After a bus ride to Santa Cruz, a ferry ride to and another bus ride, we arrived in de Gran Canaria. Gran Canaria is second to Tenerife in terms of population, most of the people reside in Las Plama, in the northeast corner of the relatively round island. Similar to Tenerife, Gran Canaria experienced several episodes of volcanism.

An initial alkali basaltic shield volcanoe (“Old Cycle”) developed very quickly between 14.5 and 14.1 Ma. This was followed by extrusion of large quantities of silicic lava that produced ignimbrites during and following a collapse that produced the Caldera de between 14.1 and 13 Ma. This structure is about 20 kms in diameter but only sections of the caldera margin are exposed today. After this, around 13 Ma, there was resurgence of volcanism and a stratovolcano develped in the caldera. Lava flows and ignimbrites emitted from 12.6 to 9 Ma consisted of trachytes and phonolites (silica undersaturated). A related syenite stock and cone sheet dikes formed below the Caldera between 12.3 and 7.3 Ma, resulting in as much as 1,400 meters of uplift in the central zone.

A later phase (“ cycle”) took place from 5.6 to 2.9 Ma, after a 3 million year quiescence, and produced less material. The phase started with alkali basaltic to phonolite eruptions (NW-SE fissure), possibly strombolian-type, that created a stratovolcano in the central island. The phase ended with more violent eruptions that generated pyroclastic flows and large quantities of unwelded ignimbrite, the Roque Nublo breccias. Some phonolites were also extruded during this phase, inlcuding Risco Blanco, and phonolitic domes were emplaced.

The next phase overlapped the end of the Roque Nuble cycle and is sometimes referred to as the Post Roque Nublo stage. The volcanic activity was restricted to the northeastern part of the island and consisted of early nephelenite rift eruptions (3.1 to 1.5 Ma) followed by low-viscosity dispersed nephelinitic and basanititic lavas (1.5 to 0.3 Ma). This phase ended with the recent small strombolian cones, basanitic and tephritic lava flows and numerous phreatomagmagic craters. The most recent eruptions on Cran Canaria involved Caldera de Pinos de Galdar (2,830 ybp), Montañon Negro (2,970 ybp), and Caldera de Bandama (1,970 ybp). A study by Rodríguez-González and others (2009) identified 24 eruptions over the past 11,000 years on Gran Canaria; these were primarily concentrated in the northern part of the island and produced small strombolian cones and occasional phreatomagmatic craters (ash eruptions). This study concluded that the greatest volcanic hazard is in the densely-populated north. The study also concluded that the interval between eruptions is decreasing Volcanic hazard map of Gran Canaria (Rodríguez-González and the eruptions are becoming more et al., 2009) violent and voluminous.

Nephelinite is a very rare igneous rock. It is a dark, extrusive lava made up almost completely by nepheline and clinopyroxene (augite) although olivine may be present. It is similar to basalt except nepheline (feldspathoid) is dominant instead of plagioclase. The proportion of nepheline/nephaline + plagioclase is over 0.9 in nephelinite indicating a very silica-undersaturated magma. Nephelinite is generated by (1) relatively high pressure partial melt, (2) low degree of partial melt in a mantle source (peridotie), and (3) high dissolved carbon dioxide in the melt (often associated with carbonatite). This rock type is usually found associated with continental rifts but occasionally it is found in ocean islands.

Saturday, March 22nd Las Palmas de Gran Caneria is located in the northeastern corner of the island and we are staying a few blocks south of Playa de las Canteras (Beach of the Quary). This low, flat region was probably a spit that connected the volcanic highlands of Gran Canaria with La Isleta (The Small Island). The reef off of the Playa de las Canteras creates a wave break that shelters the gold-sand beach. The reef is interpreted as a previous beach strand that was cemented by meteoric waters that percolated through the sands. A subsequent increase in relative sea level submerged the old beach, and allowed the reef to develop. There is a public, multi-use pathway all along the waterfront, both along Playa de las Canteras and the seawall to the south. We walked along the seawall to the old city center. Sunday, March 23rd Today we took the bus to to see the unique dune field. The Maspalomas dune field covers an area of approximately 400 hectars and has been designated a Special Natural Reserve. The dune field consists of several sets of barchan and transverse dues with a maximum height of 18 meters that formed no more than 200 years ago (Sánchez- Pérez, 2010). The dunes consist of medium to fine-grained sand that are approximately 50% biogenic carbonates and 50% lithic fragments from terrigenous sources. We walked from El Inglés beach to Maspalomas beach (about 4 kms), walking into the dune field a few times. There are some cobbles exposed at the beach and in a few places in the dune field where deflation has occurred. The pebbles are flattened dark volcanic cobbles that are part of the dune field substrate. The dunes developed on coarse alluvial deposits originating from the gully. During storm events the beach sands on El Inglés beach are often eroded, leaving a cobble beach. There is vegetation on the dune field margin but the main dune field is generally unvegetated, indicating an active and mobile environment. When we were there, there was little wind and no sand movement.

Barchan and transverse dunes are frequently found together. The barchans structure is crescent-shaped with the “horns” oriented downwind. They tend to have a lower slope facing the wind and a Maspalomas dune field in relation to steep slip-surface down wind. Sand slides down the downwind El Inglés beach (N-S tending) and face, producing a slope at the angle of repose (30-35°) for Maspalomas beach (E-W trending) medium-fine dry sand. The barchans upwind slope is about half of (Fontán et al., 2012) that. These dunes migrate with time as sand is eroded from the upwind side and deposited on the downwind side; the migration rate is on the order of meters to tens of meters per year. Transverse dune do not have the distinctive crescent-shape, they tend to be more linear and perpendicular to the prevailing wind direction. Their cross-sections are very similar to the barchans.

A study by Hernández and others (2007) suggest that the dunes are not being replenished and there is a significant reduction in the thickness of the aeolian deposits as well as more area of deflation. Sand from El Inglés beach, to the north, replenishes the Maspalomas dune field (the shoreline adjacent to the dune field is dominated by longshore currents). Subsequent work by Fontán and others (2012) agree that the aeolian transport in the dominated by the trade winds blowing from the NNE between April and September and by multidirectional winds during the rest of the year but the local topography influences the Maspalomas winds, they have a primary direction from the ENE and a minor direction from WSW.

Tuesday, March 25th Today we took a bus to Gandar, west of Las Palmas. Gandar is situated on the flanks of the very symmetrical El Pico that can be seen from Santa Catalina. The Church is constructed of a light-colored tuff that is quarried locally. The Guanches (indigenous people) had a large settlement in this location and there is an excellent museum at the excavation site. The Guanches appear to have originally migrated from northern Africa around 6000 BCE; there are similarities between the Guanches language and Berber languages of North Africa and genetic evidence supports this connection as well. What is interesting is that the Guanches must have arrived by sea but they seem to have forgotten how to sail after arriving. They brought with them goats, sheep, pigs, dogs, wheat and barley. Once colonized they didn’t seem to interact (languages and dialects were different).

The Guanches homes were constructed of round or shaped (rectangular) rocks that were arranged in a circle with one or two sleeping spaces on the sides. The roof was constructed of wood slats holding flat rock and mud (adobe). Cooking was done out-of-doors. The excavation picture shows the lower parts of the interior walls of a house with two sleeping areas. The space between the round exterior wall and interior wall was filled with rock and earth. The Gandar site has over a dozen home excavations and caves cut into the tuff that served as food storage and held a Guanche mummy. The museum contained many of the artifacts that have been discovered, including pottery and idols.

Although there is evidence that Romans visited the Canary Islands, they did not colonize. The Castilians certainly did colonize the islands beginning in 1402. Although the Guanches resisted, the islands were conquered, the last one, Tenerife, in 1496. When the Castilians arrived they found the Guanches leading a stone-age existence of shepherding, gathering and limited agriculture. The volcano in the foreground is El Pico (Gandar is located at the southwest flank) and Teide, on the island of Tenerife is the volcanoe in the clouds to the right.

Wednesday, March 26th Today we took the bus to the center of Gran Canaria. The first bus took us to San Mateo and the second bus took us to Tejeda. The road (GC-15) winds up through the rugged volcanic topography where farmers have constructed steep terraced fields to take advantage of the rich volcanic soil. San Mateo is located just north of the NW-SE main rift area and Tejeda is located to the south, within the gravitational slide of the Roque Nublo Volcano (the slide was to the south). The Roque Nublo phase (5.5 – 2.9 Ma) produced lava flow and ignimbrites that flowed away from the central island. This phase is characterized by phreatomagmatic eruptions. These are eruptions generated by the interaction between water and magma and differ from magmatic and phreatic eruptions. Phreatic eruptions do not contain juvenile clasts but phreatomagmatic eruptions do. The water associated with phreatomagmatic eruptions can be sea water (Surtsey) or lake water (Santorini) but in this case it may have been ground water. These explosive eruptions produced lithic- rich, relatively low-temperature pyroclastic flows with a phonolitic composition; in some cases the ignimbrites were cool enough that they produced non-welded deposits (block or ash flow). The ignimbrites consist of pumice, ash and rock fragments (Roque Nublo breccia). Some of these deposits were up to 60 meters thick and extended 25 kms down the flanks of the central stratovolcano. At its peak, the volcano is estimated to have been between 2,500 and 3,000 meters high with steep, symmetrical flanks. A catastrophic failure resulted in the collapse of the southern flank and the formation of the Crator de Tejeda. This collapse generated a tremendous debris avalanche. It isn’t clear how the Crator de Tejeda and the previous Caldera de Tejede are related.

Roque Nublo (Rock in the Clouds) is a stunning feature that is about 80 meters tall and one of the most famous landmarks on Gran Canaria. It is located on the margin of the Crator de Tejeda, a remnant of the central stratovolcano that once stood here. Roque Nublo is a dislodged block of breccia that is composed of tephra and phonolitic blocks.

The Canary Islands are an archipelago of exceptions. The islands are a hot spot track except volcanism has not always progressed to the west; the phonolites and nephelinites lavas are exceptionally rare, particularly for an ocean island; the indigenous Guanches sailed to the islands but stopped sailing; the islands are off Africa except they are distinctly European; the Canaries are volcanically active but the greatest geologic hazard is landslide.

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