Massive rock boulder may be the oldest remain of rockfall ever found

A massive rock boulder in Scotland may be part of the oldest rockfall ever found.

The remains of rockfalls are not preserved for many years, therefore, it is challenging to document rockfall incidents that have occurred in ancient times.

The rock boulder was found in a coastal region located in North Scotland. It measures 100, 60 and 15 meters in length, width and height, respectively. According to a research team that studied the rock, the boulder is massive and its weight is estimated at about 243.000 tons. 

The rock boulder is composed of Neoarchean Lewisian gneiss formation. Gneiss is a metamorphic rock that is formed under high pressures and temperatures through certain geologic procedures known as metamorphic processes.

According to the research team, the boulder fell off a cliff and plunged into sediments about 1.2 billion years ago. While the initial cliff eroded over the following eras, the discovery remains intact until today. 

After thorough fieldwork, Zachary Killingback, an M.Sc. student at Durham University at that time, managed to derive geological data about the gneiss formation and the surrounding geological background.

In particular, a basin was formed in the region (which is now in Scotland) about 1.2 billion years ago. In the basin, water accumulated creating lakes, rivers and streams while water flow caused erosion and sediments' deposition. An earthquake probably destabilized the rock boulder which collapsed in the sediment and twisted. This is proved by its foliation pattern that is rotated by around 90 degrees. Cracks propagated through the gneiss boulder and were filled with sediment material.

The research team conducted numerical simulations to find that the boulder fell no more than 15 meters above the ground. In their analyses, researchers had to incorporate the tensile strength of the gneiss rock, therefore, they obtained core samples and conducted laboratory tests. The tensile strength of a rock can be derived from the Direct Tensile Strength test or from the Splitting Tensile Strength Test (Brazilian Test).

Despite the fact that many scientists have visited the site, the discovery about the boulder's history is a brand-new finding. “How have we not noticed this before? It makes so much sense. I’ve literally sat on that (rock boulder) several times and eaten my lunch,” Dr. Catherine Mottram, a senior Lecturer of Structural Geology and Tectonics at the University of Portsmouth, stated.

According to Christopher Jackson, a Professor of Geology at Imperial College London, the discovery offers a "unique opportunity" to witness an ancient geological process that scientists know it existed but few remains are evident in the present.

The findings of the study were recently published in the Journal Geology.