In the gray chill of the North Atlantic, it’s not uncommon to see icebergs float by. But this one was not like the rest. A photo taken from a shrimp trawler off Labrador captured an iceberg that seems deeply out of place: a jet-black hulking piece among its white kin.

“It is all black. Almost in a diamond shape,” said Hallur Antoniussen, the Faroese-born fisher who took the shot. He was working aboard the Saputi, more than 100 kilometers off the Labrador coast in May, when he saw it from the ship’s crane. “It’s something you don’t see very often.”

The photo, posted on Facebook, went viral. Comments poured in. “It’s an Oil Berg,” joked one user. “Looks like a giant woolly mammoth!” said another. So what’s actually the deal with it?

Ice is not always nice

Antoniussen says he’s seen a lot of icebergs in his 50 years fishing off the coast of Greenland, but nothing quite like this. To get to the bottom of this, CBC asked Lev Tarasov, a Memorial University physicist and glacial earth systems modeller, if he has any ideas what could have created this dark iceberg.

According to Tarasov, the most likely answer is the simplest one: dirt.

Greenland’s glaciers aren’t just rivers of frozen water. They are giant machines, grinding their way over rock and time. As ice slowly slides from the island’s center to the sea, it scrapes the bedrock below. That friction pulverizes stone into a fine, dark powder. Some of it becomes embedded in the ice, sometimes for millennia.

Parts of the ice flow at up to 20 kilometers per year, Tarasov explained. That’s a couple of meters per hour These ice streams act like conveyor belts, funneling ancient, dirt-streaked ice to the coast, where it calves off into the sea as icebergs. Most of these bergs are white or pale blue, but occasionally, a chunk breaks off that’s so thoroughly infused with ancient debris — ash, rock, or silt — that it turns black.

Tarasov guesses the ice in there is at least 1,000 years old, but could even be 100,000 years old.

That’s not the only plausible explanation

Without any actual samples from the glacier, it’s hard to have a definitive answer. Another possibility is volcanic ash.

Icebergs sometimes carry the ghostly remnants of ancient eruptions. As glaciers flow across landscapes, they collect soot from wildfires or layers of ash laid down by distant volcanoes. Over time, these veins become frozen into the ice.

Iceland is famously volcanic, and scientists also suspect subglacial hotspots beneath Greenland’s ice sheet. So, Tarasov doesn’t dismiss the idea because we just don’t know where all the hotspots under Greenland are.

There’s an even crazier possibility still.

There is some evidence of an asteroid strike in northwest Greenland. If this iceberg originated near the Hiawatha impact crater, dust from that cosmic collision might have become frozen into the glacier, only to emerge hundreds of thousands of years later in a slab of melting ice.

What’s next for the black iceberg?

As exotic as the iceberg may be, its trajectory from now on is bound to be unspectacular.

As it drifts south through Baffin Bay and along the Labrador coast, the berg continues to melt. Of course, what we’re seeing — this uncanny wedge of black — is just the tip. Roughly 90 percent of any iceberg lies below the waterline.

Tarasov suspects that much of the clean ice has already melted away. What remains is the hardened, dirty heart — densely packed and heavy, the last survivor of a larger, slowly disintegrating body.