Mantle Plumes: A Visit to the Galapagos Hot Spot

It’s been weeks without a geology post. How did that happen? Some background. WC, semi-immobilized by a bad knee and huddled by his air-conditioning vent to escape the current heat wave, has been sorting through slides from the days before digital photography. A recent batch of scanned slides from WC’s 2002 trip to the Galápagos Islands demonstrates that WC’s interest in geology is nothing new.

Geologic map of the Galápagos Islands; note the extensive uplifted area that serves as a platform for the islands themselves.

The Galápagos Islands, 600 miles or so off the coast of Ecuador, are the product of a mantle plume, like the more familiar Hawaiian Islands. The Galapagos sit over a “hot spot,” a place where a plume of heat and molten material from the mantle pushes through the asthenosphere and crust to produce voluminous eruptions on the surface of the earth. Over geologic time, where the mantle plume surfaces in the ocean, the eruptions create islands: the Hawaiian Islands and the Galapagos Islands, to name just two. Because the crust itself is moving as plate tectonics shuffles the surface of the planet around, hot spot islands tend to go from older to younger in the direction the plate is traveling.1

For the Galápagos Islands, which sit on the Nazca Plate, the plate is moving eastward as the plume remains in place. The older islands of the Galapagos are eastwards; the new are westwards, and the active volcanos, where the mantle plume currently pushes to the surface, are in the very westernmost islands. Isabella Island, composed of six different volcanoes laid out in a sea horse shape roughly north-south, is one of the westernmost islands.

From the rim of Vol. Sierra Negra Caldera, looking west. The west caldera rim is the dark line, about six miles away; the lower slopes the paler gray and the white the Pacific Ocean. The gray areas left of the pressure ridge appeared to be low-lying vog. (scanned from slide)

The second southernmost volcano on Isabella Island is Vol. Sierra Negra (also called Vol. Santo Tomas).2 WC’s visit to the Galapagos in 2002 included an excursion to the 3,612 foot high summit of Volcan Sierra Negra.3 The volcano is a shield volcano, with mostly gentle slopes (five degrees or less), with a large caldera at the summit that’s about six miles in diameter. We were on the top of the easterly rim of the caldera, site of an eruption in 1979. The terrain offered pretty much every volcanic feature a volcanologist could want: fumaroles, cinder cones, vents, pressure ridges, pahoehoe and aa lavas, kipukas, lava tunnels, cinders, ash flows, sulfur deposits and more. The caldera itself is perhaps 100 meters deep, and as rugged and barren as the moon.

Volcan Chico, the vent for the 1979 eruption, and in 2002 the newest lava flow on Sierra Negra (scanned from slide)

Volcan Sierra Negra has erupted dozens of times in recorded history, and twice since WC was there in 2002, most recently in 2018. The Galapagos Plume seems to be less intense than the Hawaiian Plume, moving less lava to the surface. Even though the Nazca Plate is moving more slowly than the Pacific Plate under the Hawaiian Islands, the volcanic edifices are smaller. But still, very impressive.

Large fumarole on Volcan Chico, the 1979 eruption site on Vol. Sierra Negra. Note the chemical alteration of the sides of the fumarole by the sulfuric acid-laden steam in just three years (scanned from slide)

While the underlying mechanism creating the Galápagos Islands and their volcanoes may be the same as the Hawaiian Islands, the lava is strikingly different. WC couldn’t find any on-line literature explaining why.

Multiple smaller fumaroles near the summit of Volcan Chico. The yellow-green on the wall of the upper left center fumarole is elemental sulfur. In fumarole at the bottom of the photo, note the focus distortion by the hot, steamy air blasting out of the fumarole.

Remember this is just one of six active volcanoes on Isabella Island, with another volcano on Fernadina Island, immediately to the west. Volcano Sierra Negra is about 530,000 years old, a geologic neonate, as it were. And while the volcanoes created by the Galapagos Plume are very impressive, consider that the Big Island in Hawaii is about the same age, and 13,700 feet above sea level at its summit, four times higher than Vol. Sierra Negra.5 All mantle plumes, you can see, are not the same.

It’s been 20 years since WC was at the summit of Vol. Sierra Negra, but the memory is still fresh. Seeing these slides is yet another reason why WC would really like to get back there some day.

1 One of the theories of why the plates move is wrapped around plumes. The idea is that the upward pressure of the plume, spreading against the underside of the crust, drags the plates along. The Galapagos Plume is a thumb in the eye of that theory. About another 600 miles west of the Galápagos Islands is the Galapagos Triple Junction, where the Pacific, Cocos and Nazca Plates all meet, and the three ridges, the spreading centers pushing those three plates apart, are active. So far, describing a geometry that explains the Galapagos Triple Junction in relation to the Galapagos Plume has proven impossible for even the most fevered geologist’s imagination.

2 A curious feature of the Galápagos Islands is that the islands themselves and many of their features each has two or three names, given by the various Western explorers who saw them. It seems to be a point of pride for Galapagos guides to know all of the names, rather than settling on just one.

3 The excursion itself was remarkable, involving ramshackle trucks, sadly mis-treated horses, the most uncomfortable “saddles” you can imagine and blast furnace hot temperatures. Perhaps a future blog post.

4 Another naming complication: Spanish doesn’t seem to differentiate between volcanoes and vents on volcanoes. Volcano Chico is a vent on Volcano Sierra Negra; it’s a part of the larger edifice. Not a separate volcano.

5 Remember when we are talking about volcanic mountains, the volume of the volcano goes up as the cube of its height. The portion of Mauna Loa above sea level on the Big Island is sixty-four times bigger than Vol. Sierra Negra.