A crisis by land, a revolution by sea

This month marks the anniversary of a seismic event that severed the lines of communication between the US and Europe.  This catastrophe revealed deep, powerful currents hidden beneath the surface, and changed our view of the landscape. But this not a story about American politics: it’s the story of the 1929 Grand Banks earthquake.

On November 18th of that year, a magnitude 7.2 earthquake struck offshore eastern Canada. The earthquake triggered a tsunami that caused heavy damage in Newfoundland and claimed 28 lives.

Several houses in Lord's Cove, Canada, displaced and toppled by the tsunami following the 1929 Grand Banks earthquake.
Houses toppled by the tsunami in Lord’s Cove, Canada. Image credit: H.M. Mosdell, W.M. Chisholm collection; Natural Resources Canada.

Though the link was obscured at the time, the catastrophe on land was caused by a dramatic upheaval on the ocean floor. The earthquake triggered an enormous submarine landslide, which generated the tsunami. A later study estimated the volume of the landslide at 200 cubic kilometers enough to fill a Texas-sized football stadium 10,000 times over! Beyond generating this staggering statistic, the events surrounding the Grand Banks earthquake transformed our understanding of the seafloor.

At the time of the earthquake, a network of transmission cables ran along the seafloor between North America and Europe. These cables originally conveyed the 19th century equivalent of a text message, and introduced the concept of a shared “now” between far-off places. Following the earthquake, 12 of these cables failed; some all at once, and others in a sequence moving roughly from north to south, with the furthest break more than 1000 km from the earthquake epicenter. Why did the second set of cables fail sequentially, and over such a long distance?

A bathymetric map of the seafloor offshore eastern Canada. A large gray area shows the extent of a key sedimentary deposit. The locations of transmission cable breaks are also shown.
Map of the seafloor offshore Newfoundland. Circles show the locations of cable breaks immediately following the earthquake (white) and those that broke sequentially thereafter (black). Reproduced from Piper et al. (1988), Figure 1.

The answer came in 1952, when researchers Bruce Heezen and Maurice Ewing argued that the submarine landslide initiated a current of sediment and water that raced across the seafloor and knocked out the cables. Reginald Daly had first proposed that these flows, known as turbidity currents, could occur in the ocean. Heezen and Ewing’s analysis revealed the cable breaks as the first direct evidence of a turbidity current. Later work by David Piper and colleagues used clues from the sediment deposit built by the flow to estimate its velocity. Imagine this: a billowing cloud of sediment moving as fast as a commuter train (60 to 100 km/hr), and as tall as a skyscraper (hundreds of meters).

An artist's rendition of a turbidity current: a dilute mixture of water and sediment moving across the seafloor.
An artist’s rendition of a turbidity current. Image credit: Open University and Wired Magazine.

At that time, more than a century of geologic thought had held firmly that the Earth evolved slowly and steadily. In the deep ocean, sediments were thought to pile up gradually and largely from the bodies of tiny creatures like diatoms, which fell to the seafloor like a “gentle rain from heaven.” A major shift in geologic thinking in the 20th century was that rare and catastrophic events, like the Grand Banks turbidity current, may write most of Earth’s history in sedimentary rock. The geologist Derek Ager wrote:

“In effect, in one step the concept of the turbidity current took us back to the catastrophism of earlier geological thought. In place of the comfortable doctrine of sedimentation keeping pace with subsidence, we were now faced with the notion of empty troughs forming in the sea-floor, with only occasional rushes of sediment to fill them.”

Sixty-five years after turbidity currents pushed to the forefront of seafloor research, the ocean still holds its secrets tightly. The sedimentary deposits built by these currents are the largest on Earth, and are the final resting place of place of bygone mountains. At St. Anthony Falls Laboratory, I am working with colleagues to shrink the flows and their deposits down to laboratory scale to observe their behavior over a matter of hours.

The transmission cables severed by the Grand Banks turbidity current were the key to building a shared present. Their destruction also transformed the way we think about the deep past. In opening lines of communication with our fellow inhabitants of Earth, we are sometimes lucky enough to listen in on the planet itself.

Reading a valley, Part 2: How can you retrace a river’s footsteps?

For geoscientists, lacking direct experience is a common quandary (and it should be noted, a happy one!) One great workaround: the thought experiment. Back in 1909, the eminent geographer William Morris Davis used his powers of imagination to visualize how the Connecticut River might have carved its valley and left terraces. Here’s a beautiful sketch […]