Encyclopedia of Islands

image image image image image image

The Geology of Canary Islands

The Canary Islands, located between 100 and 500 km from the coast of northwestern Africa (Morocco), consist of seven major volcanic islands forming a rough west-southwest to east-northeast trending archipelago. Together with the Selvagen Islands and a group of seven major seamount complexes (some of which were former Canary Islands) to the northeast, they form the Canary volcanic province. Volcanism in this ∼800-km-long and ∼400-km-wide volcanic belt (located at 33–27° N and 18–12° W) decreases in age from the northeast (Lars Seamount, 68 million years) to the southwest (Hierro Island, 1 million years) and is interpreted to represent the Canary hotspot track (Fig. 1). The Canary volcanic province is located on Jurassic ocean crust (∼150 million years old beneath the western part of the province to ∼180 million years old beneath the eastern part of the province), and contains some of the oldest ocean crust preserved in ocean basins.

GEOLOGICAL OVERVIEW OF THE EVOLUTION OF THE ISLANDS
The morphology of the Canary volcanic province show systematic changes from southwest to northeast, refl ecting an increase in age (Figs. 1) and a change in evolutionary stage. As the volcanoes age, they originally go through a constructive phase of evolution in which growth of the edifi ce through volcanic activity outpaces its destruction through mass wasting (e.g., landsliding) and erosion.

Bathymetric map showing the Canary (red) and Madeira  (blue) volcanic provinces

FIGURE 1. Bathymetric map showing the Canary (red) and Madeira (blue) volcanic provinces, including islands and associated seamounts, in the eastern central North Atlantic. Thick dashed lines mark centers of possible hotspot tracks. For clarity, only depth contours above 3500 m are shown. Bathymetric data from Smith and Sandwell (1997); ages and location of the Azores–Gibraltar fracture zone from Geldmacher et al. (2005) and Guillou et al. (1996).

The constructive phase occurs primarily during the shield-building (or shield) cycle of activity, during which eruptive rates are high, and most of the volcanic edifi ce is formed. Even though mass wasting is an important process during the shield stage, the volcano continues to increase in size, despite short-term setbacks. The
constructive phase of island/volcano evolution can extend into the fi rst late (also commonly referred to as post-erosional or rejuvenated) cycle of volcanism, during which volcanic eruptive rates are drastically lower, but magmas
can be more evolved (silicious and thus more viscuous), contributing to an increase in volcano height. Late cycles of volcanism are generally separated from the shield stage of volcanism by extended periods of volcanic inactivity or
drastically reduced activity. During the destructive phase of evolution, mass wasting and erosion outpace volcanic growth, and the volcanoes (islands) decrease in size until they are eroded to sea level. As the plate moves away from the magma source, it cools and subsides, and the now fl at-topped volcanic edifi ces sink beneath sea level, forming guyots. Despite the differences in age of the volcanoes, all of the islands have had Holocene acitivity except La Gomera and Fuerteventura. The islands of La Palma, Tenerife, and Lanzarote have had historical volcanic activity, and thus youthful volcanic structures can be found across the entire archipelago, even on the oldest islands.

The two youngest and westernmost islands of Hierro (1500 m above sea level, with oldest dated subaerially erupted rocks at 1.2 million years) and La Palma (2426 m; 1.8 million years old) have been the most active within the Holocene. Both volcanic islands are characterized by mafi c alkaline volcanism, high eruptive rates, and volcanism along magmatic rift zones, commonly associated with the shield cycle of volcanism on ocean island volcanoes. Both volcanoes (and associated islands) are expected to continue to grow in size in the future.

The three central islands, intermediate in age, were in their shield stage in the Middle to Late Miocene and have had low levels of late or rejuvenated volcanism in the Pliocene and/or Quaternary. Tenerife in the central western part of the archipelago forms the largest island and is also the third largest and highest (more than 7000 m in elevation above the sea fl oor and 3718 m in elevation above sea level; 11.9 million years old) volcanic structure on Earth after the Hawaiian volcanoes of Mauna Loa and Mauna Kea. The highest peak of Tenerife is formed by the highly differentiated (phonolitic) Teide volcano, nested in a lateral collapse caldera (formed through mass wasting), indicating that this volcano is at the transition from its constructive to destructive phase. Considering the significantly lower eruption rates of the Canaries as compared to the Hawaiian volcanoes, the similarity in size refl ects Tenerife’s older age (longer life-span), most likely related
to the almost-order-of-magnitude slower motion of the plate beneath the Canary Islands (∼12 mm/year) as compared to the motion of the plate beneath the Hawaiian Islands. Therefore, it has taken much longer for Tenerife to move away from its magmatic source than has been the case with the Hawaiian volcanoes. Although crudely round in outline, the central volcanoes of La Gomera (1487 m; 9.4 million years old) and Gran Canaria (1950 m; 14.5 million years old) no longer have conical shapes and are characterized by deeply incised canyons, indicating that these two volcanoes are well into their destructive phase of evolution, with erosion and mass wasting outpacing growth through magmatic activity. Erosion has
exposed intrusive complexes and dike swarms on both islands. The compositions of volcanic rocks are highly variable, ranging from mafi c (transitional tholeiite to melilite nephelinite) to highly evolved (peralkaline rhyolite to trachyte to phonolite), refl ecting the mature nature of these volcanoes.

On the two easternmost, oldest, and lowest islands of Fuerteventura (807 m; 20.2 million years old) and Lanzarote (670 m; 15.6 million years old), erosion is clearly the main process shaping the morphology of the islands, even though both islands have had rejuvenated volcanism within the last 150,000 years, and Lanzarote even within historical times. Both islands are realtively fl at, with little of the original shield volcano morphology being preserved. Instead, they are characterized by isolated older volcanic sequences or erosional remnants and broad valleys. Surprisingly, the largest historical eruption in the Canary Islands and the second largest basaltic fi ssure eruption ever recorded (after the 1783 Laki eruption on Iceland) was the 1730–1736 Timanfaya eruption on Lanzarote, which produced ∼1 km3 of primarily tholeiitic basalts.