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Lanzarote). Thick sand deposits cover extensive areas of Fuerteventura (Fig. 8.22). The sand is formed by a complex aeolian system supplied by Pliocene and Pleistocene marine deposits, partially covered by carbonate crusts (calcrete). Sands are formed mainly by a Mio-Pliocene biogenic component comprising skeletal calcareous algae, shell fragments, and foraminifera that thrived on Fuerteventura during a period of equatorial climate (ie, lacking annual seasonal cycles) that extended from approximately 9–4 Ma (Meco et al., 2007). The closing of the Panama isthmus at approximately 4.6 Ma then initiated the cold Canary current that brought a drastic change in climate to the Canaries. Climate became colder, arid, and with marked annual seasonal cycles (see also Chapter: The Geology of Lanzarote, Fig. 7.9). This led to “mass extinction” of the marine equatorial fauna in the Canaries and provides the source of the jables and, hence, the spectacular beaches of Fuerteventura.
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Figure 8.22. Distribution of aeolian sand deposits (jables) on Fuerteventura. Notably, the eastern (leeward) coast largely lacks this type of deposit.
A related feature is the calcrete, or caliche, a hard carbonate crust that partially covers the aeolian sands and is several meters thick in some places, reaching elevations of up to 400 meters above sea level (masl). The calcrete was initially interpreted as a result of dissolution of plagioclase from basalts (Hausen, 1967). This, however, causes difficulties to explain encrusted sand dunes, for example. A more consistent explanation is the encrusting of the sands and sand dunes by repeated dissolution and precipitation (evaporation) events of the biogenic calcium carbonate particles, gradually forming a hard crust that protects the underlying sand from wind erosion (Meco et al., 2007).
This type of “calcrete” mantled large parts of the island of Fuerteventura, although in some areas only vestiges are preserved, implying that during the late Miocene and part of the Pliocene the island was probably completely covered with white sand, likely with only the highest peaks of the Miocene basaltic shields forming black “islets” or “nunataks” in a sea of sand.
A leading industry of the island since the 17th century was therefore obtaining lime (CaO) from the abundant calcareous crusts. Several lime kilns were in operation until the mid-20th century, and one of these will be inspected on Day 4 (Stop 4.3).
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Water at the Surface of the Earth
In International Geophysics, 1977
Geographic Distribution
These data on potential evapotranspiration derived from a few series of lysimeter measurements have been supplemented by calculated values of potential evapotranspiration. There are many ways to make these calculations, e.g., those developed in the Glavnaia Geofizi-cheskaia Observatoriia in Leningrad by Budyko (1974, pp. 335−346), Zubenok, and others in this large research group.* They continually are being improved as more is found out about the physical and biological processes at work. For our purposes here, it will suffice to look at some of the patterns that result when one or another of these methods is applied.
At Midsummer
The smallest values in June—less than 20 mm for the month—are found at the shores of the Arctic Ocean, which are swept by cold maritime air streams. Tundra vegetation loses a large amount of sensible heat into this cold air at the expense of evapotranspiration. Lands farther from this cold ocean have values of evapotranspiration of 75−100 mm, more in accord with their supply of radiant energy.
In most agricultural regions of the western Soviet Union and Europe, potential evapotranspiration ranges between 120 and 150 mm during June, i.e., 4−5 mm day−1. The agricultural regions of eastern North America have potential evapotranspiration rates in summer of 4−6 mm day−1. In oases, this quantity ranges up to 8 mm day−1, aided no doubt by hot air off the adjacent desert.
Annual Total
Excluding the ice sheets, the lowest amounts are found on the northern shores of North America and Eurasia. They are smaller than 200 mm.
A marked latitudinal gradient, which was not seen on the map for June, appears in the annual map. In Europe, potential evapotranspiration decreases from 1300 mm or more in lands near the Mediterranean to 500 mm in Baltic lands, indicating the longer summers and the mild winters of the south.
In North America, potential evapotranspiration on the coastal plains of the Gulf of Mexico exceeds 1500 mm. From this there is a decrease northward to 1000 mm in the corn belt and 700 mm in the dairy belt, and to still smaller values in the boreal forest and tundra.