A Look at the geology of the Galapagos Islands

There are a few things that you might want to look up as you peruse this page: hot spot, tuff, lava, uplift, and plate tectonics come quickly to mind. However, once you start on these topics and begin to see the earth’s surface as a living, moving, dynamic skin you may not find time to return to this page – so hold off a minute.

The Galapagos Islands are located above a hot spot; a place in the earth’ surface where (for some reason not well understood) molten material streams upward and oozes through the harder drier surface of the earth’s crust. This may be easy to understand as we are all familiar with volcanoes; those outward expressions of the inner-earth’s energy. But the plot thickens when you factor in the continually (slowly but continually) moving plates that make up the surface of the earth the way scutes cover the shell of a turtle (or tortoise). the plate moves, the hot magma flows upward, the resulting volcanoes, over time, get strung out like beads on a necklace.

The Hawaiian Islands are also hotspot islands. In the case of the Hawaiian Islands the tectonic plate that they are riding on is moving to the northwest making the easterly islands the youngest and the low islands in the west the oldest. As you will see the plate the Galapagos are riding on is moving to the southeast making the easterly islands the oldest. Read on.

Flying in to the Galapagos from the South American mainland the first islands appear low and flat. Being about 600 miles off the coast of Ecuador you might think that the only way they got there was through volcanism. It is unlikely that they would be built from glacial rubble or a broken-off edge of a continental land mass. They are volcanic. Why are they low and flat if they are volcanic? Lava is forced upward from the inner earth in many ways. It moves upward through a single channel spurting  lave from the top end creating a typical cone-shaped volcanic hill or mountain. But, it can also ooze up through an array of cracks, eventually seeping out over the ground in a low flat flow. Many of the lava seeps never reach the surface but are stopped for one reason or another deep in the ground; these are plutonic plumes that are occasionally exposed by subsequent erosion. (The koppjes in the Serengeti are such granitic plumes.)

So, why are the eastward islands low and flat? They are old and wearing down. The signs of their volcanic origins have eroded away leaving a slowly decayed landmass that sooner or later will be claimed by the sea and disappear below the ocean’s surface.

The volcanic origin of the Galapagos can still be seen as you move westward. The tectonic plate is sliding to the east over a hot spot that remains stationary. Thus the newer, more volcanic looking, islands are in the west near the hot spot. The large island (Isabela) is pocked with five large volcanos and the westernmost island (Fernandina) consists almost entirely of the volcano called La Cumbre.
Not all volcanos exude lava, some blow molten spatter into the air and others give of huge bursts of ash. This view of Bartolome shows a remnant bit of a tuff cone. Once there was the footprint of volcanic activity shown by a ring made up of spatter and ash. This cone fell apart leaving only this iconic spire behind. It is hard and holds its shape but it isn’t a lava rock.
The Daphnes are best known as the finch research site of Rosemary and Peter Grant who have studied bird populations on Daphne for decades. However, keeping with the geologic tone of this page, it is easy to see that Daphne Major is a large hollowed out volcanic remnant. The collapse into itself that creates the vast interior can happen in a few ways; inner collapse back into the lava tube that once fed the volcanic buildup and an explosive finish to volcanic activity are two of the most likely ways. Daphne Major is a very nice looking caldera. (There are about 18 Galapagos Shearwaters in the foreground water. This is a very common bird of the islands but they are rarely seen away from the archipelago.)
The images above and below show layers of ash. A volcano can exude lava which flows downhill until it hardens. Those sorts of volcanoes are rather clean overall. On other occasions volcanoes blow tiny shards of rock into the area which descends according to its weight; larger chunks fall near the source and lighter smaller particles can drift on the jet stream. We call this stuff ash but in reality it is mineral and often a gritty sharp mineral. It is not ash you can blow off your car it is more akin to a carbide dust that will easily remove the finish from your car if you try to wipe it off.
Ash varies in color depending on its mineral makeup. Lava is usually a dark gray or black and ash is usually gray but can be a tawny brown as well.
Much the way ash colors vary the size and shape of volcanic features vary as well. Not all are explosive. Not all give off ash. Not all originate in the same place in the hot spot. Not all bring the same minerals (gasses and water vapor) to the surface. Thus there are many many different forms and shapes to be seen in a hot spot array. The bottom line is that they originate from the depths of the earth and the keep their heads above water for a while and then they slowly decay until they surface is below sea level.

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