Here's one common definition of geomythology by L. Piccardi (2007):
Geomythology indicates every case in which the origin of myths and legends can be shown to contain references to geological phenomena and aspects, in a broad sense including astronomical ones (comets, eclipses, meteor impacts, etc.). As indicated by Vitaliano (1973) "primarily, there are two kinds of geologic folklore, that in which some geologic feature or the occurrence of some geologic phenomenon has inspired a folklore explanation, and that which is the garbled explanation of some actual geologic event, usually a natural catastrophe"
I would define geomythology a little more broadly than this and break it into three general categories:
1. A garbled explanation of some actual geologic event, usually a natural catastrophe
2. A geologic feature or phenomenon which has inspired a folklore explanation
3. An association between a mythic figure or object and a geologic feature or phenomenon
An example of the first category might be the story of Noah's flood. Geologists have known for over a century that there's no geological evidence for a global flood as recounted in Genesis. Floods, however, are a common occurrence and most cultures have flood legends. Some geologists have argued that the flooding of the Black Sea around 5600 BCE have have inspired the story in the Mesopotamian cultures.
The Great Flood by Bonaventura Peeters the Elder
Another might be the story of Atlantis as recounted by Plato in 360 BCE. While his story just doesn't work historically, it may be a garbled remembrance of a massive volcanic eruption on the Aegean island of Thera (also called Santorini) around 1600 BCE which may have triggered the downfall of the Minoan Civilization.
Santorini (Thera) caldera from space in 2000
A final example is the eruption of Mount Mazama in 5677 BCE to form Crater Lake in Oregon. The Klamath Indians, who lived in the area at the time, had a legend passed down to recent times of a battle between gods of the mountains who threw fiery rocks off the summit.
Crater Lake caldera
In this way, myths may shed some light on real historical events (but have to be taken with a grain of salt because they're overlain with cultural influences and religious beliefs).
The Lakota (Sioux), for example, have stories about how the grooves on the side of Devil's Tower in eastern Wyoming were formed by a giant bear. Grooves geologists attribute to cooling of magma and formation of columnar jointing.
The Lakota name - Mato Tipilia or Bear Lodge - is a much better name, by the way, than the stupid "Devils Tower" (most likely named after the Devil because the Lakota viewed it as a sacred place).
Another example might be Giant's Causeway in Northern Ireland attributed to Fionn mac Cumhaill (Finn McCool) who supposedly built the causeway to walk to Scotland.
Of course geologists just talk about boring old magma intrusions and columnar jointing again.
The final category of geomythology I mentioned at the beginning of the post is a bit more odd. I'm going to leave it for tomorrow's post...