{"document_url": "https://en.wikipedia.org/wiki/Gal%C3%A1pagos_hotspot"}
Pacific volcanic hotspot
The Galápagos hotspot is a volcanic hotspot in the East Pacific Ocean responsible for the creation of the Galápagos Islands as well as three major aseismic ridge systems, Carnegie, Cocos and Malpelo which are on two tectonic plates. The hotspot is located near the Equator on the Nazca Plate not far from the divergent plate boundary with the Cocos Plate. The tectonic setting of the hotspot is complicated by the Galapagos Triple Junction of the Nazca and Cocos plates with the Pacific Plate. The movement of the plates over the hotspot is determined not solely by the spreading along the ridge but also by the relative motion between the Pacific Plate and the Cocos and Nazca Plates.

The hotspot is believed to be over 20 million years old and in that time there has been interaction between the hotspot, both of these plates, and the divergent plate boundary, at the Galapagos Spreading Centre. Lavas from the hotspot do not exhibit the homogeneous nature of many hotspots; instead there is evidence of four major reservoirs feeding the hotspot. These mix to varying degrees at different locations on the archipelago and also within the Galapagos Spreading Centre.

Hotspot theory

In 1963, Canadian geophysicist J. Tuzo Wilson proposed the "hotspot" theory to explain why although most earthquake and volcanic activity occurs at plate boundaries, some occurs far from plate boundaries. The theory claimed that small, long-lasting, exceptionally "hot" areas of magma are located under certain points on Earth. These places, dubbed "hotspots", provide localized heat and energy systems (thermal plumes) that sustain long-lasting volcanic activity on the surface. This volcanism builds up seamounts that eventually rise above the ocean current, forming volcanic islands. As the islands slowly moved away from the hotspot, by the motion of sliding plates as described by the theory of plate tectonics, the magma supply is cut, and the volcano goes dormant. Meanwhile, the process repeats all over again, this time forming a new island, on and on until the hotspot collapses. The theory was developed to explain the Hawaiian-Emperor seamount chain, where historic islands could be traced to the northwest in the direction that the Pacific Plate is moving. The early theory put these fixed sources of heat for the plumes deep within the Earth; however, recent research has led scientists to believe that hotspots are actually dynamic, and able to move on their own accord.

Tectonic setting

The Galapagos hotspot has a very complicated tectonic setting. It is located very close to the spreading ridge between the Cocos and Nazca plates; the hotspot interacts with both plates and the spreading ridge over the last twenty million years as the relative location of the hotspot in relation to the plates has varied. Based on similar seismic velocity gradients of the lavas of the Carnegie, Cocos and Malpelos Ridges there is evidence that the hotspot activity has been the result of a single long mantle melt rather than multiple periods of activity and dormancy.

In Hawaii the evidence suggests that each volcano has a distinct period of activity as the hotspot moves under that portion of the Pacific plate before becoming dormant and then extinct and eroding under the ocean. This does not appear to be the case in the Galapagos, instead there is evidence of concurrent volcanism over a wide area. Nearly all Galapagos Islands show volcanism in the recent geological past, not just at the current location of the hotspot at Fernandina. The list below gives the last eruption dates for the Galapagos volcanoes, ordered from West to East.

The movement of the Nazca and Cocos plates have been tracked. The Nazca plate moves at 90 degrees at a rate of 58±2 km per million years. The Cocos Plate moves at 41 degrees at a rate of 83±3 km per million years. The location of the hotspot over time is recorded in the oceanic plate as the Carnegie and Cocos Ridges.

The Carnegie Ridge is on the Nazca plate is 600 km (373 mi) long and up to 300 km (186 mi) wide. It is orientated parallel to the plate movement, and its eastern end is approximately 20 million years old. There is a prominent saddle in the ridge at 86 degrees West where the height drops much closer to the surrounding ocean floor. The Malpelo Ridge, which is 300 km (186 mi) long was once believed to be part of the Carnegie Ridge.

The Cocos Ridge is a 1000 km long feature located on the Cocos plate and is orientated parallel to the plates motion from the 91 degree west transform fault at the Galapagos Spreading Centre towards the Panamanian coast. The north eastern end of the ridge dates from about 13–14.5 million years ago. However, Cocos Island at the northern end of the ridge is only 2 million years old, and was therefore created at a time well after the ridge had moved away from the hotspot. The presence of a pronounced sedimentary hiatus in sediments on the Cocos Ridge indicates that the Cocos Ridge was probably buckled upon its initial shallow subduction along the Middle American Trench.

The current model for the interaction of the hotspot and the spreading centre between the Cocos and Nazca plates attempts to explain the ridges on both plates; the split between the Carnegie and Malpelo Ridge and subsequent volcanic activity away from the hotspot. There have been eight major phases in the last 20 million years.

Lava chemistry

Analysis of the radioactive isotopes of the lavas on the islands of the Galapagos archipelago and on the Carnegie Ridge shows that there are four major reservoirs of magma that mix in varying combinations to form the volcanic province.

The four types are: