Is your question how are geodes formed in nature? The simple explanation is the process involves 1) cavity formation and 2) mineral-rich underground or hydrothermal solutions percolating and forming mineral masses and/or crystals.
However, the entire process is complex and not well understood. Also, it takes a long time, usually thousands to hundreds of millions of years.
This post discusses how geodes are formed. It focuses on how cavities form in volcanic and sedimentary rocks and the mineral masses and crystal filling process.
However, before that, here is a quick recap of what geodes are.
What are geodes?
Geodes are hollow or partly filled, nearly round or globular bodies that contain mineral masses and sometimes mineral crystals.
These minerals include quartz, chalcedony, agate, amethysts, opal, calcite, and other exotic minerals.
Usually, geodes are found in weathered volcanic lava flows, layered ashes, and tuff or sedimentary rocks, mainly limestone. However, they can also occur in dolomite, shale, and other sedimentary rocks.
In the us, these rocky bodies are common in Keokuk in Iowa, Dugway in Utah, and Woodbury in Tennessee, among other places. Also, they occur in countries like Mexico, Brazil, Uruguay, and Namibia.
How are geodes formed?
The general idea is that mineral-rich hydrothermal fluid, rain, or groundwater percolates or seeps into cavities and crystallizes or precipitates mineral masses or crystals. It means that cavities must exist.
Let us look at how cavities form in volcanic and sedimentary rocks as stage one and the partial filling of these cavities as stage two.
1. Stage one: Cavity formation
How geode cavities form depends on which rocks they occur. Are they in igneous or sedimentary rocks? A less common way is fractures or cracks in any rock.
a). Volcanic geodes
Cavities in geodes of volcanic origin are vesicles or lava tubes. These are common in places with volcanic activities, including lava flows.
Vesicles form when solidifying lava traps air or gas bubbles. These bubbles form from exsolving volatiles.
Usually, gases exsolve from lava or magma melt due to depressurization as magma rises. It is like how carbonated drinks bubble when you open them.
These vesicles will have a nearly spherical-shaped geode. Their sizes vary, i.e., they can be small or large. However, most form relatively larger geodes than those in sedimentary rocks.
On the other hand, lava tubes are tunnels inside solidified lava. They form when lava flows below the hardened surface.
Lava tubes produce elongated and longest or cathedral geode kids, some nearly a meter long or more. At times, these geodes are branched.
Lastly, volcanic geodes are much sought after and are often filled with agate, quartz, opal, chalcedony, and other minerals.
b). Sedimentary geodes
Cavities in sedimentary rock like limestone, dolomite, and calcareous shale form when 1) an existing structure in rocks dissolves or 2) enclosed organic matter decays and dissolves.
For instance, groundwater can dissolve mineral masses like nodules or concretions in limestones or sandstones. An example is Keokuk geodes from Iowa, where such structures dissolve in surrounding shale, creating voids.
On the other hand, voids can form when shells, roots, wood, branches, coral, or other organic matter buried in layers of sediments or sedimentary host rocks decay and dissolve.
Also, some voids or cavities can form from buried animals’ burrows. The thick sediment layers will lithify, i.e., compact and cement-forming sedimentary rocks with these voids.
Usually, cavities in sedimentary rocks form geodes of various shapes, including oblong ones. However, they are relatively smaller than those of volcanic origin.
Lastly, they often have opal, quartz, agate, and carbonate minerals. However, they can have other exotic minerals.
2. Stage two: Partial filling of cavities
Once cavities form, irrespective of the way, mineral-rich hydrothermal fluid, groundwater, and rainwater will seep into these cavities through cracks and pores in host rocks.
Then, evaporation, change in pressure and temperature, or fluid chemistry in these cavities will make these fluids crystallize or precipitate minerals.
For instance, the alkaline environment created by anaerobic conditions as organic matter decomposes without oxygen can encourage calcite precipitation.
The precipitation or crystallization of minerals will partially fill cavities to form geodes. This process takes a very long time, up to hundreds of millions of years. Therefore, it is not a mere partial filling of cavities.
Usually, geode formation occurs inward from the cavity wall. It will start with forming the thick layer, shell, or rind of chalcedony that will line the cavity.
Chalcedony is a hard cryptocrystalline quartz made of silica (SiO2), which is more weather-resistant than the host rock.
Once the rind forms, more water-rich fluids will seep into the cavity and deposit minerals like quartz, agate, chalcedony, and opal.
These will form delicate, colorful layers growing inward due to cyclic changes in fluid chemistry, temperature, and other factors.
Also, these fluids may precipitate or crystallize minerals that will grow inward on formed layers. These are well-formed and include amethysts, calcite, quartz, and other exotic minerals. They are well-formed as they grow into the void unimpeded.
During formation, some geodes can burst during geode growth and expansion, destroying the original vesicle or fossil cavity. Evidence includes fractured original fossil cavity. Also, expansion can distort laminations around geodes.
How long it takes depends on the size of the cavity and other factors. For instance, you expect a larger one to take longer.
Lastly, bedrock around geodes eventually decomposes and weathers, forming these discrete structures.
Frequently Asked Questions (FAQs)
The visible bands correspond to varying precipitation or crystallization stages or episodes. Some can show patterns suggesting fluid entered cavities. Similarly, color change can show a change in the chemistry of fluids, including impurities.
Yes. Dissolution of nodules and concretion can co-occur as partial filling of the created void, i.e., syngenetic. The filling minerals can include some that dissolve or are brought in by hydrothermal solutions, rain, or groundwater.
Quartz, chalcedony, agate, and opal are the most common minerals. Also, it can have calcite, amethysts, and dolomite, and they can also form exotic minerals, including celestine, citrine, millerite, and pyrite, among others.