dark
September 2022
Written by Jenny
Cave of the Mounds is well known for its spectacular speleothems. The formations are rich in color because of the impurities, or minerals, present in calcite. In Cave of the Mounds, we see white calcium-rich calcite, black or grey manganese-rich calcite, and orange or brown iron-rich calcite deposits. These displays of color and texture amaze countless visitors time and time again.
Many visitors experience the cave at this colorful level. Some visitors add to this experience and view the cave in an otherworldly glow. Many of the calcite deposits in the Cave of the Mounds are photoluminescent. Guests who carry our black lights with them have a chance to see the cave in a new light.
During photoluminescence, a substance absorbs energy, in the form of a photon, from a light source. This forces the electrons into a higher energy state. As the electron returns to a lower energy state the photon is released in a way that produces visible light. There are two kinds of photoluminescence experienced here, fluorescence and phosphorescence.
Fluorescent formations absorb and release energy very quickly. When an ultraviolet, or black light, is directed at these deposits the manganese molecules become activated and we see the calcite release light as a blue-green glow because it is at a different wavelength than the surrounding light sources. Once the light is removed the glow on the rock disappears.
Some formations are fluorescent only, others are also phosphorescent. When these phosphorescent formations are exposed to the ultraviolet light they also appear blue or green. However, when the black light is turned off, the rocks continue to glow in the dark. This happens because there is additional excitation at the atomic level which causes additional steps on the flow back to a more stable state. The intermediate state releases energy as visible light and provides guests with the chance to see our rocks glowing in the dark after being charged with ultraviolet light.
Not all of the speleothems in the cave respond to the black lights. This is because calcite itself is not inherently photoluminescent. It only becomes so when certain activating elements are present. Manganese is one such activator. Iron on the other hand is not. This is why guests have fun exploring the various formations to discover changes in color and intensity as they move through different areas of the cave.
All guests are invited to notice the luminescent creations near Surprise Cave as part of the tour experience. In this area there is a permanent ultraviolet light illuminating the flowstone so we can see the blue-green glow immitted by the calcite. Other adventurous spelunkers are delighted to discover how their caving experience is enhanced by renting a black light to carry through the cave during their tour. Finally, there is a black light tour available for adventurers who want to explore the cave using only ultraviolet light. This unique tour includes black light rental and a guided experience using only the light from ultraviolet flashlights as we wind our way through the cave to glimpse both large and small glowing formations.