Photo Essay: Rising Islands, Monster Wave

Cape Verde is a small island chain off the coast of West Africa. Fed by a plume of magma beneath the Atlantic Ocean, these islands are slowly inching upward. (NASA)

Charles Darwin stopped on the largest and most populated of the islands, Santiago, in 1832 on his famous Beagle voyage. (R.T. Pritchett)

At these cliffs, Darwin figured out that lava once submerged in seawater now stood high and dry. He failed to recognize Santiago was rising, but his observation that submarine lava flows had somehow wound up on land was correct.

Ricardo Ramalho, a postdoctoral researcher at Lamont-Doherty and University of Bristol, is studying uplift on Cape Verde. Cape Verde rests on a slow-moving tectonic plate which may explain why it is rising while the Hawaiian islands, situated on the faster-moving Pacific plate, are sinking.

Cape Verde’s youngest and most active volcano, Fogo, has erupted every 20 years or so since the Portuguese settled here in the 1500s. Recent eruptions in 1995 and 1951 have dramatically built up the landscape.

In late 2014, Fogo roared to life, nearly two decades after its last eruption. (Ramalho)

Within two weeks, lava flows had bulldozed two villages high in the caldera leaving 1,200 people homeless, including a volcano observatory worker (pictured) who often hosted geologists in his home. (Ramalho)

Over the course of hundreds, even thousands, of eruptions, volcanic cones swell with fresh lava. As they grow top-heavy, the chances of a spectacular collapse increase.

Geologists think that Fogo’s eastern slope crashed into the sea in this way some 65,000 to 124,000 years ago, leaving a giant scar where a new volcano can be seen growing in this satellite image. (NASA)

The flank collapse on Fogo may have generated a wave bigger than any recorded by humans, and possibly bigger than prehistoric megatsunamis off the Hawaiian and Canary islands. (Katsushika Hokusai)

Ramalho and his colleague, Jose Madeira (left), a geologist at Lisbon University, independently stumbled on evidence for such a wave while doing fieldwork in Santiago.

On a clear day, from these cliffs in northern Santiago, it is possible to see a silhouette of Fogo, nearly 40 miles away. Ramalho and Madeira think that Fogo’s partial collapse triggered a giant wave that swept over this ridge.

The wave may have hurled dozens of boulders like this one a half-mile or more inland.

The boulders appear utterly alien in a landscape of dry grasslands. Here, three goats take cover from the midday sun in the shadow of what appears to be a tsunami deposit.

The wave did more than sweep massive boulders into remote fields. It also scooped up sand, mollusks and rocks and hurled them against cliffs. Here, on a slope above the shoreline, Ramalho inspects a mishmash of debris, or conglomerate, overlaying Santiago’s native red soil.

Key to testing their hypothesis is figuring out when the big wave, exactly, broke up shoreline cliffs into boulders carried inland. Ramalho and his colleagues have been chiseling away samples to find out. (Ramalho)

Here, Katherine Adena hammers away a chunk of basalt. At Lamont-Doherty, the rocks are analyzed for their helium-isotope content, indicating when it broke away and became exposed to cosmic radiation from space. (Ramalho)

The geologists need to read the Cape Verde landscape to understand its history. Here a village on a young volcanic plateau is framed by much older, highly eroded volcanic plateaus in the distance.

Volcanic eruptions have paced migrations across the Cape Verde islands, sometimes at great cost. The population on Fogo has been growing thanks to rich soils that support vineyards and wine-making, and an uptick in tourism, that has fueled a demand for lodging and restaurants. The November eruption wiped out the grapevines pictured here and a new national park welcome center nearby.

Cape Verde’s volcanoes provide in other ways. Here, volcanic sand collected along streambeds is turned into concrete for building houses.

The local roads, made of volcanic rock cut and laid by hand, are beautiful works of art. Here, a rare volcanic rock akin to limestone, divides the road into two lanes. (Ramalho)

At the same latitude as the Sahara Desert, Cape Verde is water-limited, with villagers walking long distances to fill jugs for cooking and drinking.

Despite the rigors of island life, Cape Verdeans are known for their exuberant culture, which includes Portuguese-African music rooted in Cape Verde’s historic tie to the trans-Atlantic slave trade. Here, a dancer leads a group of women pounding on cloth drums in call and response, in a form of traditional music and dance called batuku.

The Cape Verde landscape continues to evolve. Fogo’s 1995 eruption left this mountain of pahoehoe lava resembling toothpaste squeezed from the tube. In November, Fogo dished out a new layer of lava that reworked this scene into something totally new.