It’s the last Science Wednesday of 2020 – wishing all our readers a very Happy New Year! Last week, my nieces received some geodes for Christmas and they are the inspiration for this week’s post! It’s been fun breaking the rocks open (safely!) together and seeing what resides inside.
A geode is a spherical or sub-spherical rock with a hollow space inside that is lined with minerals such as quartz or calcite. Geodes get their start as bubbles in volcanic rock or as hollows in sedimentary rock where organic materials such as shells and tree branches have decayed away, leaving behind voids waiting to be filled. Over millions of years, the outer part of the spherical holes hardens and hydrothermal fluid or groundwater containing elements of dissolved silicates and carbonates starts to slowly precipitate (grow) minerals on the inner walls of the cavity of the geode.
The most common minerals to form are quartz, but amethyst and calcite also can form. Different types of silica cool at different temperatures, thus layers of different crystals can form in a single geode. The host rock – bedrock – that contains these geodes eventually weathers with time, leaving behind these erosion-resistant crystal-filled treasures in streams and soils. Geodes are found in the western and central US (especially Iowa, of which the geode is the state rock) and around the world in Mexico, Brazil and Namibia (Africa).