A landslide is a rapid form of mass wasting, defined as the downslope movement of rock, debris, or earth under the influence of gravity. These events range from small rockfalls to colossal movements that reshape mountain ranges and ocean floors. Determining the single “biggest” landslide is complicated because the designation can be based on several metrics. A simple answer based on volume is challenging since the largest movements occurred millions of years ago and are only partially preserved.
Defining the Scale of Mass Wasting Events
The primary geological metric used to quantify mass wasting events is the total volume of material displaced, typically measured in cubic kilometers. This quantitative basis allows scientists to rank these natural phenomena. For instance, a small landslide might involve less than one cubic kilometer of material, while a megaslide can exceed thousands of cubic kilometers.
Geologists categorize these massive movements based on their environment, distinguishing between subaerial and submarine events. Subaerial landslides occur on land, moving down mountain slopes. Submarine landslides, by contrast, take place entirely underwater, often along continental shelves.
This distinction is important because submarine mass movements frequently dwarf their terrestrial counterparts. The sheer volume of unstable sediment accumulating on the ocean floor, combined with the lack of restraining friction, allows for colossal events. Although submarine slides are difficult to measure, they consistently represent the largest movements of material on Earth.
The Heart Mountain Slide: Earth’s Largest Subaerial Event
The Heart Mountain Slide, located in northwestern Wyoming and southeastern Montana, is the largest known subaerial landslide on Earth. This event occurred approximately 48.87 million years ago during the Eocene epoch, concurrent with volcanic activity. The movement covered an area of at least 3,400 square kilometers.
The total volume of rock that moved is cited to be around 3,400 cubic kilometers, consisting of Paleozoic carbonate rocks and Eocene volcanic material. This block, up to two kilometers thick, traveled a horizontal distance of at least 45 kilometers from its original position. The slide initiated near Silver Gate, Montana, with debris scattered as far as Cody, Wyoming.
The mechanism by which this mass of rock moved across a relatively flat surface is still studied. The detachment fault, the surface along which the rock slid, had a remarkably shallow slope of only about two degrees. This low angle suggests that the friction between the sliding block and the underlying rock must have been extremely low.
Current theories suggest the movement was facilitated by a cushion of material that acted as a lubricant. One prominent hypothesis involves the rapid, high-temperature decomposition of carbonate minerals along the fault plane. This process may have produced carbon dioxide gas, creating a temporary, near-frictionless layer that allowed the block to glide rapidly.
Other Megaslides and Their Triggers
While the Heart Mountain Slide is the largest terrestrial event, the biggest overall mass wasting events are found on the ocean floor. The colossal Storegga Slide off the coast of Norway exemplifies this, having occurred about 8,150 years ago. This submarine landslide involved an estimated 3,500 cubic kilometers of debris and triggered a massive tsunami that impacted coastlines across the North Atlantic.
Megaslides are generally triggered by geological forces that destabilize large volumes of rock and sediment. One common trigger is seismic activity, where large earthquakes generate ground motion strong enough to liquefy sediments or overcome rock strength. The Storegga Slide is theorized to have been initiated by an earthquake that destabilized the continental slope.
Primary Triggers of Megaslides
Volcanic instability is another trigger, particularly for large subaerial slides like the Heart Mountain event. The rapid growth and structural weaknesses of volcanic edifices can lead to flank collapses that displace vast quantities of material. Additionally, rapid deglaciation at the end of an ice age can trigger slides by removing the weight of overlying ice sheets.
The removal of ice weight causes the underlying land to rebound, leading to ground instability. Furthermore, the influx of meltwater and sediment can overload and weaken submarine slopes. The combination of seismic events, volcanic processes, and changes in glacial loading demonstrates that the largest landslides result from Earth’s profound geological shifts.