Galapagos on Tectonic Plates

Maps of the seafloor situate the Galápagos on the Nazca tectonic plate. Spreading away from the East Pacific Rise, which is the centre of tectonic expansion, this plate shifts southeast towards South America at a speed of around 60 mm (2.4 in) per year, twice as fast as human nails grow. It is predicted that in approximately one million years, the islands will be carried about 60 km (32 Nm) southeast.

Española, the oldest and southernmost of the Galápagos Islands, is probably a remnant of a massive volcano. The lava on this island dates back 3.0–3.4 million years. This timespan gave Española ample opportunity to travel away from the hotspot in the northwest corner of the archipelago, but also subjected the island to another major force – erosion. Nowadays it stands as an almost flat rock with a maximum height of about 200 m (660 ft), and is perhaps only a shadow of its former volcanic glory. Its walls, eroded by winds and waves along its south coast, serve as a magnificent nesting site, irresistible to millions of seafaring birds. Over time the conveyor belt of the Nazca plate will push Española further away from the Galápagos hotspot, and the island, battered by the forces of the sea and weather, will sink beneath the surface of the ocean.

The Galápagos hotspot gives birth to islands that are travelling, sinking and eventually disappearing down into the depths of the Peru–Chile Trench. Here the Nazca plate slides beneath the South American continental plate, lifting the Andes to their highest peak of Aconcagua, almost 7,000 m (22,900 ft) above sea level. A chain of underwater seamounts east of the Galápagos, called the Carnegie Ridge, offers direct evidence of this protracted process.

Unlike the Hawaiian hotspot, which generates a breadcrumb-like island chain, the Galápagos hotspot is much more complicated, apparently influenced by the triple junction of the Nazca, Cocos and Pacific Plates to the west, rather than riding a large single plate. There is evidence of four major magma reservoirs involved in the Galápagos hotspot, covering a much larger area than the one that produces the Hawaiian chain. Added to that, the Galápagos Spreading Centre – the giant tear separating the Nazca and Cocos plates – changed its position over time due to the complicated fractured interaction between these two plates. It appears that between 12–20 million years ago this spreading centre was located just above the hotspot. Later, between 7.5–12 million years ago, the crust above the hotspot was welded to the Cocos plate. During that time, the hotspot spilled an ancient chain of underwater mountains (seamounts) called the Cocos Ridge. Later the seabed rifts changed again, and the hotspot began to feed the volcanoes forming the Galápagos archipelago and the Carnegie Ridge.

It is uncertain how old the Galápagos mantle plume is, because the Carnegie and Cocos Ridges disappear beneath South and Central America respectively, so their complete age record has long been obliterated. However, an eight-million-year-old seamount on the Carnegie Ridge was located by an oceanographic expedition in 1990 at a depth of 1,500 m (5,000 ft). Rounded cobbles rest on its flat top, providing evidence of wave erosion, hence this seamount had to be an island in the past. The mantle plume feeding the hotspot is certainly even older. Many scientists believe that the Galápagos mantle plume is responsible for the abundant basaltic rocks of the Cretaceous age (65.5–145.5 million years ago) that occur in the Caribbean and on the northwest margin of South America. Thus, this plume could be as old as 90 million years, and the Galápagos hotspot may even have played an important role in the creation of the land bridge connecting the Americas, isolating the marine life of the Atlantic and Pacific Oceans.