2023 June Report – Geodes

This is a report on the June, 2023 Walk and Talk: The Story of Geodes.

Dr. Tim Chowns, Professor Emeritus of Geology at the University of West Georgia, shared his own personal story of the Woodbury Tennessee geodes in a talk that both explored the origin of geodes in general as well as expressed Tim’s own love of science.

We were led into the story with “geodes are holes filled with beautiful crystals – often varieties of quartz.” We learned that geodes may be formed in ancient lava flows or in sedimentary rocks. In lava, holes are formed when gas bubbles escape from the lava. Minerals are deposited within the holes by water and water vapor circulating through the lava. Crystals from these minerals are “seeded” on the margins of the cavity and grow inwards into the open space. Some especially favored are the amethyst filled geodes from Brazil or the “Thunder eggs” with agates from Washington and Oregon.

Tim’s story for us focused on geodes that form in sedimentary rocks, such as the limestones of Iowa, Kentucky and Tennessee. Holes are unusual in sedimentary rocks but sometimes occur inside fossil shells. Gas bubbles are clearly unlikely in loose sediment. In our case, the particular location centered on a farm near Woodberry, Tenn.
In the early 70’s, Tim was a professor at the University of Georgia, where he became acquainted with a graduate student in the geology department. Jerry “Geode,” as he became known, lived on that farm in Woodbury. Which had an abundance of geodes which he collected and brought back to the University. Jerry asked Tim to supervise a project to discover the origin of these specific Woodbury geodes. Tim visited the farm and suggested that Jerry cut some of the smaller geodes into slides that could be studied under a microscope. Jerry then cut and polished “tens of slides” but couldn’t figure out what was happening of significance in them. Tim then had a go at studying slide after slide. The first thing he noticed was what looked like the relics of needles enclosed within the earliest quartz crystals around the margins of the geodes. Though dubious, he was reminded of the opal spicules that make up glass sponges. Turns out these sponge spicules were abundant in some of the sedimentary rock surrounding the geodes. (Sponge spicules are structural elements found in most sponges. The meshing of multiple spicules forms the sponge’s skeleton which provides structural support)

So, at 10:30 at night Tim zeroed in on those needles and soon hit on a slide where the nature of the needles was obvious. He left a note for Jerry; “I know the origin of your geodes” and went home to bed. That “Aha” minute was in 1971, It took another three years to confirm their results, show their applicability to other localities and get their paper published. (As he stated it: “One second of elation followed by three years of grind.”)
To fill in the story: The “tell tale needles” of the Woodbury geodes came from the mineral anhydrite, a salt related to gypsum (calcium sulfate). Gypsum is used for Plaster of Paris and Wallboard, for example. It comes from salts evaporated from sea water.

The geodes from Jerry’s farm turned out to be relics of nodular masses of calcium sulfate which hardened within the sediment as a consequence of evaporation. Because the calcium sulfate is ultimately soluble, it later dissolved to leave cavities (vugs) in the compressed sediments. These cavities were filled eventually with the mineral quartz. In this case the quartz came from the redistribution of the silica derived from sponge spicules in sediment surrounding the cavities.

As it happened, opal in the sponge spicules was dissolved and carried into the geodes where it precipitated as quartz. The quartz then partially engulfed the anhydrite needles that had not completely dissolved as the calcium sulfate slowly dissolved away. In his Aha minute, Tim found just the spot where the anhydrite was covered just right with the quartz deposits.

The ultimate importance of Jerry’s geodes: Using the principle of Uniformity (the Present is the key to the Past) we can know that when the rocks around Jerry’s farm were deposited, the area was flooded by shallow seas with tidal flats like those around the Persian Gulf, with warm dry climates. The same goes for other localities where geodes are discovered in sedimentary rocks.

As an addendum to this story:

As can happen with true life science (and we love it when it happens as a result of an FOMR Walk and Talk) interests are aroused and new adventures happen. Wanting to collect geodes for herself, Tim sent Chrissie Jones and her husband to Taylor Ridge near Summerville, where they collected a roundish rock that resembled the specimens Chrissie had seen at McIntosh. Tim agreed to cut it open with a rock saw. While perhaps not the most beautiful of geodes, Chrissie was proved correct and her group of friends had an exciting afternoon in the West Georgia geology labs.