Cave formations tell tales of ancient earthquakes

Stalagmites in ancient caves may hold clues about some of the oldest earthquakes known to science, a new study has found.

A team from the US and Japan studied four stalagmites in a cave in southern Indiana in the US, searching for signs of earthquakes over hundreds of thousands of years.

Stalagmites, the rocky prongs that shoot up from cave floors, are formed by the minerals left over from centuries of water dripping from above.

Interruptions in the development of these historic spikes can offer clues of geological unrest, as University of Illinois at Urbana‐Champaign geologist and study lead author Samuel Panno explains: “When you take a stalagmite and slice it down its middle the long way and open it up like a book, you can see these shifts in the axis of its growth.”

These shifts can be signs of bushfires, extreme flooding and other changes in climate, as well as, apparently, earthquake activity.

Donnehue’s Cave in southern Indiana, the site of the study, lies in close proximity to the Wabash Valley fault system – a hotspot for earthquakes known to strike over the past 20,000 years.

The researchers used dating techniques to analyse four stalagmites showing significant shifts in their growth axes, monitoring when growth stopped, started and moved off-kilter. 

The findings, published in the Bulletin of the Seismological Society of America, focus on a pair of stalagmites lifted from the cave, the first of which began growing around 300,000 years ago. 

According to analysis, the first stalagmite experienced a growth-shift around 246,000 years ago – a period that coincides with a significant flood event – and then hit pause 170,000 years ago. 

Its twin, however, began growing 170,000 years ago and ceased 100,000 years ago. Both stalagmites then resumed growth around 60,000 years ago – coinciding with a 7+ magnitude earthquake in the region.

These ancient quakes are usually evidenced by analysis of changes in river streams or folds in rock formations, or, in the case of Donnehue’s Cave, by shifts in soil makeup, a process known as liquefaction. 

As Panno notes, using stalagmites could help to extend the existing record further back in time.

“Most of the evidence for palaeoearthquakes comes from liquefaction features that are fairly easy to date,” he explains. 

“The problem is that you are doing this in sediments that are usually on the order of several hundred to up to 20,000 years old, so to go beyond that, to get older and older earthquake signatures, we decided to look into caves.”

A younger stalagmite analysed in the study contained no evidence of flooding, and began its life around 1,800 years ago, at approximately the time a 6.2-magnitude quake struck the Wabash Valley. 

This stalagmite also showed shifts in growth dating back to quake events from the nearby New Madrid seismic zone, around 900 CE.

Because changes in the growth of stalagmites can also reflect flooding episodes and other examples of climate change, the team emphasise that more research is needed to figure out how to distinguish these factors.