We are staring down the face of a submarine cliff, a wall twice the height of the Grand Canyon, and wondering “How did this get here?”

We began our sonar surveys of the ocean floor early this morning. We will be “mowing the lawn,” as the oceanographers say, for about four days as we transit northwest and southeast along three survey lines south of Isabela Island.

Within 12 kilometers (about 6.5 miles) from shore, the island’s slope descends to a depth of 3.5 kilometers. “Features this big are not unknown in the ocean,” Dennis Geist says. But what fascinates us, even more than its shear relief, are the lava flows and landslides we are mapping along the submarine flanks of the volcano that will help explain how Isabela formed. “It’s unclear whether Isabela was constructed on top of an older island, after a series of eruptions or after a long period of time,” he says.

The southern sides of Cerro Azul and Sierra Negra, the southernmost of Isabela’s five volcanoes, rise like upside down soup bowls from the ocean. Off the southeast corner of Isabela is Tortuga Island - a reminder of the Galápagos archipelago’s violent past.

When lava erupts deep underwater, the enormous pressure keeps the water from turning to steam, similar to a pressure cooker. But when lava erupts in shallow water less than 200 meters deep, the gases can escape and the heated water turns to steam causing dangerous explosions. The magma is broken up into little pieces of volcanic ash, called tuff. Tortuga Island is one of many tuff cones that erupted millions of years ago when the Galápagos Islands first started forming.

With whales spouting in the distance and Revelle steaming along at 8 knots, we are looking at our maps for the keys to unlock the secrets of how the Galápagos Islands formed.

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