An earthquake and a delayed disaster
At 5:02 p.m. on November 18, 1929, a magnitude 7.2 earthquake occurred on the Grand Banks, about 250 kilometers south of Newfoundland's Burin Peninsula. The shaking was felt as far away as Montreal and New York City, but on the sparsely populated peninsula, it caused little direct damage. For about two and a half hours, the main effect seemed to be the ground tremors. Then, the sea suddenly receded, exposing the ocean floor in the harbors before a series of three massive waves struck the coast. This tsunami, with waves reaching heights of 13 meters in the narrow bays, destroyed homes and fishing stages, killing 28 people and leaving hundreds homeless.
A blizzard struck the following day, and with the peninsula's single telegraph line already out of service, news of the tsunami did not reach the outside world for three days. While the human tragedy unfolded on land, another, much larger disturbance was racing through the deep ocean, unnoticed by anyone. The event would not be understood for decades, but it would fundamentally change geology.
A deep-sea detective story
Telegraph companies quickly discovered that 12 of their transatlantic submarine cables, which connected North America and Europe, had gone dead. Repair crews found that the cables were broken in more than one place near the earthquake's epicenter, but in multiple locations stretching for hundreds of kilometers across the seafloor. The breaks occurred in a distinct sequence. Cables closest to the epicenter snapped almost instantly, while those farther away, in deeper water, broke hours later. The final cable break happened more than 13 hours after the earthquake, over 500 kilometers from the initial slide.
This pattern puzzled scientists. The earthquake alone could not explain the sequential, hours-long series of breaks. In 1952, geologists Bruce Heezen and Maurice Ewing of Columbia University studied the cable repair logs. By mapping the location and exact time of each break, they calculated the speed of whatever had destroyed them. They concluded that the earthquake had triggered a massive submarine landslide on the continental slope, dislodging an estimated 200 cubic kilometers of sediment. This sediment mixed with water to form a dense, fast-moving underwater avalanche known as a turbidity current.
The current started at a speed of nearly 100 kilometers per hour, snapping the first cables, and continued its journey into the abyss, slowing as it went but still powerful enough to sever the final cable 13 hours later. It was the first time a turbidity current had ever been documented, providing direct evidence for these powerful, seafloor-shaping events that had previously only been a theoretical concept.